Homo sapiens evolved in Africa around 150,000 years ago. The modern human, known as Homo sapiens sapiens, is a sub species of the original Home sapiens. We are constantly thinking about our origins. Where are we from? How did we develop into the most common species on the planet? Paleoanthropologists have all sorts of theories for every aspect of our origins, including where are the first modern humans from? There are two major theories to answer this question, the first is Multiregional Evolution, and the second is Complete Replacement.
The first Homo arose from the continent of Africa 2.4 million years ago which has been estimated from evidence found in Ethiopia. From this and other evidence collected over decades it has been found that Africa is definitely the home of the first Homo. (Jurmain et al, 2005) From this point on, ideas start to fly in all directions. How did this Homo develop into the modern human we have today, the Homo sapiens sapiens?
Multiregional Evolution is a theory developed by Milford H. Wolpoff, who argues 2 million years ago, the early Homo migrated across land into different regions of the world, and then separately over time evolved into the Homo sapiens sapiens. The idea is that every species of Homo evolved relatively at the same time, with each group choosing practically the same traits to make us into the human we are today. (Wolpoff, 1999)
The opposing theory is Complete Replacement, also known as, “Out of Africa 2”, “Eve theory”, or “Recent African Evolution”. This theory suggest that Homo sapiens sapiens evolved only in Africa 200,000 years ago. They then migrated out of Africa and moved into regions of the world and eventually replaced any of the Homo inhabitants they encountered. (Lewin, 1987) These two theories are debated time after time and both have compelling arguments. Lets first take a look at the Multiregional Evolution theory by Wolpoff.
In an article, The multiregional evolution of humans, published in the July 2003 issue of Scientific America, Wolpoff and collaborator Alan Thorne, say that the evolution to modern human began 2 million years ago when the first Homo started to leave Africa. They settled and “through an interconnected web of ancient lineages in which the genetic contributions to all living peoples varied regionally and temporally”. (Thorne & Wolpoff, 2003) They continue to say this was very much like today’s society where people from different regions still exhibit traits from those regions even though interbreeding and large movement takes place. Wolpoff explains from his book, Paleoanthropology, that 2 million years ago, Homo continued to evolve and the modern features we see today all occurred at different times. Each of these traits later spread through genetic exchanges as they were the most advantageous. (1999) These advantageous traits can be easily addressed to Darwin’s theory of survival of the fittest. (Darwin, 1859) These fittest traits continued to spread successfully through populations that came in contact with other groups. This contact sometimes brought an intermixing of groups, but other times brought a complete population replacement. Modern human features also started to commonly appear as each population responded to changes within their own society. Wolpoff sees these changes as similar but sometimes very different. (Wolpoff, 1999) Over time these traits would have evolved into Homo sapiens sapiens, as this large diversity of modern traits appeared in different regions around the world.
Opposing this is the Complete Replacement, or Out of Africa 2 theory. The latter name summaries the theory quite well. The first modern Humans came out of Africa 200,000 years ago. This theory believes that Homo sapiens sapiens evolved in Africa and were truly the first modern humans. (Lewin, 1987) They then moved North out of Africa for the second time around 60,000-45,000 years ago moving into other locations of the world. (Ehrilich, 2000) Within time the modern humans increased their population and grew dense outnumbering the pre-existing earlier Homo which eventually became extinct. From this point on as time progressed the modern human became more dense into the population of the world we have today. (Lewin, 1987)
The Complete Replacement theory is based upon research within the science of molecular biology. Mitochondrial DNA, a DNA structure found in the mitochondria of a cell, are direct copies of themselves and are passed on through the mother. Looking at Mitochondrial DNA (mtDNA) allows us to look back at all the maternal ancestral traits. (Jenson, 1998)
Examining the mtDNA, one can help to see where the early modern humans migrated and settled down. There are many ways early modern humans could have actually left Africa but there is little evidence to make conclusions of their path. However, in December 1999, an article published in Nature Genetics, discusses evidence of mtDNA that shows how the early Homo sapiens sapiens left through Ethiopia to South Asia, instead of through the Levant – Greater Syria. From studies, a high frequency of the haplogroup M, along with two other variants from the mtDNA has been found in Ethiopians and Indian populations. This suggests that the Asian haplogroup M separated from eastern-African haplogroup M more than 50,000 years ago. To further support this idea, there is practically a nonexistence of haplogroup M in the Levant – even though their fossils remain – but a high frequency in South-Arabian peninsula that suggests that the only successful exit was through east Africa. (Quintana-Murci et al., 1999)
