Human Evolution

The possibility of linking humans with earlier apes by descent only became clear after 1859 with the publication of Charles Darwin’s On the Origin of Species. This 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 agree that the origin of the mental capacities and the moral sensibilities of humans could be explained by natural selection. Darwin applied the theory of evolution and sexual selection to humans when he published The Descent of Man in 1871.

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 Asutralopithecines, early Homo, and even Homo erectus. These finds cemented Africa as the cradle of human kind. 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 Don 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 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 paper in 1967 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. Application of the molecular clock principle revolutionized the study of molecular evolution.

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.

The Out-of-Africa model proposed that modern H. sapiens speciated in Africa recently (approx. 200,000 years ago) and their subsequent migration through Eurasia resulted in 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 like 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.

Progress in DNA sequencing, specifically mitochondrial DNA (mtDNA) and then Y-chromosome DNA advanced the understanding of human origins. 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 dispersed across the Sinai peninsula into Asia, resulting in the bulk of human population for Eurasia. This second group 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.

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 analyses 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

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.

Recent research casts doubt on the “Out of Africa” hypothesis. The high genetic diversity in Sub-Saharan Africa has been shown to be a result of interbreeding with non-human lineages after Sapiens migrated there from Eurasia, where they arose.

Genetics

The closest living relatives of humans are gorillas (genus Gorilla) and chimpanzees (Genus Pan). With the sequencing of both the Human and Chimpanzee genome, current estimates of similarity between human and chimpanzee 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 (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 arrive 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.

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, and 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 cases the claims have been contested. It is also possible that either 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 relation 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 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 are best represented in the fossil record is Australopithecus Afarensis with more than a 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 members of the genus Homo are Homo habilis which evolved around 2.3 million years ago. Homo habilis is the first species for which we have positive evidence of use of stone tools. They developed the oldowan lithic technology, named after the Olduvai gorge where the first specimens were found. Some scientists consider Homo rudolfensis, a group 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. habilis – 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 these 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 are 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 already 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. Recent studies of the Human and Neanderthal genomes indicate that there was in fact gene flow between archaic Homo sapiens and Neanderthals and Denisovans.

This migration out of Africa is estimated to have begun about 70,000 years BP. 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 at least 14,500 years BP.

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 species—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.[64] 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 29–28 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 distant—Otavipithecus from cave deposits in Namibia, and Pierolapithecus and Dryopithecus from France, Spain and Austria—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.

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 (5.5–4.4 Ma), with species Ar. kadabba and Ar. ramidus.

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.

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 20 watts (400 kilocalories per day), a fifth of body’s total energy consumption.[citation needed] 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 species 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. 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 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 350,000–300,000 years ago), the Middle Paleolithic (Middle Stone Age, until 50,000–30,000 years ago), and the Upper Paleolithic.

The period from 700,000–300,000 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.

Until about 50,000–40,000 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 “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 Homo sapiens already 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.

Introduction to Human Evolution

The Genographic Project

Becoming Human

Science Daily: Human Evolution News

Human Evolution – New Scientist

Journal of Human Evolution

Human Evolution Research Center

Human Genome Project