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172-589: Humans ( Homo sapiens , meaning "thinking man" or "wise man") or modern humans are the most common and widespread species of primate , and the last surviving species of the genus Homo . They are great apes characterized by their hairlessness , bipedalism , and high intelligence . Humans have large brains , enabling more advanced cognitive skills that enable them to thrive and adapt in varied environments, develop highly complex tools , and form complex social structures and civilizations . Humans are highly social , with individual humans tending to belong to

344-415: A multi-layered network of cooperating, distinct, or even competing social groups – from families and peer groups to corporations and political states . As such, social interactions between humans have established a wide variety of values, social norms , languages , and traditions (collectively termed institutions ), each of which bolsters human society . Humans are also highly curious , with

516-423: A mutation–selection balance . It is predicted that a viral quasispecies at a low but evolutionarily neutral and highly connected (that is, flat) region in the fitness landscape will outcompete a quasispecies located at a higher but narrower fitness peak in which the surrounding mutants are unfit, "the quasispecies effect" or the "survival of the flattest". There is no suggestion that a viral quasispecies resembles

688-588: A promoter sequence. The promoter is recognized and bound by transcription factors that recruit and help RNA polymerase bind to the region to initiate transcription. The recognition typically occurs as a consensus sequence like the TATA box . A gene can have more than one promoter, resulting in messenger RNAs ( mRNA ) that differ in how far they extend in the 5' end. Highly transcribed genes have "strong" promoter sequences that form strong associations with transcription factors, thereby initiating transcription at

860-667: A theory of mind . The human mind is capable of introspection , private thought , imagination , volition , and forming views on existence . This has allowed great technological advancements and complex tool development through complex reasoning and the transmission of knowledge to subsequent generations through language . Humans have had a dramatic effect on the environment . They are apex predators , being rarely preyed upon by other species. Human population growth , industrialization, land development, overconsumption and combustion of fossil fuels have led to environmental destruction and pollution that significantly contributes to

1032-542: A two-part name , called a "binomial". The first part of a binomial is the genus to which the species belongs. The second part is called the specific name or the specific epithet (in botanical nomenclature , also sometimes in zoological nomenclature ). For example, Boa constrictor is one of the species of the genus Boa , with constrictor being the species' epithet. While the definitions given above may seem adequate at first glance, when looked at more closely they represent problematic species concepts. For example,

1204-400: A "classical" method of determining species, such as with Linnaeus, early in evolutionary theory. However, different phenotypes are not necessarily different species (e.g. a four-winged Drosophila born to a two-winged mother is not a different species). Species named in this manner are called morphospecies . In the 1970s, Robert R. Sokal , Theodore J. Crovello and Peter Sneath proposed

1376-424: A 'smallest clade' idea" (a phylogenetic species concept). Mishler and Wilkins and others concur with this approach, even though this would raise difficulties in biological nomenclature. Wilkins cited the ichthyologist Charles Tate Regan 's early 20th century remark that "a species is whatever a suitably qualified biologist chooses to call a species". Wilkins noted that the philosopher Philip Kitcher called this

1548-428: A connected series of neighbouring populations, each of which can sexually interbreed with adjacent related populations, but for which there exist at least two "end" populations in the series, which are too distantly related to interbreed, though there is a potential gene flow between each "linked" population. Such non-breeding, though genetically connected, "end" populations may co-exist in the same region thus closing

1720-437: A consequence of better nutrition, healthcare, and living conditions. The average mass of an adult human is 59 kg (130 lb) for females and 77 kg (170 lb) for males. Like many other conditions, body weight and body type are influenced by both genetic susceptibility and environment and varies greatly among individuals. Humans have a far faster and more accurate throw than other animals. Humans are also among

1892-445: A continuous messenger RNA , referred to as a polycistronic mRNA . The term cistron in this context is equivalent to gene. The transcription of an operon's mRNA is often controlled by a repressor that can occur in an active or inactive state depending on the presence of specific metabolites. When active, the repressor binds to a DNA sequence at the beginning of the operon, called the operator region , and represses transcription of

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2064-432: A different species from its ancestors. Viruses have enormous populations, are doubtfully living since they consist of little more than a string of DNA or RNA in a protein coat, and mutate rapidly. All of these factors make conventional species concepts largely inapplicable. A viral quasispecies is a group of genotypes related by similar mutations, competing within a highly mutagenic environment, and hence governed by

2236-498: A double-helix run in opposite directions. Nucleic acid synthesis, including DNA replication and transcription occurs in the 5'→3' direction, because new nucleotides are added via a dehydration reaction that uses the exposed 3' hydroxyl as a nucleophile . The expression of genes encoded in DNA begins by transcribing the gene into RNA , a second type of nucleic acid that is very similar to DNA, but whose monomers contain

2408-488: A few genes and are transferable between individuals. For example, the genes for antibiotic resistance are usually encoded on bacterial plasmids and can be passed between individual cells, even those of different species, via horizontal gene transfer . Whereas the chromosomes of prokaryotes are relatively gene-dense, those of eukaryotes often contain regions of DNA that serve no obvious function. Simple single-celled eukaryotes have relatively small amounts of such DNA, whereas

2580-419: A few other places), saw the emergence of agriculture and permanent human settlement ; in turn, this led to the development of civilization and kickstarted a period of continuous (and ongoing) population growth and rapid technological change . Since then, a number of civilizations have risen and fallen, while a number of sociocultural and technological developments have resulted in significant changes to

2752-434: A gene - surprisingly, there is no definition that is entirely satisfactory. A gene is a DNA sequence that codes for a diffusible product. This product may be protein (as is the case in the majority of genes) or may be RNA (as is the case of genes that code for tRNA and rRNA). The crucial feature is that the product diffuses away from its site of synthesis to act elsewhere. The important parts of such definitions are: (1) that

2924-573: A gene corresponds to a transcription unit; (2) that genes produce both mRNA and noncoding RNAs; and (3) regulatory sequences control gene expression but are not part of the gene itself. However, there's one other important part of the definition and it is emphasized in Kostas Kampourakis' book Making Sense of Genes . Therefore in this book I will consider genes as DNA sequences encoding information for functional products, be it proteins or RNA molecules. With 'encoding information', I mean that

3096-410: A gene may be split across chromosomes but those transcripts are concatenated back together into a functional sequence by trans-splicing . It is also possible for overlapping genes to share some of their DNA sequence, either on opposite strands or the same strand (in a different reading frame, or even the same reading frame). In all organisms, two steps are required to read the information encoded in

3268-404: A gene's DNA and produce the protein it specifies. First, the gene's DNA is transcribed to messenger RNA ( mRNA ). Second, that mRNA is translated to protein. RNA-coding genes must still go through the first step, but are not translated into protein. The process of producing a biologically functional molecule of either RNA or protein is called gene expression , and the resulting molecule

3440-578: A gene: that of bacteriophage MS2 coat protein. The subsequent development of chain-termination DNA sequencing in 1977 by Frederick Sanger improved the efficiency of sequencing and turned it into a routine laboratory tool. An automated version of the Sanger method was used in early phases of the Human Genome Project . The theories developed in the early 20th century to integrate Mendelian genetics with Darwinian evolution are called

3612-439: A gene; however, members of a population may have different alleles at the locus, each with a slightly different gene sequence. The majority of eukaryotic genes are stored on a set of large, linear chromosomes. The chromosomes are packed within the nucleus in complex with storage proteins called histones to form a unit called a nucleosome . DNA packaged and condensed in this way is called chromatin . The manner in which DNA

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3784-508: A genetic boundary suitable for defining a species concept is present. DNA barcoding has been proposed as a way to distinguish species suitable even for non-specialists to use. One of the barcodes is a region of mitochondrial DNA within the gene for cytochrome c oxidase . A database, Barcode of Life Data System , contains DNA barcode sequences from over 190,000 species. However, scientists such as Rob DeSalle have expressed concern that classical taxonomy and DNA barcoding, which they consider

