Human evolution is the evolutionary process leading up to the appearance of anatomically modern humans. The topic usually covers the evolutionary history of primates, in particular the genus Homo, and the emergence of Homo sapiens as a distinct species of hominids (or "great apes") rather than studying the evolutionary history that led to primates. The study of human evolution involves many scientific disciplines, including physical anthropology, primatology, archaeology, paleontology, ethology, linguistics, evolutionary psychology, embryology and genetics.
Genetic studies show that primates diverged from other mammals about 85 million years ago, in the Late Cretaceous period, and the earliest fossils appear in the Paleocene, around 55 million years ago. The family Hominidae diverged from the Hylobatidae (Gibbon) family 15-20 million years ago, and around 14 million years ago, the Ponginae (orangutans) diverged from the Hominidae family. Bipedalism is the basic adaption of the Hominin line, and the earliest bipedal Hominin is considered to be either Sahelanthropus or Orrorin, with Ardipithecus, a full bipedal, coming somewhat later. Either Sahelanthropus or Orrorin may be the last shared ancestor between chimps and humans. The early bipedals eventually evolved into the australopithecines, and later into the genus Homo.
The earliest documented members of the genus Homo are Homo habilis, which evolved around 2.3 million years ago; the earliest species for which there is positive evidence of use of stone tools. The brains of these early hominins were about the same size as that of a chimpanzee, although it has been suggested that this was the time in which the human SRGAP2 gene doubled, producing a more rapid wiring of the frontal cortex. During the next million years a process of encephalization began, and with the arrival of Homo erectus in the fossil record, cranial capacity had doubled to 850 cm3. This increase in human brain size is equivalent to every generation having an additional 125,000 neurons more than their parents. It is believed that these species were the first to use fire and complex tools. Homo erectus and Homo ergaster were the first of the hominina to leave Africa, and these species spread through Africa, Asia, and Europe between 1.3 to 1.8 million years ago. According to the Recent African Ancestry theory, modern humans evolved in Africa possibly from Homo heidelbergensis, Homo rhodesiensis or Homo antecessor and migrated out of the continent some 50,000 to 100,000 years ago, replacing local populations of Homo erectus, Homo denisova, Homo floresiensis and Homo neanderthalensis.
Archaic Homo sapiens, the forerunner of anatomically modern humans, evolved between 400,000 and 250,000 years ago. Recent DNA evidence suggests that several haplotypes of Neanderthal origin are present among all non-African populations, and Neanderthals and other hominids, such as Denisova hominin may have contributed up to 6% of their genome to present-day humans, suggestive of a limited inter-breeding between these species. Anatomically modern humans evolved from archaic Homo sapiens in the Middle Paleolithic, about 200,000 years ago. The transition to behavioral modernity with the development of symbolic culture, language, and specialized lithic technology happened around 50,000 years ago according to many anthropologists although some suggest a gradual change in behavior over a longer time span.
History of study
Before Darwin
The word homo, the name of the biological genus to which humans belong, is Latin for "human". It was chosen originally by Carolus Linnaeus in his classification system. The word "human" is from the Latin humanus, the adjectival form of homo. The Latin "homo" derives from the Indo-European root *dhghem, or "earth". Linnaeus and other scientists of his time also considered the great apes to be the closest relatives of humans based on morphological and anatomical similarities.
Darwin
The possibility of linking humans with earlier apes by descent became clear only after 1859 with the publication of Charles Darwin's On the Origin of Species, in which he argued for the idea of the evolution of new species from earlier ones. Darwin's book did not address the question of human evolution, saying only that "Light will be thrown on the origin of man and his history".
The first debates about the nature of human evolution arose between Thomas Huxley and Richard Owen. Huxley argued for human evolution from apes by illustrating many of the similarities and differences between humans and apes, and did so particularly in his 1863 book Evidence as to Man's Place in Nature. However, many of Darwin's early supporters (such as Alfred Russel Wallace and Charles Lyell) did not initially agree that the origin of the mental capacities and the moral sensibilities of humans could be explained by natural selection, though this later changed. Darwin applied the theory of evolution and sexual selection to humans when he published The Descent of Man in 1871.
First fossils
A major problem at that time was the lack of fossil intermediaries. Neanderthal remains were discovered in a limestone quarry in 1856, three years before the publication of the Origin of the Species, and Neanderthal fossils had been discovered in Gibraltar even earlier, but it was originally claimed that these were human remains of a creature suffering some kind of illness. Despite the 1891 discovery by Eugène Dubois of what is now called Homo erectus at Trinil, Java, it was only in the 1920s when such fossils were discovered in Africa, that intermediate species began to accumulate. In 1925, Raymond Dart described Australopithecus africanus. The type specimen was the Taung Child, an Australopithecine infant which was discovered in a cave. The child's remains were a remarkably well-preserved tiny skull and an endocranial cast of the brain.
Although the brain was small (410Â cm3), its shape was rounded, unlike that of chimpanzees and gorillas, and more like a modern human brain. Also, the specimen showed short canine teeth, and the position of the foramen magnum was evidence of bipedal locomotion. All of these traits convinced Dart that the Taung baby was a bipedal human ancestor, a transitional form between apes and humans.
The East African fossils
During the 1960s and 1970s, hundreds of fossils were found, particularly in East Africa in the regions of the Olduvai gorge and Lake Turkana. The driving force in the East African researches was the Leakey family, with Louis Leakey and his wife Mary Leakey, and later their son Richard and daughter in-law Meave being among the most successful fossil hunters and palaeoanthropologists. From the fossil beds of Olduvai and Lake Turkana they amassed fossils of australopithecines, early Homo and even Homo erectus.
