Out of Eden: The Spread of Homo sapiens
Behavioral/Cultural Traits Shared by All Living Humans
The Biblical story of Eden talks of how the first two humans Adam and Eve lived in the Garden of Eden without having to work for a living, but after the Fall lived in a world with suffering (including having to labor to feed themselves). From this couple all populations of the world descended. In fact, Genesis 10 explains how the children of Noah were the founders of the various populations around the known world of Biblical times (i.e., the Near and Middle East, and the parts of Europe, Central Asia, and Africa with which these peoples had contact in the Bronze Age.) As it turns out, however, humans were even more widespread than Genesis implied: from the Arctic to the Tropics, from rainforests to deserts. Yet everywhere, these people represent the same species.
All peoples of the world share certain behavioral and cultural traits in common, which were thus almost certainly present in the common ancestral population of crown-group humans. These are beyond Linnaeus's description of our most important trait being our intelligence (i.e., Homo sapiens, "thinking humans"), and I have tongue-in-cheek described alternative names that are probably equally good for our species:
How do we trace the spread of humans over the world? There are three primary lines of evidence:
This idea flies in the face of our understanding of how speciation works. If true, than "H. sapiens" would be polyphyletic! Furthermore, it suggests that the majority of the genetic heritage at each of these regions was established locally nearly 2 million years ago (although admittedly with some intermixing ever since.) For this reason, the multiregional hypothesis is also called the regional continuity hypothesis.
In contrast, the Recent Out-of-Africa hypothesis considers all living humans as descending from a relatively recent (perhaps only 50-60 kyr old) ancestral population from Africa, spreading out around the world and displacing the previous local inhabitants. (Again, there is no reason to assume that there wasn't limited intermixing.) This is much more in line with the fossil, archaeological, genetic, and other data, and with basic evolutionary biology.
For example, it has been observed that the genetic distance between members of the same troop of chimpanzees in Africa is 4-6 times the genetic distance among any given members of the 7.4 billion humans! So for all our apparent diversity, all humans are extremely closely related with a very recent common ancestral population. Some analyses suggest that we are the descendants of a genetic "bottleneck" where the effective population size of humans some 70 ka or so was a mere 2000 or so individuals, and that this small population size existed for hundreds or thousands of years.
Mitochondrial Eve, Y-Chromosome Adam, and the Genetic Phylogeny of Modern People
Geneticists have examined the genomes of people from all over the world to map out various aspects of our genetic diversity and our genetic interrelatedness. One of the important early discoveries was by looking at the mitochondria. Mitochondria are passed on only from the mother, so they can be used as markers to look down the maternal line. It turns out that the most recent common mother of all living humans seems to have been a woman who lived about 200 ka (152-234 ka), probably in Africa and long after the split between the H. heidelbergensis-neanderthalensis and H. rhodesiensis-sapiens lines. NOTE: although named "mitochondrial Eve" this would NOT have been the very first H. sapiens nor the only woman around at the time! It is simply the last mother through whom all mothers can be traced. She would have been a member of a larger population, some of whom only gave birth to sons (and the same is true of many other mothers at points in between.)
A similar discovery using the Y-Chromosome (which is passed down only from father to son) helps us find the last patrilineal ancestor of all men. Again, this is NOT the oldest H. sapiens nor the only man in his environment. The original analyses placed the so-called Y-Chromosome Adam at around 142-60 ka (and thus thousands of years younger than mitochondrial Eve!), and still most likely Africa. A report from early 2013 suggests that an individual (whose ancestry is from the Mbo people of Cameroon) pulls Y-Chromosome Adam back to 338 ka (long before the oldest evidence of H. sapiens or H. rhodesiensis!! Future work will hopefully sort this out.
Furthermore, geneticists have been able to map out phylogenies of peoples from around the world. Some of the main discoveries:
This genetic phylogeny strongly conforms to the Recent Out-of-Africa model. But examination of fossil genomes added some interesting additions.
Neanderthals and Denisovan "Mystery Men"
Neanderthal genetic material has been recovered from numerous fossil throughout their history. Some interesting discoveries have been found:
A very recent discovery, based on 2016 analyses of the Neanderthal genome, is that there were significant differences between the Y-chromosome of Neanderthals and modern humans. This could explain why the Y-chromosome of modern humans lacks Neanderthal genes, and would mean that nearly all successful Neanderthal-modern hybrids would be between male moderns and female Neanderthals: crosses the other direction might have been fairly inviable.
Perhaps more unusual was the discovery at the Denisova site in southcentral Siberia, between Kazakhstan and Mongolia. Here a handful of fossils (a toe bone, a finger bone, some teeth, lots of fragments) have been found, dated to mostly around 41 ka (but some as old or older than 51 ka and as young as 28.9 ka.) Essentially nothing is known of their anatomy, but their genetic material is preserved and discovered to be closer to H. neanderthalensis than to us. (In fact, they might arguably be an eastern variety of Neanderthal.)