3956-461: A great capacity for altering their habitats by means of technology, irrigation , urban planning , construction, deforestation and desertification . Human settlements continue to be vulnerable to natural disasters , especially those placed in hazardous locations and with low quality of construction. Grouping and deliberate habitat alteration is often done with the goals of providing protection, accumulating comforts or material wealth, expanding

4128-448: A high rate. Others genes have "weak" promoters that form weak associations with transcription factors and initiate transcription less frequently. Eukaryotic promoter regions are much more complex and difficult to identify than prokaryotic promoters. Additionally, genes can have regulatory regions many kilobases upstream or downstream of the gene that alter expression. These act by binding to transcription factors which then cause

4300-612: A higher body fat percentage. At puberty , humans develop secondary sex characteristics . Females are capable of pregnancy , usually between puberty, at around 12 years old, and menopause , around the age of 50. As omnivorous creatures, they are capable of consuming a wide variety of plant and animal material, and have used fire and other forms of heat to prepare and cook food since the time of Homo erectus . Humans can survive for up to eight weeks without food and several days without water . Humans are generally diurnal , sleeping on average seven to nine hours per day. Childbirth

4472-473: A human can lose personhood (such as by going into a persistent vegetative state ). Humans are apes ( superfamily Hominoidea ). The lineage of apes that eventually gave rise to humans first split from gibbons (family Hylobatidae) and orangutans (genus Pongo ), then gorillas (genus Gorilla ), and finally, chimpanzees and bonobos (genus Pan ). The last split, between the human and chimpanzee–bonobo lineages, took place around 8–4 million years ago, in

4644-465: A lineage should be divided into multiple chronospecies , or when populations have diverged to have enough distinct character states to be described as cladistic species. Species and higher taxa were seen from the time of Aristotle until the 18th century as categories that could be arranged in a hierarchy, the great chain of being . In the 19th century, biologists grasped that species could evolve given sufficient time. Charles Darwin 's 1859 book On

4816-492: A misnomer, need to be reconciled, as they delimit species differently. Genetic introgression mediated by endosymbionts and other vectors can further make barcodes ineffective in the identification of species. A phylogenetic or cladistic species is "the smallest aggregation of populations (sexual) or lineages (asexual) diagnosable by a unique combination of character states in comparable individuals (semaphoronts)". The empirical basis – observed character states – provides

4988-577: A more barrel-shaped chests in contrast to the funnel shape of other apes, an adaptation for bipedal respiration. Apart from bipedalism and brain size, humans differ from chimpanzees mostly in smelling , hearing and digesting proteins . While humans have a density of hair follicles comparable to other apes, it is predominantly vellus hair , most of which is so short and wispy as to be practically invisible. Humans have about 2 million sweat glands spread over their entire bodies, many more than chimpanzees, whose sweat glands are scarce and are mainly located on

5160-499: A much clearer picture of life at the time. Between the 8th and 6th century BCE, Europe entered the classical antiquity age, a period when ancient Greece and ancient Rome flourished. Around this time other civilizations also came to prominence. The Maya civilization started to build cities and create complex calendars . In Africa, the Kingdom of Aksum overtook the declining Kingdom of Kush and facilitated trade between India and

5332-572: A new expanded definition that includes noncoding genes. However, some modern writers still do not acknowledge noncoding genes although this so-called "new" definition has been recognised for more than half a century. Although some definitions can be more broadly applicable than others, the fundamental complexity of biology means that no definition of a gene can capture all aspects perfectly. Not all genomes are DNA (e.g. RNA viruses ), bacterial operons are multiple protein-coding regions transcribed into single large mRNAs, alternative splicing enables

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5504-449: A particular species, including which genus (and higher taxa) it is placed in, is a hypothesis about the evolutionary relationships and distinguishability of that group of organisms. As further information comes to hand, the hypothesis may be corroborated or refuted. Sometimes, especially in the past when communication was more difficult, taxonomists working in isolation have given two distinct names to individual organisms later identified as

5676-400: A perfect model of life, it is still a useful tool to scientists and conservationists for studying life on Earth, regardless of the theoretical difficulties. If species were fixed and clearly distinct from one another, there would be no problem, but evolutionary processes cause species to change. This obliges taxonomists to decide, for example, when enough change has occurred to declare that

5848-400: A process known as RNA splicing . Finally, the ends of gene transcripts are defined by cleavage and polyadenylation (CPA) sites , where newly produced pre-mRNA gets cleaved and a string of ~200 adenosine monophosphates is added at the 3' end. The poly(A) tail protects mature mRNA from degradation and has other functions, affecting translation, localization, and transport of the transcript from

6020-419: A protein-coding gene consists of many elements of which the actual protein coding sequence is often only a small part. These include introns and untranslated regions of the mature mRNA. Noncoding genes can also contain introns that are removed during processing to produce the mature functional RNA. All genes are associated with regulatory sequences that are required for their expression. First, genes require

6192-450: A result, humans are a cosmopolitan species found in almost all regions of the world, including tropical rainforest , arid desert , extremely cold arctic regions , and heavily polluted cities; in comparison, most other species are confined to a few geographical areas by their limited adaptability. The human population is not, however, uniformly distributed on the Earth 's surface, because

6364-627: A series of holy wars to regain control of the Holy Land from Muslims . In the Americas, between 200 and 900 CE Mesoamerica was in its Classic Period , while further north, complex Mississippian societies would arise starting around 800 CE. The Mongol Empire would conquer much of Eurasia in the 13th and 14th centuries. Over this same time period, the Mali Empire in Africa grew to be

6536-400: A short way of saying that something applies to many species within a genus, but not to all. If scientists mean that something applies to all species within a genus, they use the genus name without the specific name or epithet. The names of genera and species are usually printed in italics . However, abbreviations such as "sp." should not be italicised. When a species' identity is not clear,

6708-639: A similar species. Migrating out of Africa , they gradually replaced and interbred with local populations of archaic humans. Multiple hypotheses for the extinction of archaic human species such as Neanderthals include competition, violence , interbreeding with Homo sapiens , or inability to adapt to climate change . Humans began exhibiting behavioral modernity about 160,000–60,000 years ago. For most of their history, humans were nomadic hunter-gatherers. The Neolithic Revolution , which began in Southwest Asia around 13,000 years ago (and separately in

6880-412: A single genomic region to encode multiple district products and trans-splicing concatenates mRNAs from shorter coding sequence across the genome. Since molecular definitions exclude elements such as introns, promotors, and other regulatory regions , these are instead thought of as "associated" with the gene and affect its function. An even broader operational definition is sometimes used to encompass

7052-404: A specialist may use "cf." before the epithet to indicate that confirmation is required. The abbreviations "nr." (near) or "aff." (affine) may be used when the identity is unclear but when the species appears to be similar to the species mentioned after. With the rise of online databases, codes have been devised to provide identifiers for species that are already defined, including: The naming of

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7224-523: A species as groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups. It has been argued that this definition is a natural consequence of the effect of sexual reproduction on the dynamics of natural selection. Mayr's use of the adjective "potentially" has been a point of debate; some interpretations exclude unusual or artificial matings that occur only in captivity, or that involve animals capable of mating but that do not normally do so in

7396-400: A species as determined by a taxonomist. A typological species is a group of organisms in which individuals conform to certain fixed properties (a type), so that even pre-literate people often recognise the same taxon as do modern taxonomists. The clusters of variations or phenotypes within specimens (such as longer or shorter tails) would differentiate the species. This method was used as

7568-491: A species. All species definitions assume that an organism acquires its genes from one or two parents very like the "daughter" organism, but that is not what happens in HGT. There is strong evidence of HGT between very dissimilar groups of prokaryotes , and at least occasionally between dissimilar groups of eukaryotes , including some crustaceans and echinoderms . The evolutionary biologist James Mallet concludes that there

