Origins of Anatomically Modern Humans

Anatomically Modern Humans (AMH, also known as AMHss, or Anatomically Modern Homo sapiens sapiens) are the most recent of the Middle Pleistocene Humans to appear. Neandertal and Denisovan fossils can be as old as 800,000 years, but AMH fossils are no more than 200,000 years old. Actually, it is more accurate to say that certain traits associated with AMH appear around 200,000 years ago, since (as you'll see below), all of the Middle Pleistocene Humans were one species.

How Different Were Anatomically Modern Humans from other Middle Pleistocene Humans?

Not very. The one definitive difference between AMH and other Middle Pleistocene Humans is that AMH have chins. Seriously, that's it. Now, I'm as fond of my chin as the next person, but it's not something I would want to hang my hat on as a major difference between me and a Neandertal.

There are other differences between AMH and other Middle Pleistocene Humans, some of which represent adaptations to different environments. Middle Pleistocene Humans in Eurasia mostly lived in colder environments than those from Africa, so they had some of the traits that we associate with cold-adapted people from Europe or northern Asia today, such as large noses, relatively robust bodies, and shorter limbs relative to body size. Some other traits of Middle Pleistocene Humans in Eurasia include a longer, lower brain-case ("lemon-shaped") compared to AMH from Africa, whose brain-cases were more "orange-shaped", like all humans today. Middle Pleistocene Humans in Eurasia also had larger faces, while AMH had shorter, flatter faces. This, in fact, is where the chin comes from. As the AMH face got shorter, the lower margin of the jaw stayed where it was, while the face shrunk backward from it.

Having said they aren't very different, I do want to point out two potentially important differences: 1) the shorter face and less rugged skull may suggest a much softer diet. Perhaps AMH cooked their foods more, or focused on different foods than non-African Middle Pleistocene Hominins; and 2) Even after AMH moved into the colder climates of Eurasia, we continued to have lower robusticity than non-AMH Middle Pleistocene Humans, suggesting that AMH had cultural adaptations that were able to maintain a less rugged lifestyle.

It's easy to overestimate the differences between AMH and other Middle Pleistocene Humans, though. Physically, they were very, very similar, and behaviorally, as we'll see next week, it's increasingly clear that they weren't categorically different in any way we can recognize. Keep in mind, after all, that Eurasian Middle Pleistocene Humans had a brain just as big (or bigger!) than ours.

Where Did Anatomically Modern Humans Originate, and How Did They Spread Across the World?

The fist Anatomically Modern Human fossils are found in East Africa, dating to 200,000 years ago, and fossils dating to no later than 30,000 years ago are found throughout Africa, Asia, Europe, and Australia. However, it's more accurate to say that certain traits associated with AMH (such as a chin, an "orange-shaped" cranium, etc.) originated in Africa 200,000 years ago, and spread across the Old World by 30,000 years ago.

Until quite recently (within the last five years), there was still a lot of debate in Anthropology over the origin and spread of AMH. With our current genetic knowledge, we've answer a lot of the outstanding questions that we had only a few years ago. Older models of the origin and spread of AMH, however, are still widely discussed in the media, so I will lay these out below, along with the evidence for and against each.

The Multiregional Evolution Model

What it is: The Multiregional Evolution Model is the "original" model, in that it represents the thinking of anthropologists since it first became clear that multiple groups of humans lived during the Middle Pleistocene. This model suggests that local populations in Europe, Asia, and African show direct evolutionary development from earlier Middle Pleistocene Humans (like Neandertals) to AMH. This model denies a solely African origin for AMH. Instead, it emphasizes role of genetic continuity over time within regions, assuming that AMH developed throughout the Old World, invoking gene flow between regional populations to account for similar anatomical transitions from earlier Middle Pleistocene Humans to AMH in different regions. This model considers all Middle Pleistocene Humans (Neandertals, AMH, Homo heidelbergensis, and the Denisovans) to be members of same species. Neandertals, in this model, would be ancestral to European populations, while Denisovans would be ancestral to Asian populations.

Who supports it: The biggest proponent of the Multiregional Evolution Model is Dr. Milford Wolpoff, of the University of Michigan. (Full disclosure: I attended the University of Michigan as an undergraduate, and my first human evolution class was taught with this perspective, so I've always been partial to it, although I, like many of his students, prefer the Partial Replacement Model. Go Blue!)

What evidence supports it: Most supporters of the Multiregional Evolution Model are fossil experts. The fossil record from Asia has been used to support the Multiregional Evolution Model. At the Chinese site of Zhoukoudien, for example, there are "transitional" fossils, ones that combine traits of AMH with other/earlier Middle Pleistocene Humans, which is exactly what would be expected under the Multiregional Evolution Model. Furthermore, certain traits, like shovel-shaped incisors, are commonly found in Asian Homo erectus, Asian Middle Pleistocene Humans, and Asia Homo sapiens sapiens, but are rarely found in those species outside of Asia. Again, this would suggest that the earlier humans of Asia were ancestral to the later humans of Asia. However, as discussed below, not all regions show evidence for a smooth transition from Homo erectus to AMH, and, in fact, this transition is debated in Asia as well.

