Human evolution is the study of the biological evolution of humans as a distinct species from its common ancestors with other animals. Analysis of fossil evidence and genetic distance are two of the means by which scientists understand this evolutionary history.
Fossil evidence suggests that humans’ earliest hominoid ancestors may have split from other primates as early as the late Oligocene, circa 26-24 million of years ago (Ma). By the early Miocene, the adaptive radiation of many different hominoid forms was well underway. Evidence from the molecular dating of genetic differences indicates that the gibbon lineage (family Hylobatidae) diverged between 18 and 12 Ma, and the orangutan lineage (subfamily Ponginae) diverged about 12 Ma. While there is no fossil evidence thus far clearly documenting the early ancestry of gibbons, fossil proto-orangutans may be represented by Sivapithecus from India and Griphopithecus from Turkey, dated to around 10 Ma. Molecular evidence further suggests that between 8 and 4 Ma, first the gorillas, and then the chimpanzee (genus Pan) split from the line leading to the humans. We have no fossil record of this divergence, but distinctively hominid fossils have been found dating to 3.2 Ma and possibly even earlier, at 6 or 7 Ma. Comparisons of chimpanzee and human DNA show the two are approximately 98.4 percent identical, and are taken as strong evidence of recent common ancestry. Today, only one distinct human species survives, but many earlier species have been found in the fossil record, including Homo erectus, Homo habilis, and Homo neanderthalensis.
Creationists dispute there is evidence of shared ancestry in the fossil evidence. They argue that these are misassigned ape fossils (e.g. that Java man was a gibbonor too similar to modern humans to designate them as distinct or transitional forms).
Creationists also dispute science’s interpretation of genetic evidence in the study of human evolution. They do not agree that genetic similarities between various animals imply a common ancestral relationship. They state that scientists are coming to this interpretation only because they have preconceived notions that such shared relationships exist. Creationists also argue that genetic mutations are strong evidence against evolutionary theory because the mutations required for major changes to occur would almost certainly be detrimental.
Human evolution, or anthropogenesis, is the part of biological evolution concerning the emergence of Homo sapiens as a distinct species from other hominans, great apes and placental mammals. It is the subject of a broad scientific inquiry that seeks to understand and describe how this change occurred.
The term “human”, in the context of human evolution, refers to the genus Homo, but studies of human evolution usually include other hominins, such as the australopithecines. The Homo genus diverged from the australopithecines about 2 Ma in Africa. Scientists have estimated that humans branched off from their common ancestor with chimpanzees, the only other living hominins—about 5–7 Ma.
Archaic Homo sapiens evolved between 400,000 and 250,000 years ago. The dominant view among scientists is the recent African origin of modern humans (RAO). Homo sapiens evolved in Africa and spread across the globe, replacing populations of Homo erectus and Homo neanderthalensis. Scientists supporting the alternative hypothesis on the multiregional origin of modern humans (MTO) view modern humans as having evolved as a single, widespread population from existing Homo species, particularly Homo erectus.
The first known 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.
A major problem was the lack of fossil intermediaries. It was only in the 1920s that such fossils were discovered in Africa. In 1925, Raymond Dart described Australopithecus africanus. The type specimen was the Taung Child, an australopithecine infant discovered in a cave. The child’s remains were a remarkably well-preserved tiny skull and an endocranial cast of the individual’s brain. Although the brain was small (410 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.
11.6.2 Hominin species distributed through time
Note: 1e +06 years = 1 million years = 1 Ma.
11.6.3 Before Homo
The evolutionary history of the primates can be traced back for some 85 million years, as one of the oldest of all surviving placental mammal groups. The oldest known primates come from North America, but they were widespread in Eurasia and Africa as well, during the tropical conditions of the Paleocene and Eocene.
With the beginning of modern climates, marked by the formation of the first Antarctic ice in the early Oligocene around 30 Ma, primates went extinct everywhere but Africa and southern Asia. One such primate from this time was Notharctus. Fossil evidence found in Germany in the 1980s was determined to be about 16.5 million years old, some 1.5 million years older than similar species from East Africa. It suggests that the primate lineage of the great apes first appeared in Eurasia and not Africa.