7740-685: A species. Generally the term includes the unknown element of a distinct act of creation. Many authors have argued that a simple textbook definition, following Mayr's concept, works well for most multi-celled organisms , but breaks down in several situations: Species identification is made difficult by discordance between molecular and morphological investigations; these can be categorised as two types: (i) one morphology, multiple lineages (e.g. morphological convergence , cryptic species ) and (ii) one lineage, multiple morphologies (e.g. phenotypic plasticity , multiple life-cycle stages). In addition, horizontal gene transfer (HGT) makes it difficult to define

7912-475: A strict definition of the word "gene" with which nearly every expert can agree. First, in order for a nucleotide sequence to be considered a true gene, an open reading frame (ORF) must be present. The ORF can be thought of as the "gene itself"; it begins with a starting mark common for every gene and ends with one of three possible finish line signals. One of the key enzymes in this process, the RNA polymerase, zips along

8084-450: A successful labor increased significantly during the 20th century in wealthier countries with the advent of new medical technologies. In contrast, pregnancy and natural childbirth remain hazardous ordeals in developing regions of the world, with maternal death rates approximately 100 times greater than in developed countries. Both the mother and the father provide care for human offspring, in contrast to other primates, where parental care

8256-528: A taxonomic decision at the discretion of cognizant specialists, is not governed by the Codes of Zoological or Botanical Nomenclature, in contrast to the PhyloCode , and contrary to what is done in several other fields, in which the definitions of technical terms, like geochronological units and geopolitical entities, are explicitly delimited. The nomenclatural codes that guide the naming of species, including

8428-517: A traditional biological species. The International Committee on Taxonomy of Viruses has since 1962 developed a universal taxonomic scheme for viruses; this has stabilised viral taxonomy. Most modern textbooks make use of Ernst Mayr 's 1942 definition, known as the Biological Species Concept as a basis for further discussion on the definition of species. It is also called a reproductive or isolation concept. This defines

8600-409: A true gene, by this definition, one has to prove that the transcript has a biological function. Early speculations on the size of a typical gene were based on high-resolution genetic mapping and on the size of proteins and RNA molecules. A length of 1500 base pairs seemed reasonable at the time (1965). This was based on the idea that the gene was the DNA that was directly responsible for production of

8772-469: A unit of biodiversity . Other ways of defining species include their karyotype , DNA sequence, morphology , behaviour, or ecological niche . In addition, paleontologists use the concept of the chronospecies since fossil reproduction cannot be examined. The most recent rigorous estimate for the total number of species of eukaryotes is between 8 and 8.7 million. About 14% of these had been described by 2011. All species (except viruses ) are given

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8944-447: A variation on the morphological species concept, a phenetic species, defined as a set of organisms with a similar phenotype to each other, but a different phenotype from other sets of organisms. It differs from the morphological species concept in including a numerical measure of distance or similarity to cluster entities based on multivariate comparisons of a reasonably large number of phenotypic traits. A mate-recognition species

9116-515: A variety of reasons. Viruses are a special case, driven by a balance of mutation and selection , and can be treated as quasispecies . Biologists and taxonomists have made many attempts to define species, beginning from morphology and moving towards genetics . Early taxonomists such as Linnaeus had no option but to describe what they saw: this was later formalised as the typological or morphological species concept. Ernst Mayr emphasised reproductive isolation, but this, like other species concepts,

9288-403: A wide variety of plant and animal material. Human groups have adopted a range of diets from purely vegan to primarily carnivorous . In some cases, dietary restrictions in humans can lead to deficiency diseases ; however, stable human groups have adapted to many dietary patterns through both genetic specialization and cultural conventions to use nutritionally balanced food sources. The human diet

9460-438: Is "an entity composed of organisms which maintains its identity from other such entities through time and over space, and which has its own independent evolutionary fate and historical tendencies". This differs from the biological species concept in embodying persistence over time. Wiley and Mayden stated that they see the evolutionary species concept as "identical" to Willi Hennig 's species-as-lineages concept, and asserted that

9632-506: Is 38 weeks, but a normal pregnancy can vary by up to 37 days. Embryonic development in the human covers the first eight weeks of development; at the beginning of the ninth week the embryo is termed a fetus . Humans are able to induce early labor or perform a caesarean section if the child needs to be born earlier for medical reasons. In developed countries, infants are typically 3–4 kg (7–9 lb) in weight and 47–53 cm (19–21 in) in height at birth. However, low birth weight

9804-451: Is a basic unit of heredity . The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA . There are two types of molecular genes: protein-coding genes and non-coding genes. During gene expression (the synthesis of RNA or protein from a gene), DNA is first copied into RNA . RNA can be directly functional or be the intermediate template for

9976-400: Is a group of sexually reproducing organisms that recognise one another as potential mates. Expanding on this to allow for post-mating isolation, a cohesion species is the most inclusive population of individuals having the potential for phenotypic cohesion through intrinsic cohesion mechanisms; no matter whether populations can hybridise successfully, they are still distinct cohesion species if

10148-484: Is a mixture of the two parental sets. Among the 23 pairs of chromosomes, there are 22 pairs of autosomes and one pair of sex chromosomes . Like other mammals, humans have an XY sex-determination system , so that females have the sex chromosomes XX and males have XY. Genes and environment influence human biological variation in visible characteristics, physiology, disease susceptibility and mental abilities. The exact influence of genes and environment on certain traits

10320-422: Is a population decline in the winter months of this realm. Humans established nation-states in the other seven realms, such as South Africa , India , Russia , Australia , Fiji , United States and Brazil (each located in a different biogeographical realm). By using advanced tools and clothing , humans have been able to extend their tolerance to a wide variety of temperatures, humidities , and altitudes. As

10492-458: Is a set of organisms adapted to a particular set of resources, called a niche, in the environment. According to this concept, populations form the discrete phenetic clusters that we recognise as species because the ecological and evolutionary processes controlling how resources are divided up tend to produce those clusters. A genetic species as defined by Robert Baker and Robert Bradley is a set of genetically isolated interbreeding populations. This

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10664-414: Is called speciation . Charles Darwin was the first to describe the role of natural selection in speciation in his 1859 book The Origin of Species . Speciation depends on a measure of reproductive isolation , a reduced gene flow. This occurs most easily in allopatric speciation, where populations are separated geographically and can diverge gradually as mutations accumulate. Reproductive isolation

10836-456: Is called a gene product . The nucleotide sequence of a gene's DNA specifies the amino acid sequence of a protein through the genetic code . Sets of three nucleotides, known as codons , each correspond to a specific amino acid. The principle that three sequential bases of DNA code for each amino acid was demonstrated in 1961 using frameshift mutations in the rIIB gene of bacteriophage T4 (see Crick, Brenner et al. experiment ). Additionally,

11008-451: Is common in developing countries, and contributes to the high levels of infant mortality in these regions. Compared with other species, human childbirth is dangerous, with a much higher risk of complications and death. The size of the fetus's head is more closely matched to the pelvis than in other primates. The reason for this is not completely understood, but it contributes to a painful labor that can last 24 hours or more. The chances of

11180-421: Is dangerous, with a high risk of complications and death . Often, both the mother and the father provide care for their children, who are helpless at birth . Humans have a large, highly developed, and complex prefrontal cortex , the region of the brain associated with higher cognition. Humans are highly intelligent and capable of episodic memory ; they have flexible facial expressions, self-awareness , and

11352-403: Is described formally, in a publication that assigns it a unique scientific name. The description typically provides means for identifying the new species, which may not be based solely on morphology (see cryptic species ), differentiating it from other previously described and related or confusable species and provides a validly published name (in botany) or an available name (in zoology) when