Some additional considerations: This model of the origins of AMH assumes that speciation occurs through natural selection. If you remember from earlier in the semester, some evolutionary scientists believe that speciation occurs through gradualism, the gradual accumulation of changes through natural selection. That is the underlying assumption of this model.

The Complete Replacement Model (aka "Out of Africa" or the "Eve Hypothesis")

What it is: According to this model, AMH first arose in East Africa about 200,000 years ago when a small population of Middle Pleistocene Humans was isolated and underwent rapid selection. This new species of human (us), then spread throughout the Old World, replacing all other Middle Pleistocene Humans through competitive exclusion. According to this view, the only place where AMH evolved was in East Africa. In the rest of the world, the non-anatomically modern Middle Pleistocene Humans were completely replaced by migrant AMH. According to this model, the different groups of Middle Pleistocene Humans (AMH, Neandertals, Denisovans, etc.) were all separate species, not capable of interbreeding. All people today are descendants of that one population in Africa, 200,000 years ago, and the other Middle Pleistocene Humans did not leave any descendants.

Who supports it: There are a number of researchers whose research has supported this model. On the fossil side, it's principle proponents are Dr. Chris Stringer (Natural History Museum, London), and Dr. Richard Klein (Stanford University). This perspective on human evolution, though, really began with modern genetic evidence (see below), and the scientist most closely associated with that work is Dr. Rebecca Cann of the University of Hawai'i Manoa.

What evidence supports it:

Fossil Evidence: It is widely accepted that all the earliest AMH fossils are from East Africa, just as would be predicted by this model. Some of the important early AMH finds from Africa include Omo 1 and 2, found in Ethiopia and dating to 190,000 years ago. These early finds still retain a number of primitive traits. They have chins, but often heavy brow ridges and occipital buns, similar to other Middle Pleistocene Humans.

Modern Genetics: Studies of modern genetics are what really fueled the development of this model of modern human origins. The initial evidence for this model came from the work of Rebecca Cann, who looked at mtDNA (mitochondrial DNA) in contemporary human populations. Mitochondria are organelles inside your cells that have their own set of DNA. This mtDNA isn't a combination of your mother's and father's DNA, like nuclear DNA is. Instead, you inherit your mtDNA directly from your mother. Theoretically, your mtDNA is an exact copy of your mother's, which is an exact copy of her mother's, which is an exact copy of her mother's, and so on and so on, all the way up the maternal line. In reality, however, mutations occur from time to time within mtDNA, so that one generation is not always identical to the previous. Rebecca Cann used those mutations to try to identify how long it has been since all contemporary people shared a common maternal ancestor. She assumed that, at one point, all people had identical mtDNA sequences. If all of the different mtDNA sequences of today were the result of random mutations occurring generation after generation, then how many generations would it take to create the sequences we see today? To answer this question, Cann had to set a molecular clock: basically she had to use proxy measures to determine how common mutations are, so that the number of mutations could be translated into a number of years.

Cann reached two conclusions: 1) mtDNA shows the most diversity in Africa, which suggests that all of the mtDNA lineages we see today originated in Africa; and 2) that the contemporary lineages of mtDNA shared a common ancestor 280,000-117,000 years ago. This is exactly what is suggested by the Complete Replacement model. You may have heard media coverage about "Mitochondrial Eve" or the "Eve Hypothesis". Those phrases are referring to Cann's work. But it is important to understand, before getting too carried away with Cann's research, that the single maternal ancestor is merely an assumption of the analysis, it's how the technique works. There's no actual evidence that only one female's mtDNA was handed down into our species, or, if there was only one female, that the female in question was an AMH. The other assumption that Cann had to make is that mtDNA is not under selection. It was long assumed that mtDNA was neutral, junk DNA. Therefore, it was not under selection, so the changes that occurred over time would all be random mutations. We have learned since Cann's work, however, that mtDNA is important for regulating metabolism and brain growth, and it appear there's been significant selection in those two functions over the last 200,000 years.

Ancient Genetics: DNA has been extracted from Eurasian Middle Pleistocene Humans, as well as early AMH. We do know that Neandertal mtDNA lineages did not survive to modern times, and were in fact fully replaced by the mtDNA of AMH out of East Africa. This is consistent with the Complete Replacement Model. However, as will be shown below, other evidence from both ancient and modern DNA has recently overturned all support for the Complete Replacement Model.

Some additional considerations: The Multiregional Evolution Model assumes gradualism (that AMH developed through a long period of slow, graduate change), but the Complete Replacement Model assumes punctuated equilibrium (that AMH developed rapidly due to an isolating event in East Africa, but then spread out and competitively excluded all other species of Middle Pleistocene Humans).