Curiously, Denisovan DNA markers show up in modern humans, too, but ONLY in peoples of Melanesia, Polynesia, and Australasia. So there must have been some interbreeding between these "mystery people" and the ancestors of the southeastern branch of modern humans.
An April 2014 study of the genomes and epigenomes of modern humans, Neanderthals, and Denisovans reveals that we differ from the other species in a few traits associated with the digestive system, but especially with genes activated in higher brain function. (In fact, some of these genes, in a malfunctioning form, contribute to Alzheimer's or to schizophrenia!) These may be the genes associated with the "Great Leap Forward".
And a mystery within a mystery: there is evidence that Denisovans interbred with a population that Neanderthals and moderns never directly interbred with, and that this other group (presumably in eastern Asia) diverged long before the modern vs. Neanderthal-Denisovans split. Might this mystery population be the Maludong or Dali peoples?
But what about the fossil evidence of the spread of humans?
Here is a traditional map for the timing of the spread of Homo sapiens. So in general, we see:
But let's take at the finer scale details:
Sahul: Sahul is a name given to the landmass comprised of New Guinea, Australia, and Tasmania (but NOT New Zealand: thus, it is a subset of "Australasia"). During glacial maxima, there are direct land connections between all three landmasses, although the Sahul is still isolated from the Malay Peninsula (the fully-exposed region of Indonesia). The oldest well-dated human remains in the Sahul are 46 ka (previously considered 41-40 ka) specimens from Lake Mungo. There are other fossils that have been dated as old as 68 ka, but the uncertainty of these measurements are very high. However, arrival of humans in the 60-45 ka range seems most likely. A newly discovered chip of an axe at around 49-45 ka is consistent with the Lake Mungo body fossil date.
East Asia: Northern China by 40-36 ka, but some teeth from South China from over 42 ka and probably as old as 80 ka.
Boreal Siberia: 45 ka.
Alaska: possibly as old as 28 ka, but definitely by 14 ka. Note that during the Last Glacial Maximum the conjoined ice sheets from the Rockies (the Cordillera ice sheet) and eastern Canada (the Laurentide ice sheet) blocked Alaska from the rest of the America. In contrast, though, the low sea level meant that there was a continuous land bridge between Siberia and Alaska: the land of Beringia.
Americas South of the Ice: Here is place of great debate. The traditional model was that the oldest presence of humans south of the ice was around 13.5-13 ka, when an "Ice-Free Corridor" opened up between the Cordilleran and Laurentide ice sheets through which the Paleo-Indians (as the ancestors of the Native Americans have been called) arrived (somewhere near present day Calgary, Alberta). Furthermore, the traditional model considered that these people quickly developed the Clovis tool kit with it's distinctive spearpoint for heavy spears developed to take down very large prey. This Clovis toolkit was widespread in central North America for a flash-in-the-pan: 13.2 to 12.7 ka. Afterwards, it was replaced by the Folsom toolkit (once thought to be as young as 10.5-8 ka, but brand new relating shows lasted merely from 12.61 to 12.17 ka) with thinner spear points for javelins and darts, thrown with atlatls. These were adapted for smaller (bison or smaller) prey. When Folsom disappears, it is replaced by many different diverse cultures.
But over the last few decades a number of sites have been discovered which put doubt on traditional "Clovis first". Among these are the Manis site in Washington State, well-dated to 13.8 ka (and thus pre-Clovis), where a a hunting point was found in the vertebra of a specimen of Mammut, and the newly-dated Page-Ladson site dating from 14.55 ka in the Aucilla River, Florida, which also has butchered mastodons. Other sites with good dates in the 14.8-14.1 ka range are found in the Americas; moderate levels of support for dates in 15-16.8 ka; and possible dates as old as 22 ka. (Some problematic dates older than 30 ka, but there are concerns that the material being dated is non-human [for example, forest fires rather than charcoal pits].)
But if the ice blocked the way, how would these pre-Clovis people arrive? They could have arrived prior to the coalescence of the Laurentide and Cordilleran ice sheets (which might have been as late as 24 ka). Recall that we are "Homo maritimus": it may be that coastal fishers rather than walking hunters were the First Americans. In fact, if the latter, some of the First Americans might not have been from Asia along the Pacific Northwest, but from Europe to Greenland to eastern North America. (In fact, there is a European toolkit called the Solutrean from 22-17 ka which resembles the Clovis, although this could very well be convergences for hunting similar animals in a similar style.) Furthermore, the genomes of a human skeleton associated with Clovis tools shows the characteristics of Native Americans (and more broadly Asians) rather than Europeans.
A new model is that coastal fishers moved down the Pacific seacoast south of Beringia beginning 17 ka or so, and reaching as far south as Monte Verde, Chile by 14.6 ka. These populations began to move inland and encountering big game, eventually developing Clovis tools by 13.2 ka. There might have been a second wave of immigration at or just before this time as the Ice-Free Corridor opened up.
So, regardless of when they arrived, what happened as humans first arrived in the Brave New Worlds of Sahul and the Americas? We'll see that in the next lecture.
The following is a May 2015 animation of the spread of Homo sapiens:
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