11524-671: Is further weakened by the existence of microspecies , groups of organisms, including many plants, with very little genetic variability, usually forming species aggregates . For example, the dandelion Taraxacum officinale and the blackberry Rubus fruticosus are aggregates with many microspecies—perhaps 400 in the case of the blackberry and over 200 in the dandelion, complicated by hybridisation , apomixis and polyploidy , making gene flow between populations difficult to determine, and their taxonomy debatable. Species complexes occur in insects such as Heliconius butterflies, vertebrates such as Hypsiboas treefrogs, and fungi such as

11696-476: Is generally aging, with the median age around 40 years. In the developing world , the median age is between 15 and 20 years. While one in five Europeans is 60 years of age or older, only one in twenty Africans is 60 years of age or older. In 2012, the United Nations estimated that there were 316,600 living centenarians (humans of age 100 or older) worldwide. Humans are omnivorous , capable of consuming

11868-726: Is hard or even impossible to test. Later biologists have tried to refine Mayr's definition with the recognition and cohesion concepts, among others. Many of the concepts are quite similar or overlap, so they are not easy to count: the biologist R. L. Mayden recorded about 24 concepts, and the philosopher of science John Wilkins counted 26. Wilkins further grouped the species concepts into seven basic kinds of concepts: (1) agamospecies for asexual organisms (2) biospecies for reproductively isolated sexual organisms (3) ecospecies based on ecological niches (4) evolutionary species based on lineage (5) genetic species based on gene pool (6) morphospecies based on form or phenotype and (7) taxonomic species,

12040-438: Is inherited only from the mother, geneticists have concluded that the last female common ancestor whose genetic marker is found in all modern humans, the so-called mitochondrial Eve , must have lived around 90,000 to 200,000 years ago. Most human reproduction takes place by internal fertilization via sexual intercourse , but can also occur through assisted reproductive technology procedures. The average gestation period

12212-403: Is mostly done by the mother. Helpless at birth , humans continue to grow for some years, typically reaching sexual maturity at 15 to 17 years of age. The human life span has been split into various stages ranging from three to twelve. Common stages include infancy , childhood , adolescence , adulthood and old age . The lengths of these stages have varied across cultures and time periods but

12384-400: Is nearly the same for all known organisms. The total complement of genes in an organism or cell is known as its genome , which may be stored on one or more chromosomes . A chromosome consists of a single, very long DNA helix on which thousands of genes are encoded. The region of the chromosome at which a particular gene is located is called its locus . Each locus contains one allele of

12556-403: Is no easy way to tell whether related geographic or temporal forms belong to the same or different species. Species gaps can be verified only locally and at a point of time. One is forced to admit that Darwin's insight is correct: any local reality or integrity of species is greatly reduced over large geographic ranges and time periods. The botanist Brent Mishler argued that the species concept

12728-478: Is not valid, notably because gene flux decreases gradually rather than in discrete steps, which hampers objective delimitation of species. Indeed, complex and unstable patterns of gene flux have been observed in cichlid teleosts of the East African Great Lakes . Wilkins argued that "if we were being true to evolution and the consequent phylogenetic approach to taxa, we should replace it with

12900-514: Is not well understood. While no humans – not even monozygotic twins  – are genetically identical, two humans on average will have a genetic similarity of 99.5%-99.9%. This makes them more homogeneous than other great apes, including chimpanzees. This small variation in human DNA compared to many other species suggests a population bottleneck during the Late Pleistocene (around 100,000 years ago), in which

13072-412: Is prominently reflected in human culture and has led to the development of food science . Species A species ( pl. : species) is a population of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring , typically by sexual reproduction . It is the basic unit of classification and a taxonomic rank of an organism, as well as

13244-586: Is similar to Mayr's Biological Species Concept, but stresses genetic rather than reproductive isolation. In the 21st century, a genetic species could be established by comparing DNA sequences. Earlier, other methods were available, such as comparing karyotypes (sets of chromosomes ) and allozymes ( enzyme variants). An evolutionarily significant unit (ESU) or "wildlife species" is a population of organisms considered distinct for purposes of conservation. In palaeontology , with only comparative anatomy (morphology) and histology from fossils as evidence,

13416-434: Is sometimes an important source of genetic variation. Viruses can transfer genes between species. Bacteria can exchange plasmids with bacteria of other species, including some apparently distantly related ones in different phylogenetic domains , making analysis of their relationships difficult, and weakening the concept of a bacterial species. Gene In biology , the word gene has two meanings. The Mendelian gene

13588-403: Is still part of the definition of a gene in most textbooks. For example, The primary function of the genome is to produce RNA molecules. Selected portions of the DNA nucleotide sequence are copied into a corresponding RNA nucleotide sequence, which either encodes a protein (if it is an mRNA) or forms a 'structural' RNA, such as a transfer RNA (tRNA) or ribosomal RNA (rRNA) molecule. Each region of

13760-399: Is stored on the histones, as well as chemical modifications of the histone itself, regulate whether a particular region of DNA is accessible for gene expression . In addition to genes, eukaryotic chromosomes contain sequences involved in ensuring that the DNA is copied without degradation of end regions and sorted into daughter cells during cell division: replication origins , telomeres , and

13932-594: Is threatened by hybridisation, but this can be selected against once a pair of populations have incompatible alleles of the same gene, as described in the Bateson–Dobzhansky–Muller model . A different mechanism, phyletic speciation, involves one lineage gradually changing over time into a new and distinct form (a chronospecies ), without increasing the number of resultant species. Horizontal gene transfer between organisms of different species, either through hybridisation , antigenic shift , or reassortment ,

14104-647: Is typified by an unusually rapid growth spurt during adolescence. Human females undergo menopause and become infertile at around the age of 50. It has been proposed that menopause increases a woman's overall reproductive success by allowing her to invest more time and resources in her existing offspring, and in turn their children (the grandmother hypothesis ), rather than by continuing to bear children into old age. The life span of an individual depends on two major factors, genetics and lifestyle choices. For various reasons, including biological/genetic causes, women live on average about four years longer than men. As of 2018,

14276-545: The British Empire expanded to become the world's largest empire . A tenuous balance of power among European nations collapsed in 1914 with the outbreak of the First World War , one of the deadliest conflicts in history. In the 1930s, a worldwide economic crisis led to the rise of authoritarian regimes and a Second World War , involving almost all of the world's countries . The war's destruction led to

14448-518: The Caral–Supe civilization is the oldest complex civilization in the Americas. Astronomy and mathematics were also developed and the Great Pyramid of Giza was built. There is evidence of a severe drought lasting about a hundred years that may have caused the decline of these civilizations, with new ones appearing in the aftermath. Babylonians came to dominate Mesopotamia while others, such as

14620-750: The Columbian Exchange . This expansion led to the Atlantic slave trade and the genocide of Native American peoples . This period also marked the Scientific Revolution , with great advances in mathematics , mechanics , astronomy and physiology . The late modern period (1800–present) saw the Technological and Industrial Revolution bring such discoveries as imaging technology , major innovations in transport and energy development . Influenced by Enlightenment ideals,

14792-471: The Homo genus. The name " Homo sapiens " means 'wise man' or 'knowledgeable man'. There is disagreement if certain extinct members of the genus, namely Neanderthals , should be included as a separate species of humans or as a subspecies of H. sapiens . Human is a loanword of Middle English from Old French humain , ultimately from Latin hūmānus , the adjectival form of homō ('man' – in

14964-530: The ICZN for animals and the ICN for plants, do not make rules for defining the boundaries of the species. Research can change the boundaries, also known as circumscription, based on new evidence. Species may then need to be distinguished by the boundary definitions used, and in such cases the names may be qualified with sensu stricto ("in the narrow sense") to denote usage in the exact meaning given by an author such as

15136-579: The Old World over the following millennia. It also occurred independently in Mesoamerica (about 6,000 years ago), China, Papua New Guinea , and the Sahel and West Savanna regions of Africa. Access to food surplus led to the formation of permanent human settlements , the domestication of animals and the use of metal tools for the first time in history. Agriculture and sedentary lifestyle led to

