Hello. I'm Tom Gilbert and I will be introducing you today to how our species spread from Africa and around the world. As with many other aspects of the Origins course, the key challenge that researchers have faced while studying this question, is the fact that it happened so long ago. Thus what is understood about the process today has been pieced together by utilizing a range of often extremely fragmentary anthropological and archaeological remains, combined with discoveries based around genetic analyses. Thus, the topic represents a synthesis of disciplines that were once very distant, and demonstrates the power of introducing evidence obtained in very different ways. So who are we, as a species? The fossil record demonstrates that approximately 200,000 years ago, a hominin species appeared in the African fossil record that had exhibited certain particularly advanced traits. Scientifically termed Homo sapiens, this species was clearly distinct from older archaic hominin forms such as Homo erectus. The identity of this species will be familiar to most people - it is, of course, the modern human, often referred in palaeoanthropological circles as the "Anatomically Modern Humans", often abbreviated to AMH. One of our ancestors' most striking features is that they differed from other late hominins not only physically, but likely also by significant cognitive traits that had not been present in prior hominins. For example, upon close study, the Anatomically Modern Human archaeological record demonstrates not only the use of tools, but the continual development of the tools throughout their use, in particular giving rise to more specialised forms as well as geographically localised variants. Furthermore, unlike previous hominins, much evidence suggests that even the earliest Anatomically Modern Humans exhibited sophisticated behaviours that included burial of the dead, the making of clothes, the development of sophisticated hunting techniques such as pit traps and fish hooks, and even the painting of intricate cave paintings. Due to this, some anthropologists have hypothesised that as a species, we differ from all prior known hominins due to four key abilities: to think abstractly, to plan, to innovate and to undertake symbolic behavior. And furthermore, it was possible such abilities that not only allowed our ancestors to outcompete and replace other contemporary hominins such as the Neanderthals (often known as Homo neanderthalensis), but ultimately successfully leave Africa and colonise much of the remainder of the Earth. So - where did this happen, when, and how? Well, today, almost all palaeoanthropologists agree that we evolved in Africa, before spreading out across the inhabitable regions of the planet as fully developed Anatomically Modern Humans, probably sometime within the last 250,000 years. Known as the "Out-of Africa" hypothesis, although widely accepted today, this has not always been the case. Although it is clear that hominids in general evolved in Africa, the recurrent spread of different hominid species out of Africa - for example Neanderthals or Homo erectus - implies that it need not necessarily be true that we, the Anatomically Modern Humans, evolved in Africa. In general there have been two competing schools of thought on the topic, based around the so-called "Out-of-Africa" and "Multiregional" hypotheses of Anatomically Modern Human evolution. Both agree that hominids originated in Africa, but differ as to how, and when, the Anatomically Modern Humans subsequently spread around the planet. Championed by researchers such as Professor Milford Wolpoff, the "Multiregional" hypothesis argues that human evolution, starting from the beginning of the Pleistocene (about 2.5 million years ago), and continuing to the present, has been within a single, continuous, globally distributed human species. Given this, Anatomically Modern Humans would have evolved globally, from pre-existing archaic hominids, with, for example, modern Europeans evolving from the Neanderthals that were already in Europe, modern Asians from the Homo erectus populations in Asia, modern Africans from the Homo erectus populations in Africa, and so on. The contrasting "Out-of-Africa" group of hypotheses argues that modern Anatomically Modern Humans only derive from an African ancestor, ca. 200,000 years ago, and subsequently these Anatomically Modern Humans spread out from Africa and replaced the existing archaic Eurasian hominids. The "Out-of-Africa" model best describes our evolution, but why? Although physical anthropological analysis of archaic specimens provides some support, the key support for the model comes from genetic analysis and comparison of modern human populations, in particularly initially through the analysis of mitochondrial and Y chromosome datasets. Specifically, from the early 1990s onwards it was shown that human genetic diversity is much greater in African, as opposed to non-African, populations. As DNA mutations arise within populations as a result of reproduction, a rough approximation is that the older a population is, the more genetic diversity it holds. Thus, the genetic diversity observations are consistent with African populations being considerably older than non-African populations, and the different populations cannot have evolved independently, yet contemporarily, with each other. In addition, other analyses have shown that most non-African genetic diversity nests within specific African populations, in particular those from the Horn of Africa (which is modern day Ethiopia and Eritrea for example). Neither of these observations are compatible with multiregionality. In summary therefore, the consensus today is that we, the Anatomically Modern Humans, originated in Africa. More specifically, although relevant paleoanthropological evidence is rare, genetic studies have been able to demonstrate that the ancestral populations to all living Anatomically Modern Humans probably originated in the south of the African continent, well over 100,000 years ago. From there our ancestors spread to fill the continent, and at some point in the last 100,000 years left to populate the non-African continents. Before exploring this topic in detail, it's perhaps worth reflecting that migration of hominids out of Africa itself appears not to have been unusual. We know with certainty that many now extinct other hominin species undertook such migrations - there is really no other plausible way to explain the presence of hominin fossils belonging to our relatives such as Homo heidelbergenisis, Homo neanderthalensis and Homo erectus in Europe and Asia. However, questions such as how many times our ancestors left, when these happened, what route they took from Africa, and finally how they spread out across Europe and Asia remain debated.
