How did AMHSS (Anatomically Modern Homo Sapiens Sapiens) come to populate the whole world? This has been a huge question and debate in anthropology longer than I have been alive. Many different theories have been proposed but as of yet none have been proven with absolute certainty. In the field of anthropology there are currently two major theories that are circulating among academics: The Out of Africa Hypothesis and the Multiregional Hypothesis. Based on genetic, fossil, and archaeological evidence the details of each theory are squabbled over by leading experts in the field. Someday there may be new evidence that leads to a third or fourth major hypothesis. Since this can be such a confusing web of information here are some of the basic points of each.

Out of Africa Hypothesis

The basic premise of this hypothesis is that modern humans developed relatively recently in a single geographic area: Africa. Generally scholars who argue in favor of this hypothesis date the development and migration of AMHSS out of Africa to between 200,000 and 60,000 years ago. Proponents of this theory argue that AMHSS gradually replaced other existing hominid groups around the world including Neanderthals and any surviving groups of Homo erectus. What kind of evidence has been used to support this theory?

Development in mtDNA research has played a major role in the advancement of this theory. To make a long story short, scientists have found that by tracing the variations and mutations of human mtDNA they can discover the routes that human groups have traveled to get to where they are today. One of the key points in this research is that there is greater mtDNA diversity found in Africa than anywhere else in the world. This is exactly what scholars would expect to find if AMHSS has been in Africa the longest, meaning that the mtDNA found in Africa has had the most time for random mutations. The Genographic Project run by National Geographic maps out the different genetic markers or mutations in mtDNA that are characteristic of certain geographic areas. Another project from the Bradshaw Foundation does the same thing and incorporates major climate changes, such as the eruption of Mt. Toba 75,000 years ago, into the equation. The hypothesis about the Mt. Toba eruption is that most of the humans in the Southeast Asia region were wiped out, both AMHSS and any archaic populations that may have still been around. This would mean that a later wave of AMHSS would have populated that region of the world after 75,000 years ago. Experts believe that a secondary effect of the eruption was a global period of cooling due to the ash and dust in the atmosphere. This would have limited the spread of humans to higher latitudes for a time and made survival for both AMHSS and archaic populations already in those areas much more difficult. The idea that climate change can drastically effect the migration and lifestyles of humans is one that may become relevant for us in the next 100 years or so with global warming.

Archaeologists have found that 40,000 to 50,000 years ago there was an explosion of cultural activity, which resulted in changes in subsistence behavior, symbolic expression, and tools. Some archaeologists argue that this is what allowed AMHSS to replace other hominids in Asia and Europe because those cultural advances would have given AMHSS an advantage in using available resources. One of the most prominent of these cultural advances is the development of more sophisticated stone tools, such as spear points. With the use of spears AMHSS would have been able to hunt more efficiently. The ability to collect more food during hunting trips would allow AMHSS to support a larger population. The hypothesis here is that AMHSS would have been able to displace archaic groups by an advantage in sheer numbers and the diversity of resources exploited.

The third line of evidence often used in favor of this theory is that there is relatively little, if any, substantial anatomic evidence for intermixing of AMHSS with either Neanderthals or H. erectus. Most of the fossil evidence of AMHSS and archaic human groups have distinct anatomical differences. For example, Neanderthal skulls have a very distinctive prominent brow ridge that does not appear on AMHSS. This is one of the more problematic lines of evidence because there have been a few fossil finds which suggest that there was the possibility of intermixing between AMHSS, H. neanderthalensis and H. erectus. Which means it’s time to talk about:

The Multiregional Hypothesis

The Smithsonian’s Human Origins Program has a very good succinct description of this that I like:

According to the Multiregional hypothesis, also known as the Continuity hypothesis, the evolution of modern humans began when Homo erectus spread throughout much of Eurasia around 1 million years ago. Regional populations retained unique anatomical features for hundreds of thousands of years, but they also mated with populations from neighboring regions, exchanging inheritable traits with each other. This exchange of inheritable traits takes place by the process known as gene flow.

Through gene flow, populations of H. erectus passed on a variety of increasingly modern characteristics, such as increases in brain size, across their geographic range. Gradually this would have resulted in the evolution of more modern looking humans throughout Africa and Eurasia. The physical differences among people today, then, would result from hundreds of thousands of years of regional evolution. This is the concept of continuity.

An important note to make is that most scholars that advocate this hypothesis do not believe that current regional physical differences denote any inherent mental attributes – this theory does not necessarily indicate the idea of human “races.”

The evidence for this hypothesis is very similar to the evidence used for the Out of Africa hypothesis, it’s just interpreted in different ways. Since the exact mutation rate of mtDNA is not known there are many different estimates of the age of origin of mtDNA found in AMHSS. It could potentially be as old at 850,000 years ago. mtDNA also makes up only about 1/400^th of our total genetic material, meaning that potentially 99.75% of our genetic material could be inherited from humans living at the same time as the “mitochondrial eve.” More recently there has been evidence that we share some of the more obscure genetic mutations that we previously assumed were only found in AMHSS with Neanderthals – such as the FoxP2 gene that might be associated with language. This raises some interesting questions about the relationship between AMHSS and archaic human groups.

Anatomic evidence is another big line of evidence used by advocates of the multiregional hypothesis. In modern human groups today there are certain anatomical differences that seem similar to earlier human groups from the same geographic areas. For example, Asian populations tend to have flatter cheek and nasal areas whereas the same features tend to project outward in European populations. Some argue that these traits are holdovers from archaic populations in the same areas. There are also a few fossil finds, such as the 25,000-year-old child skeleton found in Lagar Velho, Portugal which seems to display both AMHSS and Neanderthal traits, that might support interbreeding of AMHSS with archaic human populations. The 4-year-old had shorter stockier limb bones which were characteristic of Neanderthal skeletons, however, the skull of the child had the appearance of a typical AMHSS in that it did not have an overly pronounced brow ridge or an occipital bun.

Conclusions

Obviously there is still much research to be done in this area of anthropology. The future of this debate will undoubtedly come from the continued examination and re-interpretation of existing evidence as well as the discovery of new evidence. I for one am looking forward to seeing what come out in the next few years as technology continues to advance giving us new ways of interpreting the evidence.

Here are some more useful resources on this topic:

Smithsonian National Museum of Natural History: http://anthropology.si.edu/HumanOrigins/faq/Encarta/encarta.htm

PNAS: http://www.pnas.org/content/102/44/15942.full.pdf?ck=nck

John Hawks, Serial Founder Effects: http://johnhawks.net/weblog/reviews/genetics/ramachandran_2005_serial_founder.html

James Jacobs, Out of Africa 2: http://www.jqjacobs.net/anthro/paleo/genome.html

Donald Johanson, Origins of Modern Humans: http://www.actionbioscience.org/evolution/johanson.html

John Hawks, Multiregional vs. Out of Africa: http://johnhawks.net/weblog/topics/modern_human_origins/multiregional_vs_out_of_africa.html * This one has a particularly nice summary of both models!