15308-646: The Poverty Point culture , Minoans and the Shang dynasty , rose to prominence in new areas. The Late Bronze Age collapse around 1200 BCE resulted in the disappearance of a number of civilizations and the beginning of the Greek Dark Ages . During this period iron started replacing bronze, leading to the Iron Age . In the 5th century BCE, history started being recorded as a discipline , which provided

15480-511: The aging process. The centromere is required for binding spindle fibres to separate sister chromatids into daughter cells during cell division . Prokaryotes ( bacteria and archaea ) typically store their genomes on a single, large, circular chromosome . Similarly, some eukaryotic organelles contain a remnant circular chromosome with a small number of genes. Prokaryotes sometimes supplement their chromosome with additional small circles of DNA called plasmids , which usually encode only

15652-401: The central dogma of molecular biology , which states that proteins are translated from RNA , which is transcribed from DNA . This dogma has since been shown to have exceptions, such as reverse transcription in retroviruses . The modern study of genetics at the level of DNA is known as molecular genetics . In 1972, Walter Fiers and his team were the first to determine the sequence of

15824-419: The centromere . Replication origins are the sequence regions where DNA replication is initiated to make two copies of the chromosome. Telomeres are long stretches of repetitive sequences that cap the ends of the linear chromosomes and prevent degradation of coding and regulatory regions during DNA replication . The length of the telomeres decreases each time the genome is replicated and has been implicated in

15996-856: The final defeat of the Byzantine Empire , and the rise of the Ottoman Empire . Meanwhile, Japan entered the Edo period , the Qing dynasty rose in China and the Mughal Empire ruled much of India. Europe underwent the Renaissance , starting in the 15th century, and the Age of Discovery began with the exploring and colonizing of new regions. This included the colonization of the Americas and

16168-399: The fly agaric . Natural hybridisation presents a challenge to the concept of a reproductively isolated species, as fertile hybrids permit gene flow between two populations. For example, the carrion crow Corvus corone and the hooded crow Corvus cornix appear and are classified as separate species, yet they can hybridise where their geographical ranges overlap. A ring species is

16340-507: The jaguar ( Panthera onca ) of Latin America or the leopard ( Panthera pardus ) of Africa and Asia. In contrast, the scientific names of species are chosen to be unique and universal (except for some inter-code homonyms ); they are in two parts used together : the genus as in Puma , and the specific epithet as in concolor . A species is given a taxonomic name when a type specimen

16512-554: The modern synthesis , a term introduced by Julian Huxley . This view of evolution was emphasized by George C. Williams ' gene-centric view of evolution . He proposed that the Mendelian gene is a unit of natural selection with the definition: "that which segregates and recombines with appreciable frequency." Related ideas emphasizing the centrality of Mendelian genes and the importance of natural selection in evolution were popularized by Richard Dawkins . The development of

16684-475: The neutral theory of evolution in the late 1960s led to the recognition that random genetic drift is a major player in evolution and that neutral theory should be the null hypothesis of molecular evolution. This led to the construction of phylogenetic trees and the development of the molecular clock , which is the basis of all dating techniques using DNA sequences. These techniques are not confined to molecular gene sequences but can be used on all DNA segments in

16856-750: The operon ; when the repressor is inactive transcription of the operon can occur (see e.g. Lac operon ). The products of operon genes typically have related functions and are involved in the same regulatory network . Though many genes have simple structures, as with much of biology, others can be quite complex or represent unusual edge-cases. Eukaryotic genes often have introns that are much larger than their exons, and those introns can even have other genes nested inside them . Associated enhancers may be many kilobase away, or even on entirely different chromosomes operating via physical contact between two chromosomes. A single gene can encode multiple different functional products by alternative splicing , and conversely

17028-508: The phenotype of the individual. Most biological traits occur under the combined influence of polygenes (a set of different genes) and gene–environment interactions . Some genetic traits are instantly visible, such as eye color or the number of limbs, others are not, such as blood type , the risk for specific diseases, or the thousands of basic biochemical processes that constitute life . A gene can acquire mutations in its sequence , leading to different variants, known as alleles , in

17200-449: The population . These alleles encode slightly different versions of a gene, which may cause different phenotypical traits. Genes evolve due to natural selection or survival of the fittest and genetic drift of the alleles. There are many different ways to use the term "gene" based on different aspects of their inheritance, selection, biological function, or molecular structure but most of these definitions fall into two categories,

17372-406: The "cynical species concept", and arguing that far from being cynical, it usefully leads to an empirical taxonomy for any given group, based on taxonomists' experience. Other biologists have gone further and argued that we should abandon species entirely, and refer to the "Least Inclusive Taxonomic Units" (LITUs), a view that would be coherent with current evolutionary theory. The species concept

17544-465: The Americas and Europe experienced a period of political revolutions known as the Age of Revolution . The Napoleonic Wars raged through Europe in the early 1800s, Spain lost most of its colonies in the New World , while Europeans continued expansion into Africa  – where European control went from 10% to almost 90% in less than 50 years – and Oceania. In the 19th century,

17716-404: The DNA helix that produces a functional RNA molecule constitutes a gene. We define a gene as a DNA sequence that is transcribed. This definition includes genes that do not encode proteins (not all transcripts are messenger RNA). The definition normally excludes regions of the genome that control transcription but are not themselves transcribed. We will encounter some exceptions to our definition of

17888-450: The DNA sequence is used as a template for the production of an RNA molecule or a protein that performs some function. The emphasis on function is essential because there are stretches of DNA that produce non-functional transcripts and they do not qualify as genes. These include obvious examples such as transcribed pseudogenes as well as less obvious examples such as junk RNA produced as noise due to transcription errors. In order to qualify as

18060-766: The DNA to loop so that the regulatory sequence (and bound transcription factor) become close to the RNA polymerase binding site. For example, enhancers increase transcription by binding an activator protein which then helps to recruit the RNA polymerase to the promoter; conversely silencers bind repressor proteins and make the DNA less available for RNA polymerase. The mature messenger RNA produced from protein-coding genes contains untranslated regions at both ends which contain binding sites for ribosomes , RNA-binding proteins , miRNA , as well as terminator , and start and stop codons . In addition, most eukaryotic open reading frames contain untranslated introns , which are removed and exons , which are connected together in

18232-938: The Mediterranean. In West Asia, the Achaemenid Empire 's system of centralized governance became the precursor to many later empires, while the Gupta Empire in India and the Han dynasty in China have been described as golden ages in their respective regions. Following the fall of the Western Roman Empire in 476, Europe entered the Middle Ages . During this period, Christianity and the Church would provide centralized authority and education. In

18404-514: The Mendelian gene or the molecular gene. The Mendelian gene is the classical gene of genetics and it refers to any heritable trait. This is the gene described in The Selfish Gene . More thorough discussions of this version of a gene can be found in the articles Genetics and Gene-centered view of evolution . The molecular gene definition is more commonly used across biochemistry, molecular biology, and most of genetics —

18576-544: The Middle East, Islam became the prominent religion and expanded into North Africa. It led to an Islamic Golden Age , inspiring achievements in architecture , the revival of old advances in science and technology, and the formation of a distinct way of life. The Christian and Islamic worlds would eventually clash, with the Kingdom of England , the Kingdom of France and the Holy Roman Empire declaring

18748-626: The Origin of Species explained how species could arise by natural selection . That understanding was greatly extended in the 20th century through genetics and population ecology . Genetic variability arises from mutations and recombination , while organisms themselves are mobile, leading to geographical isolation and genetic drift with varying selection pressures . Genes can sometimes be exchanged between species by horizontal gene transfer ; new species can arise rapidly through hybridisation and polyploidy ; and species may become extinct for