The Partial Replacement Model (aka, the African-European sapiens Model)

What it is: There are actually a number of different models that can be called "Partial Replacement", but in general, these models suggest that the suite of traits that we consider representative of AMH first arose in a Middle Pleistocene Human population in East Africa around 200,000 years ago. This population grew and expanded, migrating out of East Africa into the rest of Africa and Eurasia. In some of these areas, particularly those closest to East Africa, their populations grew so fast, and their traits were so successful, that they completely outcompeted other populations of Middle Pleistocene Humans. In other areas, however, particularly those farthest from East Africa (such as northern Europe and far East Asia), other populations of Middle Pleistocene Humans were better adapted to the local environment than the newer arrivals. The AMH and these other Middle Pleistocene Human populations interbred, producing offspring that had some of the traits of both. Over time, the vast majority of older traits were replaced by the AMH traits, but some DNA from the other Middle Pleistocene Human populations remained in descendant populations, particularly those from Eurasia.

Who supports it: A number of scientists support this model. Perhaps the most effective advocate for it is Dr. John Hawks, an anthropologist at the University of Wisconsin, and a former student of Milford Wolpoff.

What evidence supports it: Fossil Evidence: The Partial Replacement Model would predict that the earliest AMH fossils are found in East Africa (which they are, see above), but that transitional or hybrid fossils would be found in Eurasia. There are some trasitional-looking fossils in Asia (discussed above, under the Multiregional Evolution Model), as well as in Europe. The fossil evidence for Middle Pleistocene Humans (both AMH and others) has been best studied in Europe. This is not because Europe has a more impressive fossil record than other continents, it's merely that European scientists have been studying their own back yards, so to speak, since the first Neandertal fossils were found in the mid-1800's. Unfortunately, Europe is probably an anomaly when it comes to the origins of AMH. There are no AMH in Europe until almost 40,000 years ago, and the Neandertals (the local population of Middle Pleistocene Humans) survived until 20,000 years ago in places. In other words, Europe was a bit of a backwater or cul-de-sac when it came to the evolution of modern humans. However, there is one site in Portugal, Abrigo do Lagar Velho, where a child's skeleton was found with both Neandertal and AMH characteristics. It is believed this child may be the result of interbreeding between the populations.

Modern Genetics: Above, you read about mtDNA and it's support for the Complete Replacement Model. The study of nuclear DNA from contemporary people, however, supports the Partial Replacement Model. Dr. Michael Hammer, from the University of Arizona, studies SNPs (Single Nucleotide Polymorphisms) in non-coding (intron) areas of the X and Y chromosomes. Not only does Hammer's work show divergence times that are much older than the mtDNA suggested, but there are also examples of SNPs on the sex chromosomes that are common in parts of East Asia and Europe, but that are absent from Africa. If, as the Complete Replacement Model suggested, all contemporary people are descendants of a small AMH population from East Africa, then there should be no segments of DNA that are found in Europeans and Asians that aren't also found in Africans (although the opposite could be true). The presence of snipets of DNA that are unique to East Asia and Europe suggests that these bits of genetic code were handed down from an earlier Middle Pleistocene ancestor. Based on the number of these SNPs that were unique to Eurasian populations, Hammer estimated that no more than 5% of the DNA of contemporary humans comes from Neandertals, Denisovans, or other Eurasian Middle Pleistocene Humans.

Ancient Genetics: In May of 2010, the journal Science published a full draft sequence of the Neandertal genome by Richard Green and colleagues, based on the extraction of DNA from three skeletons in Vindija Cave, Croatia. This genome showed that Neandertals contributed genes to European populations. Green and colleagues identified a number of "deep rooted" places in our DNA. These are places with a lot of genetic variation, suggesting that those sequences of DNA have had a lot of time to accumulate mutations. Some deep rooted areas are found in people all over the globe, or are only found in African populations. But some deep rooted areas are found only (or mostly) in non-African populations. Green and colleagues defined twelve non-African deep rooted areas (that is, twelve places where Eurasian populations had deep variation, but where people of African descent did not), and in ten cases, the Neandertals shared the deep-rooted version, not the more recent African one. This analysis showed that there are places where some non-African people have Neandertal alleles, therefore, some modern people do have Neandertal ancestors. However, most genes of non-African Middle Pleistocene Humans did not survive to modern times. For most people outside of African, only 1-4% of their genome comes from non-AMH Middle Pleistocene ancestors. This is similar to the amount of DNA that you would have from a great-great-great grandparent.

Living people have about 95% of their genes from that initial AMH population, 200,000 years ago in East Africa. But the presence of any non-African Middle Pleistocene Human DNA shows that Neandertals and other contemporary humans must have been all the same species, capable of producing fertile, viable offspring

Some additional considerations: The genetic information that has come to light in the last five years has