These finds cemented Africa as the cradle of humankind. In the 1980s, Ethiopia emerged as the new hot spot of palaeoanthropology as "Lucy", the most complete fossil member of the species Australopithecus afarensis, was found by Donald Johanson in Hadar in the desertic Middle Awash region of northern Ethiopia. This area would be the location of many new hominin fossils, particularly those uncovered by the teams of Tim White in the 1990s, such as Ardipithecus ramidus.
The genetic revolution
The genetic revolution in studies of human evolution started when Vincent Sarich and Allan Wilson measured the strength of immunological cross-reactions of blood serum albumin between pairs of creatures, including humans and African apes (chimpanzees and gorillas). The strength of the reaction could be expressed numerically as an immunological distance, which was in turn proportional to the number of amino acid differences between homologous proteins in different species. By constructing a calibration curve of the ID of species' pairs with known divergence times in the fossil record, the data could be used as a molecular clock to estimate the times of divergence of pairs with poorer or unknown fossil records.
In their seminal 1967 paper in Science, Sarich and Wilson estimated the divergence time of humans and apes as four to five million years ago, at a time when standard interpretations of the fossil record gave this divergence as at least 10 to as much as 30 million years. Subsequent fossil discoveries, notably Lucy, and reinterpretation of older fossil materials, notably Ramapithecus, showed the younger estimates to be correct and validated the albumin method.
Progress in DNA sequencing, specifically mitochondrial DNA (mtDNA) and then Y-chromosome DNA advanced the understanding of human origins. Application of the molecular clock principle revolutionized the study of molecular evolution.
On the basis of a separation from the orangutan between 10 and 20 million years ago, earlier studies of the molecular clock suggested that there were about 76 mutations per generation that they did not inherit from their parents. Using this evidence confirmed the separation (above). However, a 2012 study of 78 children and their parents in Iceland, suggests a mutation rate of only 36 mutations per generation extending the separation between to an earlier period before 7 million years ago (mya). Furthermore, research with wild 226 offspring of chimp populations in 8 locations, suggests that on average chimps reproduce when they are 26.5 years old. Based upon this number it suggests the separation from chimps occurred between 7 to 13 million years ago. This would suggest that Ardipithecus (4.5 mya, Orrorin 6 mya and Sahelanthropus 7 mya may have in fact been on the Hominin lineage, suggesting that the separation may have even occurred outside the African Rift Valley region.
Furthermore analysis of the two species' genes in 2006 provides evidence that after human ancestors had started to diverge from chimps, interspecies mating between "proto-human" and "proto-chimps" nonetheless occurred regularly enough to change certain genes in the new gene pool:
- A new comparison of the human and chimp genomes suggests that after the two lineages separated, they may have begun interbreeding... A principal finding is that the X chromosomes of humans and chimps appear to have diverged about 1.2 million years more recently than the other chromosomes.
The research suggests:
- There were in fact two splits between the human and chimp lineages, with the first being followed by interbreeding between the two populations and then a second split. The suggestion of a hybridization has startled paleoanthropologists, who nonetheless are "treating the new genetic data seriously".
The quest for the earliest hominin
In the 1990s, several teams of paleoanthropologists were working throughout Africa looking for evidence of the earliest divergence of the Hominin lineage from the great apes. In 1994, Meave Leakey discovered Australopithecus anamensis. The find was overshadowed by Tim White's 1995 discovery of Ardipithecus ramidus, which pushed back the fossil record to 4.2 million years ago.
In 2000, Martin Pickford and Brigitte Senut discovered in the Tugen Hills of Kenya a 6-million-year-old bipedal hominin which they named Orrorin tugenensis. And in 2001, a team led by Michel Brunet discovered the skull of Sahelanthropus tchadensis which was dated as 7.2 million years ago, and which Brunet argued was a bipedal, and therefore a hominid.
Human dispersal
Anthropologists in the 1980s were divided regarding some details of reproductive barriers and migratory dispersals of the Homo genus. Subsequently, genetics has been used to investigate and resolve these issues. According to the Sahara pump theory evidence suggests that genus Homo have migrated out of Africa at least three times (e.g. Homo erectus, Homo heidelbergensis and Homo sapiens).
The Out-of-Africa model proposed that modern H. sapiens speciated in Africa recently (approx. 200,000 years ago) and the subsequent migration through Eurasia resulted in nearly complete replacement of other Homo species. This model has been developed by Chris Stringer and Peter Andrews. In contrast, the multiregional hypothesis proposed that Homo genus contained only a single interconnected population as it does today (not separate species), and that its evolution took place worldwide continuously over the last couple million years. This model was proposed in 1988 by Milford H. Wolpoff.
Sequencing mtDNA and Y-DNA sampled from a wide range of indigenous populations revealed ancestral information relating to both male and female genetic heritage. Aligned in genetic tree differences were interpreted as supportive of a recent single origin. Analyses have shown a greater diversity of DNA patterns throughout Africa, consistent with the idea that Africa is the ancestral home of mitochondrial Eve and Y-chromosomal Adam.
Out of Africa has gained support from research using female mitochondrial DNA (mtDNA) and the male Y chromosome. After analysing genealogy trees constructed using 133 types of mtDNA, researchers concluded that all were descended from a female African progenitor, dubbed Mitochondrial Eve. Out of Africa is also supported by the fact that mitochondrial genetic diversity is highest among African populations.
A broad study of African genetic diversity, headed by Sarah Tishkoff, found the San people had the greatest genetic diversity among the 113 distinct populations sampled, making them one of 14 "ancestral population clusters". The research also located the origin of modern human migration in south-western Africa, near the coastal border of Namibia and Angola. The fossil evidence was insufficient for Richard Leakey to resolve this debate. Studies of haplogroups in Y-chromosomal DNA and mitochondrial DNA have largely supported a recent African origin. Evidence from autosomal DNA also predominantly supports a Recent African origin. However evidence for archaic admixture in modern humans had been suggested by some studies.