18920-405: The abbreviation "sp." in the singular or "spp." (standing for species pluralis , Latin for "multiple species") in the plural in place of the specific name or epithet (e.g. Canis sp.). This commonly occurs when authors are confident that some individuals belong to a particular genus but are not sure to which exact species they belong, as is common in paleontology . Authors may also use "spp." as

19092-433: The adenines of one strand are paired with the thymines of the other strand, and so on. Due to the chemical composition of the pentose residues of the bases, DNA strands have directionality. One end of a DNA polymer contains an exposed hydroxyl group on the deoxyribose ; this is known as the 3' end of the molecule. The other end contains an exposed phosphate group; this is the 5' end . The two strands of

19264-570: The amount of hybridisation is insufficient to completely mix their respective gene pools . A further development of the recognition concept is provided by the biosemiotic concept of species. In microbiology , genes can move freely even between distantly related bacteria, possibly extending to the whole bacterial domain. As a rule of thumb, microbiologists have assumed that members of Bacteria or Archaea with 16S ribosomal RNA gene sequences more similar than 97% to each other need to be checked by DNA–DNA hybridisation to decide if they belong to

19436-476: The available food, improving aesthetics , increasing knowledge or enhancing the exchange of resources. Humans are one of the most adaptable species, despite having a low or narrow tolerance for many of the earth's extreme environments. Currently the species is present in all eight biogeographical realms , although their presence in the Antarctic realm is very limited to research stations and annually there

19608-556: The best long-distance runners in the animal kingdom, but slower over short distances. Humans' thinner body hair and more productive sweat glands help avoid heat exhaustion while running for long distances. Compared to other apes, the human heart produces greater stroke volume and cardiac output and the aorta is proportionately larger. Like most animals, humans are a diploid and eukaryotic species. Each somatic cell has two sets of 23 chromosomes , each set received from one parent; gametes have only one set of chromosomes, which

19780-474: The biological species concept, "the several versions" of the phylogenetic species concept, and the idea that species are of the same kind as higher taxa are not suitable for biodiversity studies (with the intention of estimating the number of species accurately). They further suggested that the concept works for both asexual and sexually-reproducing species. A version of the concept is Kevin de Queiroz 's "General Lineage Concept of Species". An ecological species

19952-505: The biological species concept, a cladistic species does not rely on reproductive isolation – its criteria are independent of processes that are integral in other concepts. Therefore, it applies to asexual lineages. However, it does not always provide clear cut and intuitively satisfying boundaries between taxa, and may require multiple sources of evidence, such as more than one polymorphic locus, to give plausible results. An evolutionary species, suggested by George Gaylord Simpson in 1951,

20124-428: The boundaries between closely related species become unclear with hybridisation , in a species complex of hundreds of similar microspecies , and in a ring species . Also, among organisms that reproduce only asexually , the concept of a reproductive species breaks down, and each clone is potentially a microspecies. Although none of these are entirely satisfactory definitions, and while the concept of species may not be

20296-662: The collapse of most global empires, leading to widespread decolonization. Following the conclusion of the Second World War in 1945, the United States and the USSR emerged as the remaining global superpowers . This led to a Cold War that saw a struggle for global influence, including a nuclear arms race and a space race , ending in the collapse of the Soviet Union. The current Information Age , spurred by

20468-782: The combined eastern and western Roman Empire in the 4th century AD. Bubonic plagues , first recorded in the 6th century AD, reduced the population by 50%, with the Black Death killing 75–200 million people in Eurasia and North Africa alone. Human population is believed to have reached one billion in 1800. It has since then increased exponentially, reaching two billion in 1930 and three billion in 1960, four in 1975, five in 1987 and six billion in 1999. It passed seven billion in 2011 and passed eight billion in November 2022. It took over two million years of human prehistory and history for

20640-402: The complexity of these diverse phenomena, where a gene is defined as a union of genomic sequences encoding a coherent set of potentially overlapping functional products. This definition categorizes genes by their functional products (proteins or RNA) rather than their specific DNA loci, with regulatory elements classified as gene-associated regions. The existence of discrete inheritable units

20812-433: The concept of a chronospecies can be applied. During anagenesis (evolution, not necessarily involving branching), some palaeontologists seek to identify a sequence of species, each one derived from the phyletically extinct one before through continuous, slow and more or less uniform change. In such a time sequence, some palaeontologists assess how much change is required for a morphologically distinct form to be considered

20984-472: The continent, gradually replacing or interbreeding with local populations of archaic humans. Humans began exhibiting behavioral modernity about 160,000–70,000 years ago, and possibly earlier. This development was likely selected amidst natural climate change in Middle to Late Pleistocene Africa. The "out of Africa" migration took place in at least two waves, the first around 130,000 to 100,000 years ago,

21156-423: The desire to understand and influence phenomena having motivated humanity's development of science , technology , philosophy , mythology , religion , and other frameworks of knowledge ; humans also study themselves through such domains as anthropology , social science , history , psychology , and medicine . There are estimated to be more than eight billion living humans . Although some scientists equate

21328-514: The development of the Internet and Artificial Intelligence systems, sees the world becoming increasingly globalized and interconnected. Early human settlements were dependent on proximity to water and – depending on the lifestyle – other natural resources used for subsistence , such as populations of animal prey for hunting and arable land for growing crops and grazing livestock. Modern humans, however, have

21500-524: The distinction between a heterozygote and homozygote , and the phenomenon of discontinuous inheritance. Prior to Mendel's work, the dominant theory of heredity was one of blending inheritance , which suggested that each parent contributed fluids to the fertilization process and that the traits of the parents blended and mixed to produce the offspring. Charles Darwin developed a theory of inheritance he termed pangenesis , from Greek pan ("all, whole") and genesis ("birth") / genos ("origin"). Darwin used

21672-410: The early 1950s the prevailing view was that the genes in a chromosome acted like discrete entities arranged like beads on a string. The experiments of Benzer using mutants defective in the rII region of bacteriophage T4 (1955–1959) showed that individual genes have a simple linear structure and are likely to be equivalent to a linear section of DNA. Collectively, this body of research established

21844-636: The early 20th century, there has been continuous human presence in Antarctica through research stations and, since 2000, in space through habitation on the International Space Station . All modern humans are classified into the species Homo sapiens , coined by Carl Linnaeus in his 1735 work Systema Naturae . The generic name " Homo " is a learned 18th-century derivation from Latin homō , which refers to humans of either sex. The word human can refer to all members of

22016-675: The emergence of early civilizations . An urban revolution took place in the 4th millennium BCE with the development of city-states , particularly Sumerian cities located in Mesopotamia . It was in these cities that the earliest known form of writing, cuneiform script , appeared around 3000 BCE. Other major civilizations to develop around this time were Ancient Egypt and the Indus Valley Civilisation . They eventually traded with each other and invented technology such as wheels, plows and sails. Emerging by 3000 BCE,

22188-435: The evidence to support hypotheses about evolutionarily divergent lineages that have maintained their hereditary integrity through time and space. Molecular markers may be used to determine diagnostic genetic differences in the nuclear or mitochondrial DNA of various species. For example, in a study done on fungi , studying the nucleotide characters using cladistic species produced the most accurate results in recognising

22360-522: The fact that both protein-coding genes and noncoding genes have been known for more than 50 years, there are still a number of textbooks, websites, and scientific publications that define a gene as a DNA sequence that specifies a protein. In other words, the definition is restricted to protein-coding genes. Here is an example from a recent article in American Scientist. ... to truly assess the potential significance of de novo genes, we relied on

22532-413: The functional product. The discovery of introns in the 1970s meant that many eukaryotic genes were much larger than the size of the functional product would imply. Typical mammalian protein-coding genes, for example, are about 62,000 base pairs in length (transcribed region) and since there are about 20,000 of them they occupy about 35–40% of the mammalian genome (including the human genome). In spite of