The discoveries suggest that the early ancestors of the hominids (the family of great apes and humans) migrated to Eurasia from Africa about 17 Ma, just before these two continents were cut off from each other by an expansion of the Mediterranean Sea. These primates flourished in Eurasia and their lineage leading to the African apes and humans—Dryopithecus—migrated south from Europe or Western Asia into Africa. The surviving tropical population, which is seen most completely in the upper Eocene and lowermost Oligocene fossil beds of the Fayum depression southwest of Cairo, gave rise to all living primates—lemurs of Madagascar, lorises of Southeast Asia, galagos or “bush babies” of Africa, and the anthropoids; platyrrhines or New World monkeys, and catarrhines or Old World monkeys and the great apes and humans.
The earliest known catarrhine is Kamoyapithecus from uppermost Oligocene at Eragaleit in the northern Kenya rift valley, dated to 24 Ma years ago. Its ancestry is generally thought to be close to such genera as Aegyptopithecus, Propliopithecus, and Parapithecus from the Fayum, at around 35 Ma. There are no fossils from the intervening 11 million years. No near ancestor to South American platyrrhines, whose fossil record begins at around 30 Ma, can be identified among the North African fossil species, and possibly lies in other forms that lived in West Africa that were caught up in the still-mysterious transatlantic sweepstakes that sent primates, rodents, boa constrictors, and cichlid fishes from Africa to South America sometime in the Oligocene.
In the early Miocene, after 22 Ma, many kinds of arboreally adapted primitive catarrhines from East Africa suggest a long history of prior diversification. Because the fossils at 20 Ma include fragments attributed to Victoriapithecus, the earliest cercopithecoid; the other forms are (by default) grouped as hominoids, without clear evidence as to which are closest to living apes and humans. Among the presently recognized genera in this group, which ranges up to 13 Ma, we find 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 9 Ma coal beds in Italy.
Molecular evidence indicates that the lineage of gibbons (family Hylobatidae) became distinct between 18 and 12 Ma, and that of orangutans (subfamily Ponginae) at about 12 Ma; we have no fossils that clearly document the ancestry of gibbons, which may have originated in a so far unknown South East Asian hominid population, but fossil proto-orangutans may be represented by Ramapithecus from India and Griphopithecus from Turkey, dated to around 10 Ma.
It has been suggested that species close to last common ancestors 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 Ma, first the gorillas, and then the chimpanzee (genus Pan) split off from the line leading to the humans; human DNA is approximately 98.4 percent identical to the DNA of chimpanzees. The fossil record of gorillas and chimpanzees is quite limited. Both poor preservation (rain forest soils tend to be acidic and dissolve bone) and sampling bias probably contribute to this problem.
Other Hominines, however, likely adapted (along with antelopes, hyenas, dogs, pigs, elephants, and horses) to the somewhat drier environments outside the equatorial belt and their fossils are relatively well known. The earliest 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;
- Australopithecus (4–2 Ma), with species Au. anamensis, Au. afarensis, Au. africanus, Au. bahrelghazali, and Au. garhi;
- Kenyanthropus (3–2.7 Ma), with species Kenyanthropus platyops
- Paranthropus (3–1.2 Ma), with species P. aethiopicus, P. boisei, and P. robustus;
- Homo (2 Ma–present), with species Homo habilis, Homo rudolfensis, Homo ergaster, Homo georgicus, Homo antecessor, Homo cepranensis, Homo erectus, Homo heidelbergensis, Homo rhodesiensis, Homo neanderthalensis, Homo sapiens idaltu, Archaic Homo sapiens, Homo floresiensis
11.6.4 Genus Homo
In modern taxonomy, Homo sapiens are the only non-extinct species of its genus Homo. Likewise, the ongoing study of the origins of Homo sapiens often demonstrates that there were other Homo species, all of which are now extinct. While some of these other species might have been ancestors of Homo sapiens, many were likely our “cousins. There is not yet a consensus as to which of these groups should count as separate species and which as subspecies of another species.
Based on archaeological and paleontological evidence, it has been possible to infer the ancient dietary practices of various Homo species and to study the role of diet in human (Homo) physical and behavioural evolution.
11.6.5 Homo habilis
Homo habilis lived from about 2.4 to 1.4 Ma. Homo habilis, the first species of the genus Homo, evolved in South and East Africa in the late Pliocene or early Pleistocene, 2.5–2 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 its 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.
11.6.6 Homo rudolfensis and Homo georgicus
These are proposed species names for fossils from about 1.9–1.6 Ma, the relation of which with Homo habilis is not yet clear.