Recent sequencing of Neanderthal and Denisovan genomes shows that some admixture occurred. Modern humans outside Africa have 2-4% Neanderthal alleles in their genome, and some Melanesians have an additional 4-6% of Denisovan alleles. These new results do not contradict the Out of Africa model, except in its strictest interpretation. After recovery from a genetic bottleneck that might be due to the Toba supervolcano catastrophe, a fairly small group left Africa and briefly interbred with Neanderthals, probably in the middle-east or even North Africa before their departure. Their still predominantly African descendants spread to populate the world. A fraction in turn interbred with Denisovans, probably in south-east Asia, before populating Melanesia. HLA haplotypes of Neanderthal and Denisova origin have been identified in modern Eurasian and Oceanian populations.
There are still differing theories on whether there was a single exodus or several. A multiple dispersal model involves the Southern Dispersal theory, which has gained support in recent years from genetic, linguistic and archaeological evidence. In this theory, there was a coastal dispersal of modern humans from the Horn of Africa around 70,000 years ago. This group helped to populate Southeast Asia and Oceania, explaining the discovery of early human sites in these areas much earlier than those in the Levant.
A second wave of humans may have dispersed across the Sinai peninsula into Asia, resulting in the bulk of human population for Eurasia. This second group possibly possessed a more sophisticated tool technology and was less dependent on coastal food sources than the original group. Much of the evidence for the first group's expansion would have been destroyed by the rising sea levels at the end of each glacial maximum. The multiple dispersal model is contradicted by studies indicating that the populations of Eurasia and the populations of Southeast Asia and Oceania are all descended from the same mitochondrial DNA lineages, which support a single migration out of Africa that gave rise to all non-African populations.
Anatomical changes
Human evolution is characterized by a number of morphological, developmental, physiological, and behavioral changes that have taken place since the split between the last common ancestor of humans and chimpanzees. The most significant of these adaptations are bipedalism, increased brain size, lengthened ontogeny (gestation and infancy), and decreased sexual dimorphism. The relationship between these changes is the subject of ongoing debate. Other significant morphological changes included the evolution of a power and precision grip, a change first occurring in H. erectus.
Bipedalism
Bipedalism is the basic adaption of the Hominin line and is considered the main cause behind a suite of skeletal changes shared by all bipedal hominins. The earliest bipedal Hominin is considered to be either Sahelanthropus or Orrorin, with Ardipithecus, a full bipedal, coming somewhat later. The knuckle-walkers, the gorilla and chimpanzee, diverged around the same time, and either Sahelanthropus or Orrorin may be our last shared ancestor.
The early bipedals eventually evolved into the australopithecines and later the genus Homo. There are several theories of the adaptation value of bipedalism. It is possible that bipedalism was favored because it freed up the hands for reaching and carrying food, saved energy during locomotion, enabled long distance running and hunting, enhanced field of vision and helped avoid hyperthermia by reducing the surface area exposed to direct sun; all this mainly for thriving in the new grassland type environment rather than the previous forest type. A new study provides support for the hypothesis that walking on two legs, or bipedalism, evolved because it used less energy than quadrupedal knuckle-walking.
Anatomically the evolution of bipedalism has been accompanied by a large number of skeletal changes, not just to the legs and pelvis, but also to the vertebral column, feet and ankles, and skull. The femur evolved into a slightly more angular position to move the center of gravity toward the geometric center of the body. The knee and ankle joints became increasingly robust to better support increased weight. To support the increased weight on each vertebra in the upright position, the human vertebral column became S-shaped and the lumbar vertebrae became shorter and wider. In the feet the big toe moved into alignment with the other toes to help in forward locomotion. The arms and forearms shortened relative to the legs making it easier to run. The foramen magnum migrated under the skull and more anterior.
The most significant changes are in the pelvic region, where the long downward facing iliac blade was shortened and became wide as a requirement for keeping the center of gravity stable while walking; bipedal hominids have a shorter but broad, bowl-like pelvis due to this. A drawback is that the birth canal of these apes is smaller than regular knuckle-walking apes, though there has been a widening of it in comparison to that of australopithecine and modern humans, permitting the passage of newborns due to the increase in cranial size but this is limited to the upper portion, since further increase can hinder normal bipedal movement.
The shortening of the pelvis and smaller birth canal evolved as a requirement for bipedalism and had significant effects on the process of human birth which is much more difficult in modern humans than in other primates. During human birth, because of the variation in size of the pelvic region, the fetal head must be in a transverse position (compared to the mother) during entry into the birth canal and rotate about 90 degrees upon exit. The smaller size of the birth canal became an obstacle when the brain size began to increase in early humans, prompted a shorter gestation period and the reason why humans give birth to immature offspring, who are unable to walk much before 12 months and have greater neoteny, compared to other primates, who are motile at a much earlier age. The increased brain growth after birth and the increased dependency of children on mothers had a big effect upon the female reproductive cycle, and the more frequent appearance of monogamous relationships in humans when compared with other hominids. Delayed human sexual maturity also led to the evolution of menopause with one explanation saying that elderly women could better pass on their genes by taking care of their daughter's offspring, as compared to having more of their own.
Encephalization
The human species developed a much larger brain than that of other primates â€" typically 1,330 cm3 in modern humans, over twice the size of that of a chimpanzee or gorilla. The pattern of encephalization started with Homo habilis, which at approximately 600 cm3 had a brain slightly larger than that of chimpanzees, and continued with Homo erectus (800â€"1,100 cm3), reaching a maximum in Neanderthals with an average size of (1,200â€"1,900 cm3), larger even than Homo sapiens. The pattern of human postnatal brain growth differs from that of other apes (heterochrony) and allows for extended periods of social learning and language acquisition in juvenile humans. However, the differences between the structure of human brains and those of other apes may be even more significant than differences in size.