This mtDNA research has been the primary biological research, however that is now changing. Although the mtDNA is not influenced by the father during fertilization, the mtDNA has a very high mutation rate. This mutation rate therefore can break the ancestral line up and can blur the lines between each ancestor, this would definitely be even more noticeable when looking back thousands if not millions of years. Therefore, scientists have had to look at other ways for analysis. (Jenson, 1998)
Looking at the Y-Chromosome switches our observations from the mother’s ancestral history to looking at the father’s history. Until 1998 attempting to study the Y-Chromosome has been unsuccessful. However, Peter Underhill and his colleague Peter Oefner both working in the department of Genetics at Stanford University, have devised a way to study the Y-Chromosome. Researchers have used this methodology and found ancestral haplotypes, that are passed on through generations, by studying the corresponding portion of the Y chromosome in chimpanzees, gorillas and orangutans. One particular haplotype matches one that was found in small populations of northern and southern Africa. Other Africans and all Europeans share a mutated version of this haplotype, and furthermore, there is a haplotype that is derived from this mutated version that is found in populations from the rest of the world. The same pattern has been found in other portions of the Y-Chromosome and support the same conclusion. These finds help to support the Complete Replacement theory, by seeing how these haplotypes have moved out across the world over time. (Jenson, 1998)
Looking at biological evidence can often cause conflicting results. According to a similar study using the X-Chromosome, Jody Hey and Eugene Harris of Rutgers University in Piscataway, New Jersey, used a key enzyme used for sugar metabolism to recalculate the time splits on the human evolutionary tree. They did this by using the method of the molecular clock. They calculated the genes mutation rate from changes between humans and chimps, and then developed a new tree for human evolution. They saw that one haplotype was only found in Africans, another haplotype eventually gave raise to another variant in Africans, but importantly they saw that another variant split about 200,000 years ago and is seen today in non-Africans. This evidence would agree with Multiregional Evolution instead of Complete Replacement. It is noted in this article that the use of molecular clocks can give inaccurate results, however, it also states that the research team believe they have found it to have a steady time. (Pennisi, 1999)
The majority of the evidence supporting Complete Replacement comes from biological evidence. In Wolpoff’s article, The multiregional evolution of humans, he constantly argues that the opposing theory has no hard evidence such as fossils. He says there is no trace of an invasion around the world and, “although it is not uncommon for one animal species to replace another locally in a fairly short time, the claim that a replacement could occur rapidly in every climate and environment is unprecedented.” (Thorne &Wolpoff, 2003) He also continues to discuss arguments that fossils show that Australia was inhabited by those coming from Java and occurred about 60,000 years ago, and the fossils found show Javan traits. This would mean that the invading modern humans from Africa would have had to evolve a second time exactly the same way the original Javans once did. “The repeated evolution of an individual feature would be conceivable but rare; the duplication of an entire set of unrelated features would be unprecedentedly improbable.” (Thorne & Wolpoff, 2003) Arguably, it could be said that some of the original Javans went to Australia however did not survive leaving fossil remains.
To further counter Wolpoff’s no fossil evidence, according to Ehrlich’s book Human natures: genes, cultures, and the human prospect, he says that there is strong evidence in Europe for a quick replacement of populations by the modern human. (Ehrlich, 2000)
There are two theories; Multiregional Evolution, and Complete Replacement. Both are two different ways we modern humans could have started on this planet. Each theory has relatively compelling arguments into why each work. Evidence and scientific research point us into different directions, yet we still can not figure out what is correct. Typically, the multiregional evolution is based on fossil and hard evidence, whereas Complete Replacement is based a lot on biological research. That said, there is now evidence that jumps from fossil to scientific research that argue against the other theory. We do not know which is correct, and as each piece of research is put together we hope to get a clearer picture. Personally, I am torn between the two theories, but definitely learn towards the Complete Replacement theory. This theory just seems to make more sense than different species of Homo coming rise to the same Homo sapiens sapiens. As research and evidence is uncovered, the truth will hopefully prevail. For now, the Complete Replacement theory seems more viable.
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