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HUMAN EVOLUTION

 

M.Tevfik Dorak, M.D., Ph.D.

 

 

Primates (order) include suborders Prosimians and Anthropoids

Anthropoids consist of New World Monkeys and Catarrhini (Old World Monkeys & Apes)

One of the superfamilies within Catarrhini, Hominoidae includes family Hominidae (Australopithecines & Homo) and Pongidae (Chimpanzee & Gorilla). Sometimes Pongidae is included in Hominidae in which case subfamily Homininae is used for Homo. Modern humans and great apes are descendants of a common ancestor, which lived about 5 million years ago. [In other words, humans did not descend from apes. Hominoids diverged from monkeys 25 mya. Gibbons and orangutans diverged from other apes 10 and 8 mya, respectively.

Hypotheses on the emergence of modern humans:

1. Multiregional evolution model: Proposes an evolutionary trend throughout the old world towards modern Homo sapiens, with little population migration and no population replacement. It emphasizes regional continuity in the evolution from H.erectus to archaic H.sapiens, and later to anatomically modern humans. The strongest piece of evidence for this hypothesis is that distinctive characteristics found today in different geographical regions are also found in H.erectus populations of the same region.

2. Out of Africa model: Evolution of Homo sapiens in one place only (east Africa), followed by extensive population migration across the old world resulting in the replacement of existing premodern population (Homo erectus). Smaller interpopulation genetic divergence than intrapopulation divergence supports this hypothesis. The greater genetic diversity among sub-Saharan African populations may indicate a longer period of separation of populations within Africa than elsewhere. Pairs of African populations are some 50% more genetically distinct from each other than are any other pairs of populations. Not only for mtDNA, but also for many nuclear loci and some regions of Y-chromosome DNA, Africans have higher diversity. This would imply a much longer population history for Africa. The initial mtDNA split was found between Africans and others, followed by progressively younger calibrated ages for specific Asian, Australian, New Guinea, and European mtDNA types. In summary, both mtDNA and Y-chromosome studies (as well as other nuclear DNA studies) indicate a recent African origin for modern humans, probably within the last 200,000 years. The oldest anatomically modern human fossils (over 100,000 years old) are found in Africa supporting the Out of Africa model as well. The latest one of those is the recent finding (Homo sapiens idÓltu) in Herto, Ethiopia which has been dated approximately 160.000 yr old (White et al, 2003; Clark et al, 2003; New Scientist Commentary; Science News Commentary).

Four major events in the course of human evolution

1. The origin of the Homininae family (5-7 mya).

2. Adaptive radiation of species of bipedal apes: bipedalism frees hands (tool making, carrying food/baby) and erect posture greatly reduces the level of solar heat load.

3. Enlargement of the brain (beginning of the genus Homo; 2 - 1.5 mya).

4. Origin of modern humans (modern Homo sapiens: humans with a flair of technology and innovation; use of language; a capacity for artistic expression; an introspective consciousness; a sense of morality; domestication of animals and plants) about 200 thousand years ago.

Other trends: disappearance of sexual dimorphism in the last million years, less projecting face, more prominent nose and chin, lighter skeleton, smaller brow-ridges.

 

Time table of speciation towards the Homo lineage

Homo sapiens sapiens occurred around 200,000 years ago

The transition from H.erectus to H.sapiens happened about 500,000 years ago

H.erectus emerged in Africa more than 1.7 million years ago

Bipedal Australopithecines appeared (5 mya)

The Homininae lineage (subfamily) diverged from Chimpanzee lineage 5-7 million years ago

The Orangutan lineage diverged from the lineage of African Apes (Hominoids) about 13 million years ago (see Pilbeam, 1996, Disotell, 1999, Stauffer et al, 2001 and Collard & Wood, 2000).

 

Anatomical differences between humans and apes

1. Brain size (three to four times larger)

2. Jaw and teeth size and shape (smaller, less projected jaws with small teeth and non-projected canines together with smaller face)

3. Mechanisms of locomotion (bipedal walking and running)

Australopithecus (Southern ape): The earliest Hominines lived in eastern and southern Africa about 5 to 1 mya (now extinct). They developed from Kenyapithecine and pre-Australopithecines. They are the transitional species between apes and humans. The adoption of bipedal locomotion may have been the motive for splitting from the ape lineage. In some ways they were similar to living apes in having small brains (450 - 550 cc) and ape-like body proportions. However, they were distinguished from them by their upright posture and bipedal (two-footed) gait. In addition, the teeth of Australopithecines included smaller canines as well as thickened enamel on the molars. These features are also found in humans. Their height was 1 - 1.5m. It comprised both gracile (ramidus, afarensis, africanus) and robust (aethiopicus, robostus, boisei) forms. The earliest well-known fossil in the homininae lineage is A.afarensis which is the ancestor of both Homo lineage and other Australopithecines [Lucy 'AL 288-1' found in Ethiopia in 1974 is a 40% complete fossil of A. afarensis from 3.6 - 3.8 mya; A. ramidus found in Aramis/Ethiopia in 1994 is from 4.4 mya and is the missing link between the common ancestor (of apes and Homo) and human family trees, and closer to chimpanzee]. A.africanus appeared about 3 mya [Taung baby (2.8 million years old) found in South Africa in 1924]. Contrary to the original idea, it may not have evolved from afarensis (see Berger Lee: Nat Geog 1998; 194(2):90) but afarensis and africanus may be sister species that share a missing-link ancestor. A.africanus is closer to H.habilis than the other Australopithecus species. Surprisingly, africanus has longer arms and shorter legs (more ape-like) than the earlier afarensis. They also have larger brain than afarensis and their skull features resemble those of the first Homo species. No tools were found with Australopithecine fossils. Whether the genus Homo split off before or after the early robust Australopithecine line is not known but Homo co-existed with robust Australopithecines. The evolutionary transition to Homo probably occurred between about 2.3 to 2.7 million years ago.