The increase in volume over time has affected areas within the brain unequally â€" the temporal lobes, which contain centers for language processing, have increased disproportionately, as has the prefrontal cortex which has been related to complex decision-making and moderating social behavior. Encephalization has been tied to an increasing emphasis on meat in the diet, or with the development of cooking, and it has been proposed that intelligence increased as a response to an increased necessity for solving social problems as human society became more complex. The human brain was able to expand because of the changes in the morphology of smaller mandibles and mandible muscle attachments to the skull into allowing more room for the brain to grow.
The increase in volume of the neocortex also included a rapid increase in size of the cerebellum. Traditionally the cerebellum has been associated with a paleocerebellum and archicerebellum as well as a neocerebellum. Its function has also traditionally been associated with balance, fine motor control but more recently speech and cognition. The great apes including humans and its antecessors had a more pronounced development of the cerebellum relative to the neocortex than other primates. It has been suggested that because of its function of sensory-motor control and assisting in learning complex muscular action sequences, the cerebellum may have underpinned the evolution of human's technological adaptations including the preadaptation of speech.
Sexual dimorphism
The reduced degree of sexual dimorphism is visible primarily in the reduction of the male canine tooth relative to other ape species (except gibbons) and reduced brow ridges and general robustness of males. Another important physiological change related to sexuality in humans was the evolution of hidden estrus. Humans and bonobos are the only apes in which the female is fertile year round and in which no special signals of fertility are produced by the body (such as genital swelling during estrus).
Nonetheless, humans retain a degree of sexual dimorphism in the distribution of body hair and subcutaneous fat, and in the overall size, males being around 15% larger than females. These changes taken together have been interpreted as a result of an increased emphasis on pair bonding as a possible solution to the requirement for increased parental investment due to the prolonged infancy of offspring.
Other changes
A number of other changes have also characterized the evolution of humans, among them an increased importance on vision rather than smell; a smaller gut; loss of body hair; evolution of sweat glands; a change in the shape of the dental arcade from being u-shaped to being parabolic; development of a chin (found in Homo sapiens alone), development of styloid processes; development of a descended larynx.
Evidence
The evidence on which scientific accounts of human evolution is based comes from many fields of natural science. The main sources of knowledge about the evolutionary process has traditionally been the fossil record, but since the development of genetics beginning in the 1970s, DNA analysis has come to occupy a place of comparable importance. The studies of ontogeny, phylogeny and especially evolutionary developmental biology of both vertebrates and invertebrates offer considerable insight into the evolution of all life, including how humans evolved. The specific study of the origin and life of humans is anthropology, particularly paleoanthropology which focuses on the study of human prehistory.
Evidence from molecular biology
The closest living relatives of humans are bonobos and chimpanzees (both genus Pan) and gorillas (genus Gorilla). With the sequencing of both the human and chimpanzee genome, current estimates of the similarity between their DNA sequences range between 95% and 99%. By using the technique called the molecular clock which estimates the time required for the number of divergent mutations to accumulate between two lineages, the approximate date for the split between lineages can be calculated. The gibbons (family Hylobatidae) and orangutans (genus Pongo) were the first groups to split from the line leading to the humans, then gorillas followed by the chimpanzees and bonobos. The splitting date between human and chimpanzee lineages is placed around 4-8 million years ago during the late Miocene epoch.
Genetic evidence has also been employed to resolve the question of whether there was any gene flow between early modern humans and Neanderthals, and to enhance our understanding of the early human migration patterns and splitting dates. By comparing the parts of the genome that are not under natural selection and which therefore accumulate mutations at a fairly steady rate, it is possible to reconstruct a genetic tree incorporating the entire human species since the last shared ancestor.
Each time a certain mutation (Single-nucleotide polymorphism) appears in an individual and is passed on to his or her descendants a haplogroup is formed including all of the descendants of the individual who will also carry that mutation. By comparing mitochondrial DNA which 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 200,000 years ago.
Genetics
Human evolutionary genetics studies how one human genome differs from the other, the evolutionary past that gave rise to it, and its current effects. Differences between genomes have anthropological, medical and forensic implications and applications. Genetic data can provide important insight into human evolution.
Evidence from the fossil record
There is little fossil evidence for the divergence of the gorilla, chimpanzee and hominin lineages. The earliest fossils that have been proposed as members of the hominin lineage are Sahelanthropus tchadensis dating from 7 million years ago, Orrorin tugenensis dating from 5.7 million years ago and Ardipithecus kadabba dating to 5.6 million years ago. Each of these have been argued to be a bipedal ancestor of later hominins but, in each case, the claims have been contested. It is also possible that one or more of these species are ancestors of another branch of African apes, or that they represent a shared ancestor between hominins and other apes.
The question of the relationship between these early fossil species and the hominin lineage is still to be resolved. From these early species, the australopithecines arose around 4 million years ago and diverged into robust (also called Paranthropus) and gracile branches, one of which (possibly A. garhi) probably went on to become ancestors of the genus Homo. The australopithecine species that is best represented in the fossil record is Australopithecus afarensis with more than one hundred fossil individuals represented, found from Northern Ethiopia (such as the famous "Lucy"), to Kenya, and South Africa. Fossils of robust australopithecines such as A. robustus (or alternatively Paranthropus robustus) and A./P. boisei are particularly abundant in South Africa at sites such as Kromdraai and Swartkrans, and around Lake Turkana in Kenya.
The earliest member of the genus Homo is Homo habilis which evolved around 2.3 million years ago. Homo habilis is the first species for which we have positive evidence of the use of stone tools. They developed the oldowan lithic technology, named after the Olduvai gorge in which the first specimens were found. Some scientists consider Homo rudolfensis, a larger bodied group of fossils with similar morphology to the original H. habilis fossils, to be a separate species while others consider them to be part of H. habilisâ€"simply representing species internal variation, or perhaps even sexual dimorphism. The brains of these early hominins were about the same size as that of a chimpanzee, and their main adaptation was bipedalism as an adaptation to terrestrial living.
