Reigniting an Tried and True Debate on Human Origins...

Today’s post is in part inspired by my Anth 102/110L students who must undertake an assignment on human evolution, specifically focusing on the origins of our species (Homo. sapiens), as well as the recent discovery in Georgia of a potentially new specimen of Homo. erectus.

There are many philosophical questions that plague us as we grow up and gain a better understanding of ourselves: What is the meaning of life?  Why is there something rather than nothing?  Is there life after death?  Does this outfit make me look fat?  But one of the most profound is:

Where do we (humans) come from?

This is a question that has baffled paleoanthropologists for many decades, and one that has yielded several answers but never the truth.  Despite years of research, endless man hours dedicated to answering this question, countless articles and undergraduate, masters, & doctoral theses written on this topic, paleoanthropologists are still unable to answer this question.  But it is one that is still popular among paleoanthropologists, particularly today given the recent find of the Dmanisi skull. 

Failure to find the truth has led to several hypotheses being brought forth as a means of trying to understand our origins.  These hypotheses are the subject of today’s blog post, and each of the three has an opposing but plausible explanation regarding the origins of Homo. sapiens.  These three hypotheses include the Multiregional Evolution (MRE) model, the Out of Africa or Recent African Origin (RAO) model, and the Mostly-Out-of-Africa model.

The Multiregional Evolution (MRE) model was spearheaded by anthropologists Milford Wolpoff and Rachel Caspari.  According to the MRE model, anatomically modern Homo sapiens evolved in Africa, but these species arose 2 million years ago (MYA), not 200,000 years ago with the departure of Homo ergaster from Africa and who then spread across the Old World, evolving genetic and phenotypic regional differences in response to their various environments and the factors of evolution (e.g. gene flow, founder’s effect, genetic drift, etc.)  These various populations remained in some contact and exchanged genetic information (through interbreeding), which maintained the production of a single species over a wide geographic distribution.  Specific traits that were adaptively beneficial for one population were spread to other populations as these individuals survived long enough to pass along their genetically advantageous traits.  In addition, ideas, cultural adaptations, and technologies were passed along through these various populations.

For the MRE model to be correct one expects to find no clear-cut evidence of a separation between anatomically modern and archaic Homo sapiens & hominids-be it morphologically or genetically speaking.  Transitional forms of the species should be found in various parts of the Old World with an admixture of traits among all the species.  As well, there should be regional continuity of traits with groups residing in Asia looking more similar than groups residing in Africa but there should never be so many differences so as to denote a separation of the species.    

Supporters of the MRE model claim that the differences seen among Homo. neanderthalensis and Homo. heidlbergensis and modern Homo sapiens are not differences in the sense of warranting a different species categorization but instead are variations within the same species.  These variations eventually gave rise to the modern human appearance we have today.   In fact, we see some features among specific modern groups, e.g. the continuous brow ridge of the Neanderthals is also present among Australian Aborigines (and according to several students and lay people, ex-significant others).  As well, there are no consistency among men or women regarding the degree they exhibit these features, so men and women will either demonstrate a strong or weak incidence of specific traits.  This sliding scale of the exhibition of traits is further evidence of possible links between humans and our hominid ancestors, supported by an independent study conducted by supporters of the MRE model.

The Out of Africa or Recent African Origin (RAO) model was spearheaded by Christopher Stringer and Ian Tattersall.  This model recognizes several different species in Homo genus, but that modern Homo sapiens branched off from other archaic Homo sapiens and hominid species around 200,000 to 150,000 years ago.  Anatomically modern Homo sapiens spread across the Old World and replaced other archaic and hominid species (e.g. Neanderthals).  This model is also known as the replacement model.

If model is correct there is an expectation of distinctive features between modern species and archaic Homo sapiens and hominid species, demonstrating a separation in species.  In other words, every single human on the planet must share specific traits that are not found in archaic Homo sapiens & hominids, and traits among archaic Homo sapiens and hominid species must not be found in modern humans.  Genetic differences should also be present.

Transitional fossils of both species should be found in one area of the world-the place of where modern Homo sapiens began, coupled with evidence of archaic and anatomically modern Homo sapiens existing in the same areas.  Eventually, archaic Homo sapiens & hominid species would have been replaced by anatomically modern Homo sapiens.

According to the supporters of the RAO model, transitional humanoid forms between hominid ancestors, Homo. neanderthalensis and Homo. heidelbergensis, and modern humans are only found in Africa.  They cite that anatomically modern humans first appear in Africa, as well, and subsequent evidence of said humans outside of Africa happens after their initial appearance in Africa.  Both human species and the hominid species exist at similar times.

With all this conflict and inability to settle on either of MRE or RAO models, John Relethford coined the Mostly-Out-of-Africa model.  The basic idea of this model is that most of our ancestors came from Africa but not all of them did.  It agrees with the MRE model in that our species began 2 MYA and gene flow among various geographically distant populations is the reason why we (humans) look so similar today, but it acknowledges the evidence cited by RAO model supporters that demonstrates that Africa was the origin of human based on fossil evidence.  

Supporters of the Mostly-out-of-African Model claim that there were two expansions out of Africa.  The first occurred with Homo ergaster leaving Africa and going to Asia (becoming the species we call Homo erectus).  Subsequent expansions may not have been African species physically leaving Africa but instead was the result of genes being passed through populations.  Specific traits that define us as human today may have started in Africa and spread across the globe through gene flow, so it was not an all out replacement of other species but a blending of them that created who we are today.     

As this post demonstrates, there remains a lot of unanswered questions concerning the origins of Homo. sapiens.  While the discovery of new hominid species, such as the Dmanisi skull, are meant to be a means to settle this debate, they have only added more fuel to the fire of this debate.  Hopefully as interest in the discipline grows and new methods realized, a resolution to this debate will be reached.  Until then, the mystery continues.