22704-630: The gene that is described in terms of DNA sequence. There are many different definitions of this gene — some of which are misleading or incorrect. Very early work in the field that became molecular genetics suggested the concept that one gene makes one protein (originally 'one gene - one enzyme'). However, genes that produce repressor RNAs were proposed in the 1950s and by the 1960s, textbooks were using molecular gene definitions that included those that specified functional RNA molecules such as ribosomal RNA and tRNA (noncoding genes) as well as protein-coding genes. This idea of two kinds of genes

22876-421: The genome. The vast majority of organisms encode their genes in long strands of DNA (deoxyribonucleic acid). DNA consists of a chain made from four types of nucleotide subunits, each composed of: a five-carbon sugar ( 2-deoxyribose ), a phosphate group, and one of the four bases adenine , cytosine , guanine , and thymine . Two chains of DNA twist around each other to form a DNA double helix with

23048-563: The genomes of 1,184 individuals from 11 populations and identified 1.6 million single nucleotide polymorphisms . African populations harbor the highest number of private genetic variants. While many of the common variants found in populations outside of Africa are also found on the African continent, there are still large numbers that are private to these regions, especially Oceania and the Americas . By 2010 estimates, humans have approximately 22,000 genes. By comparing mitochondrial DNA , which

23220-421: The genomes of complex multicellular organisms , including humans, contain an absolute majority of DNA without an identified function. This DNA has often been referred to as " junk DNA ". However, more recent analyses suggest that, although protein-coding DNA makes up barely 2% of the human genome , about 80% of the bases in the genome may be expressed, so the term "junk DNA" may be a misnomer. The structure of

23392-591: The global average life expectancy at birth of a girl is estimated to be 74.9 years compared to 70.4 for a boy. There are significant geographical variations in human life expectancy, mostly correlated with economic development – for example, life expectancy at birth in Hong Kong is 87.6 years for girls and 81.8 for boys, while in the Central African Republic , it is 55.0 years for girls and 50.6 for boys. The developed world

23564-443: The human lifestyle. Genes and the environment influence human biological variation in visible characteristics, physiology , disease susceptibility, mental abilities, body size, and life span. Though humans vary in many traits, humans are among the least genetically diverse species. Any two humans are at least 99.5% genetically similar. Humans are sexually dimorphic : generally, males have greater body strength and females have

23736-497: The human population to reach one billion and only 207 years more to grow to 7 billion. The combined biomass of the carbon of all the humans on Earth in 2018 was estimated at 60 million tons, about 10 times larger than that of all non-domesticated mammals. In 2018, 4.2 billion humans (55%) lived in urban areas, up from 751 million in 1950. The most urbanized regions are Northern America (82%), Latin America (81%), Europe (74%) and Oceania (68%), with Africa and Asia having nearly 90% of

23908-512: The human population was reduced to a small number of breeding pairs. The forces of natural selection have continued to operate on human populations, with evidence that certain regions of the genome display directional selection in the past 15,000 years. The human genome was first sequenced in 2001 and by 2020 hundreds of thousands of genomes had been sequenced. In 2012 the International HapMap Project had compared

24080-809: The largest empire on the continent, stretching from Senegambia to Ivory Coast . Oceania would see the rise of the Tuʻi Tonga Empire which expanded across many islands in the South Pacific. By the late 15th century, the Aztecs and Inca had become the dominant power in Mesoamerica and the Andes , respectively. The early modern period in Europe and the Near East ( c.  1450 –1800) began with

24252-561: The late Miocene epoch. During this split, chromosome 2 was formed from the joining of two other chromosomes, leaving humans with only 23 pairs of chromosomes, compared to 24 for the other apes. Following their split with chimpanzees and bonobos, the hominins diversified into many species and at least two distinct genera. All but one of these lineages – representing the genus Homo and its sole extant species Homo sapiens  – are now extinct. The genus Homo evolved from Australopithecus . Though fossils from

24424-413: The nucleus. Splicing, followed by CPA, generate the final mature mRNA , which encodes the protein or RNA product. Many noncoding genes in eukaryotes have different transcription termination mechanisms and they do not have poly(A) tails. Many prokaryotic genes are organized into operons , with multiple protein-coding sequences that are transcribed as a unit. The genes in an operon are transcribed as

24596-483: The numerous fungi species of all the concepts studied. Versions of the phylogenetic species concept that emphasise monophyly or diagnosability may lead to splitting of existing species, for example in Bovidae , by recognising old subspecies as species, despite the fact that there are no reproductive barriers, and populations may intergrade morphologically. Others have called this approach taxonomic inflation , diluting

24768-552: The ongoing mass extinction of other forms of life. Within the last century, humans have explored challenging environments such as Antarctica , the deep sea , and outer space . Human habitation within these hostile environments is restrictive and expensive, typically limited in duration, and restricted to scientific , military , or industrial expeditions. Humans have visited the Moon and made their presence known on other celestial bodies through human-made robotic spacecraft . Since

24940-439: The only primates to have short, relatively flush canine teeth . Humans have characteristically crowded teeth, with gaps from lost teeth usually closing up quickly in young individuals. Humans are gradually losing their third molars , with some individuals having them congenitally absent. Humans share with chimpanzees a vestigial tail, appendix , flexible shoulder joints, grasping fingers and opposable thumbs . Humans also have

25112-471: The palm of the hand and on the soles of the feet. It is estimated that the worldwide average height for an adult human male is about 171 cm (5 ft 7 in), while the worldwide average height for adult human females is about 159 cm (5 ft 3 in). Shrinkage of stature may begin in middle age in some individuals but tends to be typical in the extremely aged . Throughout history, human populations have universally become taller, probably as

25284-593: The paper is accepted for publication. The type material is usually held in a permanent repository, often the research collection of a major museum or university, that allows independent verification and the means to compare specimens. Describers of new species are asked to choose names that, in the words of the International Code of Zoological Nomenclature , are "appropriate, compact, euphonious, memorable, and do not cause offence". Books and articles sometimes intentionally do not identify species fully, using

25456-674: The person who named the species, while the antonym sensu lato ("in the broad sense") denotes a wider usage, for instance including other subspecies. Other abbreviations such as "auct." ("author"), and qualifiers such as "non" ("not") may be used to further clarify the sense in which the specified authors delineated or described the species. Species are subject to change, whether by evolving into new species, exchanging genes with other species, merging with other species or by becoming extinct. The evolutionary process by which biological populations of sexually-reproducing organisms evolve to become distinct or reproductively isolated as species

25628-431: The phosphate–sugar backbone spiralling around the outside, and the bases pointing inward with adenine base pairing to thymine and guanine to cytosine. The specificity of base pairing occurs because adenine and thymine align to form two hydrogen bonds , whereas cytosine and guanine form three hydrogen bonds. The two strands in a double helix must, therefore, be complementary , with their sequence of bases matching such that

25800-510: The population density varies from one region to another, and large stretches of surface are almost completely uninhabited, like Antarctica and vast swathes of the ocean. Most humans (61%) live in Asia; the remainder live in the Americas (14%), Africa (14%), Europe (11%), and Oceania (0.5%). Estimates of the population at the time agriculture emerged in around 10,000 BC have ranged between 1 million and 15 million. Around 50–60 million people lived in

25972-487: The result of misclassification leading to questions on whether there really are any ring species. The commonly used names for kinds of organisms are often ambiguous: "cat" could mean the domestic cat, Felis catus , or the cat family, Felidae . Another problem with common names is that they often vary from place to place, so that puma, cougar, catamount, panther, painter and mountain lion all mean Puma concolor in various parts of America, while "panther" may also mean

26144-535: The ring. Ring species thus present a difficulty for any species concept that relies on reproductive isolation. However, ring species are at best rare. Proposed examples include the herring gull – lesser black-backed gull complex around the North pole, the Ensatina eschscholtzii group of 19 populations of salamanders in America, and the greenish warbler in Asia, but many so-called ring species have turned out to be