During the next million years, a process of encephalization began and, with the arrival of Homo erectus in the fossil record, cranial capacity had doubled. Homo erectus were the first of the hominina to leave Africa, and this species spread through Africa, Asia, and Europe between 1.3 to 1.8 million years ago. One population of H. erectus, also sometimes classified as a separate species Homo ergaster, stayed in Africa and evolved into Homo sapiens. It is believed that these species were the first to use fire and complex tools.
The earliest transitional fossils between H. ergaster/erectus and Archaic H. sapiens are from Africa, such as Homo rhodesiensis, but seemingly transitional forms were also found at Dmanisi, Georgia. These descendants of African H. erectus spread through Eurasia from ca. 500,000 years ago evolving into H. antecessor, H. heidelbergensis and H. neanderthalensis. The earliest fossils of anatomically modern humans are from the Middle Paleolithic, about 200,000 years ago such as the Omo remains of Ethiopia; later fossils from Skhul in Israel and Southern Europe begin around 90,000 years ago.
As modern humans spread out from Africa, they encountered other hominins such as Homo neanderthalensis and the so-called Denisovans, who may have evolved from populations of Homo erectus that had left Africa around 2 million years ago. The nature of interaction between early humans and these sister species has been a long standing source of controversy, the question being whether humans replaced these earlier species or whether they were in fact similar enough to interbreed, in which case these earlier populations may have contributed genetic material to modern humans.
This migration out of Africa is estimated to have begun about 70,000 years BP and modern humans subsequently spread globally, replacing earlier hominins either through competition or hybridization. They inhabited Eurasia and Oceania by 40,000 years BP, and the Americas by at least 14,500 years BP.
Before Homo
Early evolution of primates
Evolutionary history of the primates can be traced back 65 million years. The oldest known primate-like mammal species, the Plesiadapis, came from North America, but they were widespread in Eurasia and Africa during the tropical conditions of the Paleocene and Eocene.
David Begun concluded that early primates flourished in Eurasia and that a lineage leading to the African apes and humans, including Dryopithecus, migrated south from Europe or Western Asia into Africa. The surviving tropical population of primates, which is seen most completely in the upper Eocene and lowermost Oligocene fossil beds of the Faiyum depression southwest of Cairo, gave rise to all living speciesâ€"lemurs of Madagascar, lorises of Southeast Asia, galagos or "bush babies" of Africa, and the anthropoids: platyrrhine or New World monkeys, catarrhines or Old World monkeys, and the great apes, including humans.
The earliest known catarrhine is Kamoyapithecus from uppermost Oligocene at Eragaleit in the northern Kenya Rift Valley, dated to 24 million years ago. Its ancestry is thought to be species related to Aegyptopithecus, Propliopithecus, and Parapithecus from the Fayum, at around 35 million years ago. In 2010, Saadanius was described as a close relative of the last common ancestor of the crown catarrhines, and tentatively dated to 29â€"28 million years ago, helping to fill an 11-million-year gap in the fossil record.
In the early Miocene, about 22 million years ago, the many kinds of arboreally adapted primitive catarrhines from East Africa suggest a long history of prior diversification. Fossils at 20 million years ago include fragments attributed to Victoriapithecus, the earliest Old World Monkey. Among the genera thought to be in the ape lineage leading up to 13 million years ago are Proconsul, Rangwapithecus, Dendropithecus, Limnopithecus, Nacholapithecus, Equatorius, Nyanzapithecus, Afropithecus, Heliopithecus, and Kenyapithecus, all from East Africa.
The presence of other generalized non-cercopithecids of middle Miocene age from sites far distantâ€"Otavipithecus from cave deposits in Namibia, and Pierolapithecus and Dryopithecus from France, Spain and Austriaâ€"is evidence of a wide diversity of forms across Africa and the Mediterranean basin during the relatively warm and equable climatic regimes of the early and middle Miocene. The youngest of the Miocene hominoids, Oreopithecus, is from coal beds in Italy that have been dated to 9 million years ago.
Molecular evidence indicates that the lineage of gibbons (family Hylobatidae) diverged from Great Apes some 18-12 million years ago, and that of orangutans (subfamily Ponginae) diverged from the other Great Apes at about 12 million years; there are no fossils that clearly document the ancestry of gibbons, which may have originated in a so-far-unknown South East Asian hominoid population, but fossil proto-orangutans may be represented by Sivapithecus from India and Griphopithecus from Turkey, dated to around 10 million years ago.
Divergence of the human clade from other great apes
Species close to the last common ancestor of gorillas, chimpanzees and humans may be represented by Nakalipithecus fossils found in Kenya and Ouranopithecus found in Greece. Molecular evidence suggests that between 8 and 4 million years ago, first the gorillas, and then the chimpanzees (genus Pan) split off from the line leading to the humans; human DNA is approximately 98.4% identical to that of chimpanzees when comparing single nucleotide polymorphisms (see human evolutionary genetics). The fossil record of gorillas and chimpanzees is limited. Both poor preservation (rain forest soils tend to be acidic and dissolve bone) and sampling bias probably contribute to this problem.
Other hominins likely adapted to the drier environments outside the equatorial belt, along with antelopes, hyenas, dogs, pigs, elephants, and horses. The equatorial belt contracted after about 8 million years ago. There is very little fossil evidence for the split of the hominin lineage from the lineages of gorillas and chimpanzees. The earliest fossils that have been argued to belong to the human lineage are Sahelanthropus tchadensis (7 Ma) and Orrorin tugenensis (6 Ma), followed by Ardipithecus (5.5â€"4.4 Ma), with species Ar. kadabba and Ar. ramidus.
