So, in the previous part, we explored that we evolved in Africa. But that leads to the question, how did we leave Africa? In the next section, we will explore this. Today, the African continent is directly connected to Eurasia via the Isthmus of Suez. However, in the past, sea levels changed in response to global ice cover. In short, the more ice on the world, the lower the sea level. Thus, during cooler times when much ice would have been present, over in particular, the northern continents, other possible migration routes would have been available. One discussed example that seems likely would have been over the Bab-El-Mandeb Straits in the Red Sea. While today these straits separate Yemen and Djibouti by a 20 kilometer expanse of sea, 50,000 years ago the sea level was probably 70 meters lower, thus considerably reducing the crossing challenge. An additional complication is that regardless of the route taken, leaving Africa does not necessarily mean successfully surviving long term after leaving. While global travel is easy today, our ancestors would have faced harsh new environments and indeed, there is fossil evidence that there were a number of failed attempts. For example, it has been argued that anatomically modern human fossils excavated in Qafzeh caves in Israel dating to approximately 75 to 100,000 years ago may represent such an attempt. Ultimately, however, our ancestors were of course successful or we would not be living in non-African regions today. While this must have happened on at least one occasion, there is a growing amount of evidence for at least two independent migrations that resulted in the long-term survival of geographically widespread, anatomically modern human populations. Three main sources of information have contributed to our understanding of the migration - archaeology, linguistics and genetics. In this regard, the role of archaeology is obvious to most. The finding, documentation and crucially dating of human remains and clearly human-made artifacts from outside of Africa have helped outline the timeline and routes that humans took. The role of linguistics and genetics is less obvious at first, but both rely on the assumption that there is a traceable continuity in both language and genetics between different modern human populations, with degree of similarity at the linguistic or genetic level indicating how closely related populations are and even whether some populations derive from others. There are two principal reasons why genetics offers a powerful tool with which to reconstruct population histories. The first is the power of phylogenetic analyses. As organisms reproduce and give rise to descendants, their DNA sequences accumulate small differences due to mutation. All individuals will differ to some degree due to such mutations, but for obvious reasons the more closely related two individuals are, the more closer their genetic sequences will appear. Given this, DNA sequence recovered from any number of individuals, whether within a species or between many different species, can be directly compared using phylogenetic methods and a phylogenetic tree can be reconstructed that shows how the individuals are related. When applied to samples taken from human populations, it is possible to cluster the populations based on similarity. The second reason is that of the so-called molecular clock. The concept underlying the molecular clock is simple. If the rate of DNA mutation can be estimated, then by counting the number of mutational differences between any two individuals, the time taken for mutations to occur can be estimated. This in turn can be used to estimate the time at which the two individuals had a shared ancestor. In the context of population analyses, if this ancestor was just before the time at which the individuals that compromise the two related populations diverged, then that is the estimated divergence time. In practical terms, both phylogenetic and molecular clock analyses are complicated and must be based on both assumptions about the way DNA mutates and factor in a number of sources of error. However, despite this, they often yield results that are remarkably consistent with observations based upon other data sources, for example, such as archaeological evidence. Of course, it is worth mentioning that while the results often agree, this is not always the case. In many cases, the results of the three branches, archaeology, linguistics and genetics do not match. For example, while interpretations from classical archaeology initially provided strong support for a multiple dispersal hypothesis, the conclusions of early genetic studies based on limited amounts of genomic information, in particular the male specific Y chromosome and the female inherited mitochondrial DNA was strongly argued to support a single expansion from Africa. Specifically, authors of these studies noted that while there is a great deal of genetic diversity within Africa, all non-African anatomically modern human populations that are not derived from recently documented migrations are closely related to just a single modern African population from North East Africa. Furthermore, statistical analyses on the genetic data demonstrate that the observation is consistent with a single small migration across the Red Sea of perhaps as few as 150 people occurring within the last 50 to 70,000 years.
