Author: Joe Liuzzi

Modern Human Origins

Joseph Liuzzi
Human Origins: Anthropology 1020-004
Teresa Potter

	The regional continuity model and the replacement model are the two leading theories on the origins of modern humans. There is much debate over which of these theories is closest to what actually happened. We cannot prove what actually happened because we cannot go back and see it all take place. Almost everything that may have taken place regarding these two theories predates recorded history, and so there is nothing to refer to other than what we have found recently and what we can find today. Technology, reason, and our knowledge of evidence can bring us to fairly specific speculations, but to prove anything is very rare. 
Regional Continuity
	The Regional Continuity theory model states that modern humans evolved in all major regions of the Old World simultaneously from local archaic humans. Milford Wolpoff advocated this theory also known as the multi-regional evolution model or the multi-regional continuity model. Four major premises outline the model. The first is that that Homo erectus was in fact the ancestor from which modern humans came nearly two million years ago. The second is after the initial dispersal of Homo erectus, there was at least some gene flow between populations segregated geographically that prevented speciation. The third states that the regional variants that we see today were caused by natural selection in those areas. The final major component in the model is that Homo sapiens emerged throughout the world as a large scale phenomenon across the range where all humans lived as opposed to being restricted to one area. (Johanson)(Olson)
	To support the regional continuity model, supporters rely on fossils that contained mixed traits of different species of pre-modern humans as well as the dates of certain skeletal remains of modern humans. Skeletal remains found in Europe show signs of Neanderthal and modern human characteristics. These show that there may have been interbreeding between populations of Neanderthals and humans as suggested by the model. This could explain the similarities between Neanderthal genes and the genes of modern humans, except that the seemingly hybrid skeletons are not found in todays humans. There are skeletal remains that were found in Liujiang Cave, China that predate modern humans that are still considered modern humans. Since the remains predate the period that the replacement theory suggests that modern humans left Africa, this supports the early spreading and inter-population breeding aspect of the multiregional theory. (Johanson)(Potts)
Replacement
	The Replacement theory model states that modern humans evolved only in Africa from archaic humans 200,000 to 150,000 years ago, where some of them migrated into the rest of the Old World, replacing the rest of the archaic humans. This theory is credited to have been developed by Christopher Stringer and Peter Andrews and is also known by three other names: the out of Africa model, the Noah’s ark model, and the African replacement model. The tenets of the replacement model are split into four critical statements. To contrast with the second stated premise of regional continuity above, the first tenet of replacement states that there was in fact independent evolution and even speciation after Homo erectus dispersed. The second premise is that Homo sapiens came from one place, (most likely Africa). Third, Homo sapiens replaced all other human populations without interbreeding after migrating out of Africa 60,000 years ago. As its fourth statement, the theory holds that modern human variation is a relatively recent occurrence. (Johanson)
	Evidence from two main sources support the replacement theory: the fossil record and DNA. To support the second tenet, the fact that the earliest Homo sapiens skeletons were found in Africa really stands out. If modern humans did originate in Africa, then it would make sense that modern human remains from anywhere else in the world would be found from only earlier times. Mitochondrial DNA, the Y chromosome, and chromosome 12 from people who were part of two DNA studies were all compared and analyzed in order to find the genetic diversity within and between human populations all across the world. The studies found that the most genetic diversity was found in the genes of native African people. This also supports the second tenet because the geographic homeland of humans should have the most genetic diversity. The potential problem with the genetic evidence is that the high level of diversity could have been not from thousands of years of evolution, but from the number of people that lived there in the last several hundred years. Genetic diversity in populations comes not only from the age of the population but from the size. Though with age, (provided success) size will usually follow. (Johanson)
	The model that I find to be the most correct is the replacement model. The replacement model suggests that we are all very homogeneous as a species, and we are. It says that we have not been around for a very long time as we are today, but have evolved relatively recently. The multiregional theory, to me, seems to suggest that there was an incredible amount of diversity that, through the interbreeding of populations, ended in very little diversity. Even though multi-regionalism allows a lot of time to get so similar through interbreeding, I still do not see how it could have been enough nor why there is still not very much genetic diversity between humans. The fact that we and Neanderthals may have looked very similar does not overcome the fact that our genes are clearly different, and so the replacement model seems to be the closer model for this reason as well. 

Works Cited
1	Johanson, Donald. "Origins of Modern Humans: Multiregional or Out of Africa?." <i>Actionbioscience</i>. N.p., 1 Jan. 2001. Web. 29 Apr. 2014. &lt;http://www.actionbioscience.org/evolution/johanson.html&gt;.
2	Olson, Steve. <i>Mapping human history: discovering the past through our genes</i>. Boston: Houghton Mifflin, 2002. Print.
3	Potts, Richard, and Christopher Sloan. <i>What does it mean to be human?</i>. Washington, D.C.: National Geographic, 2010. Print.