Genus Australopithecus
The Australopithecus genus evolved in eastern Africa around 4 million years ago before spreading throughout the continent and eventually becoming extinct 2 million years ago. During this time period various forms of australopiths existed, including Australopithecus anamensis, A. afarensis, A. sediba, and A. africanus. There is still some debate amongst academics whether certain African hominid species of this time, such as A. robustus and A. boisei, constitute members of the same genus; if so, they would be considered to be A. robust australopiths whilst the others would be considered A. gracile australopiths. However, if these species do indeed constitute their own genus, then they may be given their own name, the Paranthropus.
- Australopithecus (4â€"1.8 Ma), with species Au. anamensis, Au. afarensis, Au. africanus, Au. bahrelghazali, Au. garhi, and Au. sediba;
- Kenyanthropus (3â€"2.7 Ma), with species Kenyanthropus platyops;
- Paranthropus (3â€"1.2 Ma), with species P. aethiopicus, P. boisei, and P. robustus;
Genus Homo
Homo sapiens is the only extant species of its genus, Homo. While some other, extinct Homo species might have been ancestors of Homo sapiens, many were likely our "cousins", having speciated away from our ancestral line. There is not yet a consensus as to which of these groups should count as separate species and which as subspecies. In some cases this is due to the dearth of fossils, in other cases it is due to the slight differences used to classify species in the Homo genus. The Sahara pump theory (describing an occasionally passable "wet" Sahara Desert) provides one possible explanation of the early variation in the genus Homo.
Based on archaeological and paleontological evidence, it has been possible to infer, to some extent, the ancient dietary practices of various Homo species and to study the role of diet in physical and behavioral evolution within Homo.
According to the Toba catastrophe theory to which some anthropologists and archeologists subscribe, the supereruption of Lake Toba on Sumatra island in Indonesia roughly 70,000 years ago had global consequences, killing most humans then alive and creating a population bottleneck that affected the genetic inheritance of all humans today.
H. habilis and H. gautengensis
Homo habilis lived from about 2.4 to 1.4 Ma. Homo habilis evolved in South and East Africa in the late Pliocene or early Pleistocene, 2.5â€"2 Ma, when it diverged from the australopithecines. Homo habilis had smaller molars and larger brains than the australopithecines, and made tools from stone and perhaps animal bones. One of the first known hominids, it was nicknamed 'handy man' by discoverer Louis Leakey due to its association with stone tools. Some scientists have proposed moving this species out of Homo and into Australopithecus due to the morphology of its skeleton being more adapted to living on trees rather than to moving on two legs like Homo sapiens.
It was considered to be the first species of the genus Homo until May 2010, when a new species, Homo gautengensis was discovered in South Africa, that most likely arose earlier than Homo habilis.
H. rudolfensis and H. georgicus
These are proposed species names for fossils from about 1.9â€"1.6 Ma, whose relation to Homo habilis is not yet clear.
- Homo rudolfensis refers to a single, incomplete skull from Kenya. Scientists have suggested that this was another Homo habilis, but this has not been confirmed.
- Homo georgicus, from Georgia, may be an intermediate form between Homo habilis and Homo erectus, or a sub-species of Homo erectus.
H. ergaster and H. erectus
The first fossils of Homo erectus were discovered by Dutch physician Eugene Dubois in 1891 on the Indonesian island of Java. He originally named the material Pithecanthropus erectus based on its morphology, which he considered to be intermediate between that of humans and apes. Homo erectus lived from about 1.8 Ma to about 70,000 years ago (which would indicate that they were probably wiped out by the Toba catastrophe; however, Homo floresiensis survived it). Often the early phase, from 1.8 to 1.25 Ma, is considered to be a separate species, Homo ergaster, or it is seen as a subspecies of Homo erectus, Homo erectus ergaster.
In the early Pleistocene, 1.5â€"1 Ma, in Africa some populations of Homo habilis are thought to have evolved larger brains and made more elaborate stone tools; these differences and others are sufficient for anthropologists to classify them as a new species, Homo erectus. This was made possible by the evolution of locking knees and a different location of the foramen magnum (the hole in the skull where the spine enters). They may have used fire to cook their meat.
A famous example of Homo erectus is Peking Man; others were found in Asia (notably in Indonesia), Africa, and Europe. Many paleoanthropologists now use the term Homo ergaster for the non-Asian forms of this group, and reserve Homo erectus only for those fossils that are found in Asia and meet certain skeletal and dental requirements which differ slightly from H. ergaster.
H. cepranensis and H. antecessor
These are proposed as species that may be intermediate between H. erectus and H. heidelbergensis.
- H. antecessor is known from fossils from Spain and England that are dated 1.2 Maâ€"500 ka.
- H. cepranensis refers to a single skull cap from Italy, estimated to be about 800,000 years old.
H. heidelbergensis
H. heidelbergensis (Heidelberg Man) lived from about 800,000 to about 300,000 years ago. Also proposed as Homo sapiens heidelbergensis or Homo sapiens paleohungaricus.
H. rhodesiensis, and the Gawis cranium
- H. rhodesiensis, estimated to be 300,000â€"125,000 years old. Most current researchers place Rhodesian Man within the group of Homo heidelbergensis, though other designations such as Archaic Homo sapiens and Homo sapiens rhodesiensis have been proposed.
- In February 2006 a fossil, the Gawis cranium, was found which might possibly be a species intermediate between H. erectus and H. sapiens or one of many evolutionary dead ends. The skull from Gawis, Ethiopia, is believed to be 500,000â€"250,000 years old. Only summary details are known, and the finders have not yet released a peer-reviewed study. Gawis man's facial features suggest its being either an intermediate species or an example of a "Bodo man" female.
