Origin of Anatomically Modern Humans

The curiosity to study origin and birth of human beings has shaped a holistic subject, paleoanthropology, which mainly focuses on the origin of modern human beings or Homo sapiens (Matthew and Nitecki, 1994).For about 30,000 years, the Earth has been inhabited by humans that carry anatomical and behavioral uniformity. The situation70,000 years before was clearly different and diverse groups of hominids preceded the modern Homo sapiens; in Asia Homo erectus prospered while in Europe and the East there were Homone, erthalensis and Homo sapiens, respectively. The different populations of humans differed in their bodily phenotypes or anatomy studied through fossil record that was obtained through archaeological mining. Theories to explain this transformation have been proposed: one that suggests a single origin forall modern humansi.e., the Out-of-Africa model, and another proposing Multiregional Continuity.

Genetic studies have indicated that the last ancestor common between chimpanzees and humans lived about 6 to 7 million years ago (mya) and from among these the oldest fossils of hominids from 4.4 mya belonged to Australpithecusramidus, who possessed distinct morphological features attributed to humans (White et al. 89).However, approximately 3.8 mya, Australpithecusramidusevolved into an even better and more human-like species Australpithecusafarensis (Kimbelet al.450),which is considered an ancestor to all human species; it probably divided into two separate human lineages about 3 to 2.5 mya (Klein 169). The first of these lineages comprised of ‘robust’australopithecines, Parathropusaethiopicu that diverged to form two species: Parathropusrobustus, prevalent in South Arica, andParathropusboiseiin East Africa. Contrary to the ‘robust’ australopithecines, the second lineage was based on ‘slender’ australopithecinesAustralpithecusafricanus, an ancestor to Homo habilis. Researchhas pointed out that Homo habilisis not one species, in fact it comprises of two species on the basis of head size: humans with smaller brains and teeth were narrowly classified as Homo habilisand humans with bigger brains and teeth as Homo rudolfensis (Leaky, 1991).However, the later and modernspecies belonging to the genus Homo that are closest to modern humans in chronologydo not seem to have sufficient resemblance with these two for them to be likely ancestors which proposes the existence of a third species; Homo erectusthat evolved about 1.8 mya is a likely candidate to be that ancestor (Klein 170).

The Models for Origin of Modern Humans

Two models or hypotheses have been proposed for the issue of evolution and origin of modern humans. These two schools of thought in this respect shape two models namely Out-of-Africa Model (Stringer and McKie, 1998) and Multiregional Continuity Model (Wolpoff and Caspari, 1997) as mentioned before. A discussion on these models follows.

Out-of-Africa Model: The Out-of-Africa Model is a school of thought that holds a stance quite unique in nature. According to this model, modern human beings evolved quite recently in Africa. From there, they migrated to Eurasia and took over all the already derived populations that originated from Homo erectus.

Major components of this model are:

Once Homo erectus left Africa, all the different populations of humans in the world were reproductively isolated; they evolved independently and no gene flow existed. These populations even evolved into separate species.

Homo sapiens evolved in one place that was probably Africa, from where they left later on.

Homo sapiens left Africa and dominated all other human populations. They did not interbreed with any other.

The variations in humans now visible are a result of recent evolutionary changes.

Multiregional Continuity Model: The Multiregional Continuity Model simply states the hypothesis that Homo erectus, at first, resided in Africa and after leavingtheirregion about 2 myathey spread in all portions of the world. These populations then eventually evolved into modern Homo sapiens or humans.

This model encompasses the following components:

There was a certain level of gene flow between the species separated over the Earth and it prevented further speciation.

Homo erectus is the origin from which all humans now living were derived.

Natural selection in the populations of different regions after their spread is the cause of regional variants, now called races.

Emergence of modern human beings did not occur in any one region. Instead, this phenomenon took place throughout the world wherever humans lived.

Applicability of the Models

The Out-of-Africa Model has been modified to some extent to yield an extension: Out-of-Africa Model 2 (Stringer and Gamble, 1993). Out-of-Africa Model 2 proposes that human populations have a tendency to follow varied evolutionary paths in different regions. This tendency culminated 100, 000 years ago and resulted in the formation of three distinct human species discussed in the first section.Out-of-Africa 2 Model takes on two extremes in its suggestions (Klein, 181):

1) At one extreme it proposes that about 60,000 to 50, 000 years ago, modern people migrated from Africa and they replaced the Neanderthals and other ancient species. They did not interbreed with other populations during this replacement.

2) At the other extreme, Out-of-Africa Model 2 suggests that there must have been some gene flow between the continuously growingmodern Homo sapiens and residentarchaic populations.

The supporters of Multiregional Continuity Model, that was proposed afterwards, agree that human populations tended to diverge morphologically just after the Out-of-Africa1; however, they hold the notion that after the migration, it was continuous flow of genetic information that ensured rapid spread of adaptive characteristics and also maintained evolution of all human populations present at that time on the same pace and track hence making single type of highly similar modern humans possible (Frayeret al., 20).

