[MUSIC] In the past section we explored that we left Africa and how we did it. But how many times did we leave Africa? Was it one, or several? In the next section we explore this. While the genetic based idea of a single dispersal was influential for much of the past two decades, recently opinion has begun to swing back behind the idea of multiple successful dispersals from Africa. As I mentioned before, for many years there has been strong archaeological and indeed linguistic evidence for at least two successful migrations. Initially it was argued a first wave of anatomically modern humans migrated out of the Horn of Africa about 70,000 years ago. Termed the Southern Dispersal Theory it is believed these principally along the southern coast of continental Asia. Down through the islands of Southeast Asia and even into Australia by at least 40,000 years ago. Subsequently, and perhaps as much as 20,00 to 30,000 years later, a second migration from Africa occurred going north up through the Levant, the region today encompassed by Lebanon, Israel, Palestine, Jordan, parts of Iraq and Syria, before dispersing both west into Europe and east into Asia. Much of the support for this two wave model comes from the fact that there is clearly older archaeological evidence for an anatomically modern human presence in East Asia. Than Western Eurasia. Although obtaining concrete evidence is challenging, due to the fact that elevation of sea level is during the Holocene, approximately the last 10,000 years, would have erased much coastal evidence. Genetic evidence clearly supports this second migration wave. What has remained unclear is whether the first southern dispersal ever happened or if it did, whether these southeasterly migrating anatomically modern humans left a genetic legacy to the present. In essence, do any of their descendants still survive? Anthropologists have long pointed towards the so called Australoid peoples that live in Asia and Australasia as evidence for a long term continuity of this dispersal. As they exhibit more phonological features that are more similar to some modern day African peoples than their closest geographically located Asian contemporaries. While the Aboriginal Australians and Papua New Guineans are perhaps the most famous, other populations include at least 12 distinct Andamanese peoples The Malaysian Semang, and at least 30 distinct Phillipino peoples. Despite their outward appearance, for many years, genetic studies were unable to provide any evidence that these peoples were more closely related to African, as opposed to other Asian peoples. One possible reason for this discrepancy was that the success of such genetic analyses required the assumption that there has been no genetic mixing, we term admixture, between the Australoid and other populations that subsequently migrated into the same regions. In other words, no blending of their gene pools through interpopulation reproduction. Given both the proximity of the populations and human nature in general, these assumptions have always been problematic. But until recently there were simply no genetic methods powerful enough to take this into account. However, with very recent technological developments that enable rapid and relatively economical sequencing on analysis of complete genome sequences, this has changed. And indeed as a result our ideas have begun to change. For example, a recent analysis of Aboriginal Australian genome sequences finally provides evidence that supports the Elusive Sudden Dispersal hypothesis. Although also demonstrating that much admixture has undergone since to a level nearly masks the original signal. Intriguingly the admixture does not stop there. To the surprise of many, such studies have also shown something else that has long been debated. Not only were the first and second migration waves of anatomically modern humans mixing with each other, but the earliest anatomically modern human populations in Eurasia also underwent some degree of reproduction with other Closely related hominids, including our cousins the Neanderthals and recently discovered Denisovans. Specifically, thanks to the recent sequencing of the genomes of our extinct cousins, comparisons of their genomes with those of modern day humans indicate that all human populations from outside Africa contain small amounts of Neanderthal DNA. That can only have arisen due to a small amount of inter-breeding, as the two sister species met. Note this surprisingly can fare some degree of truth to the controversial multi regional hypothesis by demonstrating that at least a small part of our modern genomes do not originate directly from African homo erectus. So what was the timeline for these migrations? With the coupling of archaeological and genetic data, an analysis of such results in light of techniques such as carbon dating, that enable dating archaeological evidence. As well as the molecular clock enabling dating of genetic divergences. A timeline can be reconstructed for the migration of anatomically modern humans across the non-African continents. Currently evidence suggests that our ancestors were in Southeast Asia by 65 to 67,000 years ago. Europe and Australia by 40 to 50,000 years ago, and East Asia including Korea and Japan by 30,000 years ago. This left one large land mass uncolonized, the Americas. Although the Pacific Islands of Polynesia also remain uncolonized until recently, starting approximately three and a half thousand years ago and not finishing in some cases until extremely recently. This is unsurprising given the logistical challenges of crossing large ocean distances. The Americas, however, is a different matter. Genetic and archaeological evidence clearly indicates that the Native Americans originated from Siberian peoples, in contrast to other possibilities such as Pacific Islanders. While the Americas are separated today from Eurasia by the Bering Straits, the lower sea levels of the past meant that for much of the last 50,000 years, Asia and North American were directly connected by the so-called Bering Landbridge, a large landmass across which large mammals are known to have regularly moved. Thus with documented human presence in northeastern Siberia at sites such as the famous Yana woolly rhino hunting site by 32,000 years ago, the possibility existed that humans could have entered the the Americas at least by then. Although still a matter of some debate, stronger and stronger evidence is appearing that despite this window, humans only successfully entered the Americas in the last 15,000 year and Greenland via Canada, only four and a half thousand years ago. The reasons underlying this delay remain mysterious. Humans had rapidly expanded throughout the rest of Eurasia and the plethora of large mammal remains found in North America shows there was plenty to hunt. One possibility is that the large ice sheets that existed across most of modern day Canada blocked the route. Another is that humans actually entered the Americas before this but left no archaeologically detectable traces. There is some evidence that supports this notion, not least the fact that the oldest verified archaeological evidence of humans in the Americas is from approximately 14,800 years ago from the Monte Verde two site in Southern Chile. Dating just a few hundred years older than the oldest human evidence in North America. It is difficult to explain why humans would have been so far to the south at this time if they had not already established a presence in North America. Ultimately, only new archaeological evidence will shed light on the matter. Although, only time will tell whether such evidence even exists. As with other key migrations, had the migration been mainly coastal, then the raised sea levels of the Holocene may have destroyed any evidence that was left behind. Well, that's it for now. This is an introduction to the Anatomically Modern Human's Origin and Spread. To recap, as a species, we probably originated in Africa, probably in the south of Africa. We expanded north from there sometime in the last several hundred thousand years ago. And then left Africa to spread over the rest of the world in one or maybe two migration waves. Today, we have been using genetics, linguistics and archaeology to look at this and reconstruct this. And as techniques improve through time, we can only imagine that these findings will be more and more concrete through time. Thank you for your attention. [MUSIC]