Neanderthal and Denisovan
H. neanderthalensis, alternatively designated as Homo sapiens neanderthalensis, lived in Europe and Asia from 400,000 to about 30,000 years ago. Evidence from sequencing mitochondrial DNA indicated that no significant gene flow occurred between H. neanderthalensis and H. sapiens, and, therefore, the two were separate species that shared a common ancestor about 660,000 years ago. However, the 2010 sequencing of the Neanderthal genome indicated that Neanderthals did indeed interbreed with anatomically modern humans circa 45,000 to 80,000 years ago (at the approximate time that modern humans migrated out from Africa, but before they dispersed into Europe, Asia and elsewhere).
Nearly all modern non-African humans have 1% to 4% of their DNA derived from Neanderthal DNA, and this finding is consistent with recent studies indicating that the divergence of some human alleles dates to one Ma, although the interpretation of these studies has been questioned. Neanderthals and homo sapiens could have co-existed in Europe for as long as 10,000 years, during which human populations exploded vastly outnumbering Neanderthals, possibly outcompeting them by sheer numerical strength.
In 2008, archaeologists working at the site of Denisova Cave in the Altai Mountains of Siberia uncovered a small bone fragment from the fifth finger of a juvenile member of Denisovans. Artifacts, including a bracelet, excavated in the cave at the same level were carbon dated to around 40,000 BP. As DNA had survived in the fossil fragment due to the cool climate of the Denisova Cave, both mtDNA and nuclear genomic DNA were sequenced.
While the divergence point of the mtDNA was unexpectedly deep in time, the full genomic sequence suggested the Denisovans belonged to the same lineage as Neanderthals, with the two diverging shortly after their line split from that lineage giving rise to modern humans. Modern humans are known to have overlapped with Neanderthals in Europe for more than 10,000 years, and the discovery raises the possibility that Neanderthals, modern humans and the Denisova hominin may have co-existed. The existence of this distant branch creates a much more complex picture of humankind during the Late Pleistocene than previously thought. Evidence has also been found that as much as 6% of the genomes of some modern Melanesians derive from Denisovans, indicating limited interbreeding in Southeast Asia.
Alleles thought to have originated in Neanderthal and the Denisova hominin have been identified at several genetic loci in the genomes of modern humans outside of Africa. HLA types from Denisovans and Neanderthal represent more than half the HLA alleles of modern Eurasians, indicating strong positive selection for these introgressed alleles.
H. floresiensis
H. floresiensis, which lived from approximately 100,000 to 12,000 before present, has been nicknamed hobbit for its small size, possibly a result of insular dwarfism. H. floresiensis is intriguing both for its size and its age, being an example of a recent species of the genus Homo that exhibits derived traits not shared with modern humans. In other words, H. floresiensis shares a common ancestor with modern humans, but split from the modern human lineage and followed a distinct evolutionary path. The main find was a skeleton believed to be a woman of about 30 years of age. Found in 2003 it has been dated to approximately 18,000 years old. The living woman was estimated to be one meter in height, with a brain volume of just 380Â cm3 (considered small for a chimpanzee and less than a third of the H. sapiens average of 1400Â cm3).
However, there is an ongoing debate over whether H. floresiensis is indeed a separate species. Some scientists hold that H. floresiensis was a modern H. sapiens with pathological dwarfism. This hypothesis is supported in part, because some modern humans who live on Flores, the island where the skeleton was found, are pygmies. This, coupled with pathological dwarfism, could possibly create a hobbit-like human. The other major attack on H. floresiensis is that it was found with tools only associated with H. sapiens.
The hypothesis of pathological dwarfism, however, fails to explain additional anatomical features that are unlike those of modern humans (diseased or not) but much like those of ancient members of our genus. Aside from cranial features, these features include the form of bones in the wrist, forearm, shoulder, knees, and feet. Additionally, this hypothesis fails to explain the find of multiple examples of individuals with these same characteristics, indicating they were common to a large population, and not limited to one individual.
H. sapiens
H. sapiens (the adjective sapiens is Latin for "wise" or "intelligent") have lived from about 250,000 years ago to the present. Between 400,000 years ago and the second interglacial period in the Middle Pleistocene, around 250,000 years ago, the trend in skull expansion and the elaboration of stone tool technologies developed, providing evidence for a transition from H. erectus to H. sapiens. The direct evidence suggests there was a migration of H. erectus out of Africa, then a further speciation of H. sapiens from H. erectus in Africa. A subsequent migration within and out of Africa eventually replaced the earlier dispersed H. erectus. This migration and origin theory is usually referred to as the recent single origin or Out of Africa theory. Current evidence does not preclude some multiregional evolution or some admixture of the migrant H. sapiens with existing Homo populations. This is a hotly debated area of paleoanthropology.
Current research has established that humans are genetically highly homogenous; that is, the DNA of individuals is more alike than usual for most species, which may have resulted from their relatively recent evolution or the possibility of a population bottleneck resulting from cataclysmic natural events such as the Toba catastrophe. Distinctive genetic characteristics have arisen, however, primarily as the result of small groups of people moving into new environmental circumstances. These adapted traits are a very small component of the Homo sapiens genome, but include various characteristics such as skin color and nose form, in addition to internal characteristics such as the ability to breathe more efficiently at high altitudes.
H. sapiens idaltu, from Ethiopia, is an extinct sub-species from about 160,000 years ago.
Use of tools
The use of tools has been interpreted as a sign of intelligence, and it has been theorized that tool use may have stimulated certain aspects of human evolution, especially the continued expansion of the human brain. Paleontology has yet to explain the expansion of this organ over millions of years despite being extremely demanding in terms of energy consumption. The brain of a modern human consumes about 13 watts (260 kilocalories per day), a fifth of body's total energy consumption. Increased tool use would allow hunting for energy-rich meat products, and would enable processing more energy-rich plant products. Researchers have suggested that early hominids were thus under evolutionary pressure to increase their capacity to create and use tools.