Behavioralstudies of early humans from fossil evidence also function between African origin theoryand the replacement of early ancestor humans. Archaeological data suggests that fully modern human beings had a very strong capability to innovate and produce. This can of course be associated with their more developed brains (Klein, 192).A more developed brain would carry a selective advantage for the modern human beings but the proposition that it was made possible by neural change is difficult to prove because fossil skulls, although differing distinctly in morphology, do not provide evidence about brain functioning. In the coming times, new discoveries from newly found archaeological and fossil data would present new paths or refine old trajectories; in any case, one cannot be fully confident about the actual origin of modern human beings and the trajectory they followed to reach their current state.

Anatomy of Close Ancestors of Modern Humans

The early hominids after leaving Africa spread into other parts of the world. They lived in separate regions and different environmental conditions produced varied morphology and genetic changes.Fossils for Neanderthals have revealed that they possessedthe following anatomical characters (Donald, 2):

Large, long, low cranial vault

Well-developed double arched brow-ridge

They had massive facial structure.

Their mid-face projected outwards

Backwards sloping cheeks were there and the nasal aperture was quite large

Occipital region had a bulge or bun.

Their molars had enlarged pulp chambers and often heavily worn incisors.

Mandible lacked a chin and a large gap behind the last molar was there.

Thorax was massive while forearms and lower legs were quite short.

They were short but had strong and robust builds.

They had long clavicles and wide scapulas.

After a prolonged period of independent survival, this population became so anatomically distinct that it was rendered as a separate species named Homo neandethalensis. They prospered in the East.

Just at this time in Africa, the Early Homo Sapiens were evolving. They had a bodily plan quite similar to what we have today. They had gained the modern anatomy but they retained an archaic behavior. These early Homo sapiensare characterized by the following characters (Donald, 2):

A cranial vault with a vertical forehead

Rounded occipital and a reduced brow-ridge

Reduced facial skeleton lacking a projected mid-face

Lower jaw supporting a chin

More modern and less robust skeleton and structure

The anatomical differences between these different human species in different part of the world indicate the fact that as they survived in different regions, they evolved on their own anatomically. Later on, Homo sapiens would leave Africa and dominate other populations on the Earth resulting in modern human beings as discussed in earlier sections.

Other evidence regarding the existence of different populations has been obtained from study of archaeological artifacts like tools, houses, graves, art or decoration as well as genetic changes. However, majority of anatomical, archaeological and genetic evidence gives credibility to the view that modern human beings are a newer phenomenon whose best explanation is provided by the Out-of-Africa Model (Donald 2). The neurological modifications and cultural innovations seen with the appearance offully modern humans have benefited them and have resulted in their dominance at the expense of all other older and less modern hominid populations.

Unfortunately, the fossil records available to archaeologists are incomplete and irregular and of course since no living witness may ever be available of the evolution under discussion, evidences gathered for the phenomenonwould remain partially circumstantial, ambiguous or even may be contradictory in the future. Multiregional hypothesis bas been criticized because of the lack of consideration of key features that were present in distant human populations; also, many features are common in recent populations while the model supposes them to be rare (Lahr, 50). Out-of-Africa model, through Out-of-Africa 2, providesthe most radical and rational explanations for present archaeological and fossil data, hence it is much more strongly supported than the Multiregional Continuity Model (Klein, 195).

References

Frayer, David W., et al. “Theories of modern human origins: the paleontological test.” American Anthropologist 95.1 (1993): 14-50.

Kimbel, William H., Donald C. Johanson, and YoelRak.”The first skull and other new discoveries of Australopithecus afarensis at Hadar, Ethiopia.”Nature 368.6470 (1994): 449-451.

Klein, Richard G. “Anatomy, behavior, and modern human origins.”Journal of World Prehistory 9.2 (1995): 167-198.

Lahr, Marta Mirazon. “The multiregional model of modern human origins: a reassessment of its morphological basis.” Journal of Human Evolution 26.1 (1994): 23-56.

Leakey, Meave G., et al., eds. Hominid cranial remains. Vol. 4. Clarendon Press, 1991.

Nitecki, Matthew H., and Doris V. Nitecki, eds. Origins of anatomically modern humans.Plenum Press, 1994.

Stringer, Chris, and Clive Gamble. In search of the Neanderthals: Solving the puzzle of human origins. London: Thames and Hudson, 1993.

Stringer, Chris, and Robin McKie.African exodus: The origins of modern humanity. Holt Paperbacks, 1998.

White, Tim D., Gen Suwa, and BerhaneAsfaw.”Australopithecus ramidus, a new species of early hominid from Aramis, Ethiopia.”Nature 375.6526 (1995): 88.

Wolpoff, Milford H., and Rachel Caspari.Race and human evolution.Simon and Schuster, 1997.