Homo habilis (skilful person): The first species of our own genus Homo appeared in East Africa around 2.5 mya. Developed from gracile Australopithecus (africanus). They had a larger brain capacity (650 - 800 cc) than Australopithecus. [ER-1470 = H.habilis, older than 2 mya]. Some H.habilis fossils are associated with tools. The oldest stone tools are about 2.5 million years old and were found associated with the fossils of H.habilis.

Homo erectus: Appeared in East Africa 1.7 mya or earlier, eventually inhabited the Old World. By about 1.5 million years ago, descendant forms of H.erectus are known from the tropical areas of the Old World. They spread out of Africa and tropicals 1.0 to 0.7 mya (the African population continued to evolve and gave rise to modern humans). They were hunter-gatherers with a cranial capacity of 775-1250 cc. [Peking man = H.erectus pekinensis; Turkana boy found in Tanzania = H.erectus, also found in Java, India and Europe]. One descent line from H.erectus led to the Neanderthals. H.erectus may have survived in parts of Asia long after it had become extinct (or evolved into a new species) elsewhere. A number of H.erectus fossils from two sites in Java were dated to only 30 to 50 thousand years ago.

Homo heidelbergensis: By about 600,000 years ago, some H.erectus populations in Africa, and probably also in Europe, had changed sufficiently to be recognized as a new species. The first fossil belonging to this species was found near Heidelberg and dated to 500,000 years ago. A large brain size, a less projecting face, a more prominent nose and a more inflated braincase are typical of H.heidelbergensis. It is believed that H.heidelbergensis split into H.neanderthalis and H.sapiens. The common ancestor to Neanderthal and modern humans was dated between 550 and 690 thousand years.

Homo sapiens neanderthalensis: An archaic subspecies (300 - 35 thousand years ago= Mid-late Palaeolithic) lived in Europe and parts of Asia. The first Neanderthal skull was found in the Neander valley in 1857. Since then more fossils have also been found in Spain and Israel. Variation observed between the Neanderthal and human species is much greater than that observed between the human lineages themselves, but not as great as that between the chimpanzee and human lineages. This supports the idea that Neanderthals went extinct without contributing mtDNA to modern humans and were a distinct species (i.e., interbreeding between Neanderthals and H.sapiens sapiens did not occur). The physical characteristics of Neanderthals are heavy brow ridges, elongated skull and long jaw. They probably had a larger brain than modern humans. They looked after their disabled and buried their dead ritualistically. Recent evidence suggests their physical capacity for the sounds of speech.

Homo sapiens (intelligent person): The final evolutionary form of humans developed from Homo erectus. In Africa, the species H.habilis, H.erectus and H.sapiens were probably a continuum of chronospecies. These repeatedly expanded out of Africa and replaced the previous colonizers of the Eurasian land mass. The brain size is 1400 cc on average, frontal lobes are developed, face projection is decreased, the size of teeth is decreased and a chin is appeared. Homo sapiens sapiens is another separate lineage which goes back more than 100 thousand years ago. Originated in Africa and replaced the Neanderthals. The Cro-Magnon people from 35 - 40 thousand years ago are believed to be prehistoric H.sapiens sapiens who are the ancestors of Europeans. They developed aesthetic feelings and are associated with Upper Palaeolithic culture. All current human races are the same species. There are larger differences between individuals within any racial groups than there are between the averages of each race.

According to molecular studies, our closest relatives are the chimpanzee and the pigmy chimpanzee (bonobo). Our next nearest relative is the gorilla. Cladograms based on craniodental characteristics, however, place humans and orangutans in the same clade and gorilla and chimpanzees in another (see Collard & Wood, 2000 and Pilbeam, 1996 for details and the conclusion that hominoid dental characters are not reliable for phylogenetic reconstruction).

 

Primates consist of four groups

1. Prosimians (including lemur): First group evolved from shrew-like mammals about 70 mya.

2. New world monkeys: Marmoset, howler and spider monkey.

3. Old world monkeys (baboons and macaques): From which hominoids split off.

4. Hominoids: The living hominoids are human (Homo sapiens), common chimpanzee (Pan troglodytes), pygmy chimpanzee (Pan paniscus), gorilla (Gorilla gorilla), orangutan (Pongo pygmaeus) and nine species of gibbons (Hylobates spp.).

 

The sister group of the Hominidae (humans, chimps, and gorilla) is the Cercopithecoidea (old world monkeys: macaque, baboons, colobines, etc).

 

An Essay on Hominid Evolution    Hominid Species Timeline

 

PBS: Human Evolution    BBC: Human Beginnings & Human Evolution

 

New Scientist Special Reports: Human Evolution & Timeline

 

Arizona State University Institute of Human Origins : Becoming Human 

 

Smithsonian Institution: Human Origins Program:  Human Ancestors Hall

 

 

M.Tevfik Dorak, MD, PhD

 

Last updated 4 February 2007

 

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