- Homo rudolfensis refers to a single, incomplete skull from Kenya. Scientists have suggested that this was just another 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.
11.6.7 Homo ergaster and Homo erectus
The first fossils of Homo erectus were discovered by Dutch physician Eugene Dubois in 1891 on the Indonesian island of Java. He originally gave the material the name Pithecanthropus erectus based on its morphology that he considered to be intermediate between that of humans and apes. Homo erectus lived from about 1.8 Ma to 70,000 years ago. 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, 1.5–1 Ma, in Africa, Asia, and Europe, presumably, some populations of Homo habilis 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. In addition Homo erectus was the first human ancestor to walk truly upright. 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 are now using the term Homo ergaster for the non-Asian forms of this group, and reserving Homo erectus only for those fossils found in the Asian region and meeting certain skeletal and dental requirements which differ slightly from Homo ergaster.
11.6,8 Homo cepranensis and Homo antecessor
These are proposed as species that may be intermediate between Homo erectus and Homo heidelbergensis.
- Homo antecessor is known from fossils from Spain and England that are dated 1.2 Ma–500 ka.
- Homo cepranensis refers to a single skull cap from Italy, estimated to be about 800,000 years old.
11.6.9 Homo heidelbergensis
Homp heidelbergensis (Heidelberg Man) lived from about 800,000 to about 300,000 years ago. Also proposed as Homo sapiens heidelbergensis or Homo sapiens paleohungaricus.
11.6.10 Homo rhodesiensis, and the Gawis cranium
- Homo rhodesiensis, estimated to be 300,000–125,000 years old. Most current experts believe Rhodesian Man to be within the group of Homo heidelbergensis though other designations such as Archaic Homo sapiens and Homo sapiens rhodesiensis have also been proposed.
- In February 2006 a fossil, the Gawis cranium, was found which might possibly be a species intermediate between Homo erectus and Homo sapiens or one of many evolutionary dead ends. The skull from Gawis, Ethiopia, is believed to be 500,000–250,000 years old. Only summary details are known, and no peer reviewed studies have been released by the finding team. Gawis man’s facial features suggest its being either an intermediate species or an example of a “Bodo man” female.
11.6.11 Homo neanderthalensis
Homo neanderthalensis lived from about 250,000 to as recent as 30,000 years ago. Also proposed as Homo sapiens neanderthalensis: there is ongoing debate over whether the ‘Neanderthal Man’ was a separate species, Homo neanderthalensis, or a subspecies of Homo sapiens. While the debate remains unsettled, evidence from sequencing mitochondrial DNA indicates that no significant gene flow occurred between Homo neanderthalensis and Homo sapiens, and, therefore, the two were separate species that shared a common ancestor about 660,000 years ago.
11.6.12 Homo sapiens
Homo sapiens (“sapiens” means 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 Homo erectus to Homo sapiens. The direct evidence suggests there was a migration of Homo erectus out of Africa, then a further speciation of Homo sapiens from Homo erectus in Africa. Then a subsequent migration within and out of Africa eventually replaced the earlier dispersed Homo erectus. This migration and origin theory is usually referred to as the single-origin theory. However, the current evidence does not preclude multiregional speciation, either. This is a hotly debated area in paleoanthropology.
Current research has established that human beings are genetically highly homogenous; that is, the DNA of individuals is more alike than usual for most species. Distinctive genetic characteristics have arisen, however, primarily as the result of small groups of people moving into new environmental circumstances. These include various characteristics such as skin colour and nose form, in addition to internal characteristics such as the ability to breathe more efficiently in high altitudes.
11.6.13 Homo sapiens idaltu, from Ethiopia
He lived from about 160,000 years ago (proposed subspecies). It is the oldest known anatomically modern human.
11.6.14 Homo floresiensis
Homo 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. Homo floresiensis is intriguing both for its size and its age, being a concrete example of a recent species of the genus Homo that exhibits derived traits not shared with modern humans. In other words, Homo floresiensis share 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 380 cm3 (considered small for a chimpanzee and less than a third of the Homo sapiens average of 1400 cm3).
However, there is an ongoing debate over whether Homo floresiensis is indeed a separate species. Some scientists presently believe that Homo floresiensis was a modern Homo sapiens suffering from 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. The other major attack on H. floresiensis is that it was found with tools only associated with H. sapiens.