Precisely when early humans started to use tools is difficult to determine, because the more primitive these tools are (for example, sharp-edged stones) the more difficult it is to decide whether they are natural objects or human artifacts. There is some evidence that the australopithecines (4 Ma) may have used broken bones as tools, but this is debated.
It should be noted that many species make and use tools, but it is the human genus that dominates the areas of making and using more complex tools. The oldest known tools are the "Oldowan stone tools" from Ethiopia, 2.5-2.6 million years old, which predates the earliest known "Homo" species. There is no known evidence that any "Homo" specimens appeared by 2.5 Ma. A Homo fossil was found near some Oldowan tools, and its age was noted at 2.3 million years old, suggesting that maybe the Homo species did indeed create and use these tools. It is a possibility but does not yet represent solid evidence. Third metacarpal styloid process enables the hand bone to lock into the wrist bones, allowing for greater amounts of pressure to be applied to the wrist and hand from a grasping thumb and fingers. It allows humans the dexterity and strength to make and use complex tools. This unique anatomical feature separates humans from apes and other nonhuman primates, and is not seen in human fossils older than 1.8 million years.
Bernard Wood noted that "Paranthropus" co-existed with the early Homo species in the area of the "Oldowan Industrial Complex" over roughly the same span of time. Although there is no direct evidence which identifies Paranthropus as the tool makers, their anatomy lends to indirect evidence of their capabilities in this area. Most paleoanthropologists agree that the early "Homo" species were indeed responsible for most of the Oldowan tools found. They argue that when most of the Oldowan tools were found in association with human fossils, Homo was always present, but Paranthropus was not.
In 1994 Randall Susman used the anatomy of opposable thumbs as the basis for his argument that both the Homo and Paranthropus species were toolmakers. He compared bones and muscles of human and chimpanzee thumbs, finding that humans have 3 muscles which are lacking in chimpanzees. Humans also have thicker metacarpals with broader heads, allowing more precise grasping than the chimpanzee hand can perform. Susman posited that modern anatomy of the human thumb is an evolutionary response to the requirements associated with making and handling tools and that both species were indeed toolmakers.
Stone tools
Stone tools are first attested around 2.6 Ma, when H. habilis in Eastern Africa used so-called pebble tools, choppers made out of round pebbles that had been split by simple strikes. This marks the beginning of the Paleolithic, or Old Stone Age; its end is taken to be the end of the last Ice Age, around 10,000 years ago. The Paleolithic is subdivided into the Lower Paleolithic (Early Stone Age, ending around 350,000â€"300,000 years ago), the Middle Paleolithic (Middle Stone Age, until 50,000â€"30,000 years ago), and the Upper Paleolithic.
The period from 700,000â€"300,000 years ago is also known as the Acheulean, when H. ergaster (or erectus) made large stone hand axes out of flint and quartzite, at first quite rough (Early Acheulian), later "retouched" by additional, more subtle strikes at the sides of the flakes. After 350,000 BP (Before Present) the more refined so-called Levallois technique was developed, a series of consecutive strikes, by which scrapers, slicers ("racloirs"), needles, and flattened needles were made. Finally, after about 50,000 BP, ever more refined and specialized flint tools were made by the Neanderthals and the immigrant Cro-Magnons (knives, blades, skimmers). In this period they also started to make tools out of bone.
Transition to behavioral modernity
Until about 50,000â€"40,000 years ago the use of stone tools seems to have progressed stepwise. Each phase (H. habilis, H. ergaster, H. neanderthalensis) started at a higher level than the previous one, but after each phase started, further development was slow. Currently paleoanthropologists are debating whether these Homo species possessed some or many of the cultural and behavioral traits associated with modern humans such as language, complex symbolic thinking, technological creativity etc. It seems that they were culturally conservative maintaining simple technologies and foraging patterns over very long periods.
Around 50,000 BP modern human culture started to evolve more rapidly. The transition to behavioral modernity has been characterized as a Eurasian "Great Leap Forward", or as the "Upper Palaeolithic Revolution", because of the sudden appearance of distinctive signs of modern behavior in the archaeological record. Some scholars consider the transition to have been more gradual, with some features already appearing among Archaic African Homo sapiens around 200,000 years ago.
Modern humans started burying their dead, using animal hides to make clothing, hunting with more sophisticated techniques (such as using trapping pits or driving animals off cliffs), and engaging in cave painting. As human culture advanced, different populations of humans introduced novelty to existing technologies: artifacts such as fish hooks, buttons and bone needles show signs of variation among different populations of humans, something that had not been seen in human cultures prior to 50,000 BP. Typically, H. neanderthalensis populations do not vary in their technologies.
Among concrete examples of Modern human behavior, anthropologists include specialization of tools, use of jewellery and images (such as cave drawings), organization of living space, rituals (for example, burials with grave gifts), specialized hunting techniques, exploration of less hospitable geographical areas, and barter trade networks. Debate continues as to whether a "revolution" led to modern humans ("the big bang of human consciousness"), or whether the evolution was more gradual.
Recent and current human evolution
Natural selection occurs in modern human populations. For example, the population which is at risk of the severe debilitating disease kuru has significant over-representation of an immune variant of the prion protein gene G127 V versus non-immune alleles. The frequency of this genetic variant is due to the survival of immune persons. Other reported evolutionary trends in other populations include a lengthening of the reproductive period, reduction in cholesterol levels, blood glucose and blood pressure.
It has been argued that human evolution has accelerated since the development of agriculture and civilization some 10,000 years ago. It is claimed that this has resulted in substantial genetic differences between different current human populations. Lactase persistence is an example of such recent evolution. Recent human evolution seems largely however to have been confined to genetic resistance to some infectious disease, which have appeared in human populations by crossing the species barrier from domesticated animals. Without a selection pressure or opportunity for speciation, such as geographic isolation, recent human evolution has been predominantly subject to genetic drift.
Species list
This list is in chronological order across the page by genus.
