Posted Thu, Feb, 09,2017
In 1831 naturist Charles Darwin embarked on his famous and memorable voyage in the Beagle, an expedition that lasted almost 5 years. Equipped with hardly more than a telescope and insatiable curiosity to understand nature, Darwin was able to make, for the first time, one of the most important observations regarding human evolution pinpointing the origin of modern humans in Africa and developing his namesake “Darwinian” school of thought that forever transformed the scientific community with the Theory of Evolution. His claim, published in The Descent of Man (1871), was based on the presence of gorillas and chimpanzees on African soil, and on Huxley's (1863) comparative anatomy studies that showed that humans and apes shared a common ancestor.
Today, the origin of the first humans in Africa is unquestionable. Darwin was proven right. Palaeontological and recent genetic studies have largely supported the Out-of-Africa (OoA) model, according to which, the evolution from archaic hominins to anatomically modern humans occurred in Africa, followed by a later migration and conquest of the rest of the world. Unravelling the origins and migratory movements of the first humans is key to understanding the evolutionary processes that have led to the genetic diversity that we observe in present-day populations. During the past few years, archaeologists and geneticists are making huge efforts to describe the OoA dispersal in a collaborative manner. In particular, with the advent of new technologies that allow the production of large amounts of genetic data, coupled with novel and powerful statistical and bioinformatic methods, it is now possible to make inferences about past events. Although we are getting closer to understanding how this happened, some of the key questions surrounding this event are still subject to debate.
One of the most vivid debates regarding the OoA event concerns the precise timings of the exit of the first humans out of Africa. There are several trends of thinking, based on different evidences and analyses. The eruption of Mount Toba 74,000 years ago has been of extreme importance in the context of dating the OoA event, as stone tools thought to belong to anatomically modern humans were found buried in the volcanic ash. Additionally, the recent discovery of the Daoxian human remains in Southern China, dated to 80,000–120,000 years ago also supported an early dispersal. However, it remains unclear if these individuals were actually the ancestors of modern populations or if, on the contrary, they represent a failed exodus in human evolutionary history. So far, genetic studies have been unable to give precise estimates for the first dispersal, timings ranging from between 50 and 100 thousand years ago.
The route that our ancestors took in their way out of Africa is also subject to debate. A northern route, through Egypt and Sinai, has been supported by a few genetic studies and the archaeological record with the finding of the earliest remains of anatomically modern humans outside Africa in the Levant (the Skhul and Qafzeh fossils, dated to 100 and 90 thousand years ago, respectively). Other genetic studies, however, have favoured a southern route, through Ethiopia, the Bab el Mandeb strait and the Arabian Peninsula, suggesting that the Skhul and Qafzeh fossils are the remnants of another failed exodus that did not survive. Complicating things even further is whether or not there was a single migrating group that populated the rest of the world (blue line in Figure 1, with a potential northern or southern exit indicated in discontinuous lines) or whether there were multiple waves of dispersal (red lines in Figure 1). So far, neither archaeological nor genetic evidences have been able to answer these questions with utter confidence.
Chances are that the answers to these questions exploring human evolution will be brought to us by the analysis of ancient DNA (aDNA) samples, as they allow direct observations of the past. Obtaining samples from hot and humid regions, like many parts of Africa, has been challenging, as these conditions hinder the preservation of DNA. However, the development of new techniques in aDNA recovery is now allowing researchers to extract and analyze DNA from these places. In conjunction with other present-day and ancient samples from key regions and different time periods, we will likely be able to describe with unprecedented detail and accuracy the paths that our ancestors took, concluding research that Darwin started more than 150 years ago.
Figure 1: Putative migration waves out of Africa and location of some of the most relevant ancient human remains and archaeological sites; Kya=thousands of years ago (modified from López et al., 2016)
López S, van Dorp L, Hellenthal G (2016) Human Dispersal Out of Africa: A Lasting Debate. Evolutionary Bioinformatics, 11(S2): 57-68.
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