26316-508: The same species. This concept was narrowed in 2006 to a similarity of 98.7%. The average nucleotide identity (ANI) method quantifies genetic distance between entire genomes , using regions of about 10,000 base pairs . With enough data from genomes of one genus, algorithms can be used to categorize species, as for Pseudomonas avellanae in 2013, and for all sequenced bacteria and archaea since 2020. Observed ANI values among sequences appear to have an "ANI gap" at 85–95%, suggesting that

26488-529: The same species. When two species names are discovered to apply to the same species, the older species name is given priority and usually retained, and the newer name considered as a junior synonym, a process called synonymy . Dividing a taxon into multiple, often new, taxa is called splitting . Taxonomists are often referred to as "lumpers" or "splitters" by their colleagues, depending on their personal approach to recognising differences or commonalities between organisms. The circumscription of taxa, considered

26660-577: The second ( Southern Dispersal ) around 70,000 to 50,000 years ago. H. sapiens proceeded to colonize all the continents and larger islands, arriving in Eurasia 125,000 years ago, Australia around 65,000 years ago, the Americas around 15,000 years ago, and remote islands such as Hawaii , Easter Island , Madagascar , and New Zealand in the years 300 to 1280 CE. Human evolution was not a simple linear or branched progression but involved interbreeding between related species . Genomic research has shown that hybridization between substantially diverged lineages

26832-543: The sense of humanity). The native English term man can refer to the species generally (a synonym for humanity ) as well as to human males. It may also refer to individuals of either sex. Despite the fact that the word animal is colloquially used as an antonym for human , and contrary to a common biological misconception , humans are animals . The word person is often used interchangeably with human , but philosophical debate exists as to whether personhood applies to all humans or all sentient beings , and further if

27004-506: The species concept and making taxonomy unstable. Yet others defend this approach, considering "taxonomic inflation" pejorative and labelling the opposing view as "taxonomic conservatism"; claiming it is politically expedient to split species and recognise smaller populations at the species level, because this means they can more easily be included as endangered in the IUCN red list and can attract conservation legislation and funding. Unlike

27176-708: The split between the last common ancestor of humans and chimpanzees . The most significant of these adaptations are hairlessness , obligate bipedalism, increased brain size and decreased sexual dimorphism ( neoteny ). The relationship between all these changes is the subject of ongoing debate. Hylobatidae ( gibbons ) Pongo abelii Pongo tapanuliensis Pongo pygmaeus Gorilla gorilla Gorilla beringei Pan troglodytes Pan paniscus Homo sapiens (humans) Until about 12,000 years ago, all humans lived as hunter-gatherers . The Neolithic Revolution (the invention of agriculture ) first took place in Southwest Asia and spread through large parts of

27348-467: The strand of DNA like a train on a monorail, transcribing it into its messenger RNA form. This point brings us to our second important criterion: A true gene is one that is both transcribed and translated. That is, a true gene is first used as a template to make transient messenger RNA, which is then translated into a protein. This restricted definition is so common that it has spawned many recent articles that criticize this "standard definition" and call for

27520-461: The sugar ribose rather than deoxyribose . RNA also contains the base uracil in place of thymine . RNA molecules are less stable than DNA and are typically single-stranded. Genes that encode proteins are composed of a series of three- nucleotide sequences called codons , which serve as the "words" in the genetic "language". The genetic code specifies the correspondence during protein translation between codons and amino acids . The genetic code

27692-445: The synthesis of a protein. The transmission of genes to an organism's offspring , is the basis of the inheritance of phenotypic traits from one generation to the next. These genes make up different DNA sequences, together called a genotype , that is specific to every given individual, within the gene pool of the population of a given species . The genotype, along with environmental and developmental factors, ultimately determines

27864-809: The term gemmule to describe hypothetical particles that would mix during reproduction. Mendel's work went largely unnoticed after its first publication in 1866, but was rediscovered in the late 19th century by Hugo de Vries , Carl Correns , and Erich von Tschermak , who (claimed to have) reached similar conclusions in their own research. Specifically, in 1889, Hugo de Vries published his book Intracellular Pangenesis , in which he postulated that different characters have individual hereditary carriers and that inheritance of specific traits in organisms comes in particles. De Vries called these units "pangenes" ( Pangens in German), after Darwin's 1868 pangenesis theory. Twenty years later, in 1909, Wilhelm Johannsen introduced

28036-436: The term gene , he explained his results in terms of discrete inherited units that give rise to observable physical characteristics. This description prefigured Wilhelm Johannsen 's distinction between genotype (the genetic material of an organism) and phenotype (the observable traits of that organism). Mendel was also the first to demonstrate independent assortment , the distinction between dominant and recessive traits,

28208-412: The term "gene" (inspired by the ancient Greek : γόνος, gonos , meaning offspring and procreation) and, in 1906, William Bateson , that of " genetics " while Eduard Strasburger , among others, still used the term "pangene" for the fundamental physical and functional unit of heredity. Advances in understanding genes and inheritance continued throughout the 20th century. Deoxyribonucleic acid (DNA)

28380-433: The term "humans" with all members of the genus Homo , in common usage it generally refers to Homo sapiens , the only extant member. All other members of the genus Homo , which are now extinct, are known as archaic humans , and the term "modern human" is used to distinguish Homo sapiens from archaic humans. Anatomically modern humans emerged around 300,000 years ago in Africa, evolving from Homo heidelbergensis or

28552-424: The transition are scarce, the earliest members of Homo share several key traits with Australopithecus . The earliest record of Homo is the 2.8 million-year-old specimen LD 350-1 from Ethiopia , and the earliest named species are Homo habilis and Homo rudolfensis which evolved by 2.3 million years ago. H. erectus (the African variant is sometimes called H. ergaster ) evolved 2 million years ago and

28724-540: The wild. It is difficult to define a species in a way that applies to all organisms. The debate about species concepts is called the species problem. The problem was recognised even in 1859, when Darwin wrote in On the Origin of Species : I was much struck how entirely vague and arbitrary is the distinction between species and varieties. He went on to write: No one definition has satisfied all naturalists; yet every naturalist knows vaguely what he means when he speaks of

28896-438: The world's 3.4 billion rural population. Problems for humans living in cities include various forms of pollution and crime , especially in inner city and suburban slums . Most aspects of human physiology are closely homologous to corresponding aspects of animal physiology. The dental formula of humans is: 2.1.2.3 2.1.2.3 . Humans have proportionately shorter palates and much smaller teeth than other primates. They are

29068-464: Was common in human evolution. DNA evidence suggests that several genes of Neanderthal origin are present among all non sub-Saharan-African populations, and Neanderthals and other hominins, such as Denisovans , may have contributed up to 6% of their genome to present-day non sub-Saharan-African humans. Human evolution is characterized by a number of morphological , developmental , physiological , and behavioral changes that have taken place since

29240-450: Was first suggested by Gregor Mendel (1822–1884). From 1857 to 1864, in Brno , Austrian Empire (today's Czech Republic), he studied inheritance patterns in 8000 common edible pea plants , tracking distinct traits from parent to offspring. He described these mathematically as 2  combinations where n is the number of differing characteristics in the original peas. Although he did not use

29412-430: Was shown to be the molecular repository of genetic information by experiments in the 1940s to 1950s. The structure of DNA was studied by Rosalind Franklin and Maurice Wilkins using X-ray crystallography , which led James D. Watson and Francis Crick to publish a model of the double-stranded DNA molecule whose paired nucleotide bases indicated a compelling hypothesis for the mechanism of genetic replication. In

29584-400: Was the first archaic human species to leave Africa and disperse across Eurasia. H. erectus also was the first to evolve a characteristically human body plan . Homo sapiens emerged in Africa around 300,000 years ago from a species commonly designated as either H. heidelbergensis or H. rhodesiensis , the descendants of H. erectus that remained in Africa. H. sapiens migrated out of

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