Origins of the humans. Part 2

This idea of difference, “How is this structured in geography?”. What we try to explain here is why genes explain history, and why we may trace back the demographic history of humans, as species, or for specific populations through the genetic differences between populations. The idea is that the mechanisms that produce the genetic differentiation are mechanisms that are at the base of demographic factors. Mainly a small population size, isolation of the populations that will produce a differentiation drift, mainly when the population is small. And this is what happened in ancient populations, in prehistorical populations. Always had very small populations size, small mobility, meaning that migration was not very important for large distances, and thus we have the base for genetic differentiation.

So, the base of differentiation, maybe seen very easily, let’s look at a single SNP in humans, and here you have one example. Doesn’t matter which. This is a snip in which you can see that Europeans are very different from Africans, and populations in East Asia and so, and so. Here is just one SNP. But, what we are able nowadays is to look at many different SNPs, and to do an overall analysis. The first idea on that came by Luca Cavalli-Sforza, one of the founders of human population genetics, in which, when he wrote his special, his librone book on the history and geography of humans genes.

What he did, is to put together the information existing at the time, and to try to make an interpretation of the genetic variation in terms of population history. Here you have, for example, the maps of the variation of ABO. The top of the A group, or the 0 group, but, what we have to understand is that seen this variation, doesn’t tell us anything on the population by itself. This is just a single locus. What we have to do is to analyze lots of places in the genome, lots of samples, and to do proper statistical methods.

So, the strategy is, samples from all over the world, and now, including ancient DNA samples; the technology, either using genotyping arrays or sequence data; and the statistical methods, to have an overall view. The technology has been moving very fast from SNP arrays, in which you can analyze, let’s say, one million snips, just with a single experiment for each individual. Or the sequences, in which nowadays you can do by a very low price the whole genome sequence of an individual, and then to put this information in some form that we can see the overall variation, the overall distribution, the overall amount of differentiation among the human populations.

Here we have one of these examples, this is what we call Principal Component Analysis, in which, for, in this case, 240.000 SNPs, these are the two dimensions that explains the most part of the variation. So, the two independent variables that can explain more.

And here you have the distribution: bottom left, the African populations; top, Europeans; right, the Asians, and here you can see the distribution of all worldwide populations. Then, we have to understand, first, that the position of populations here in this plot, tell us the overall genetic differentiations among the individuals and populations. Because here, every dot is a single individual, and you can see very well how individual cluster in the populations they belong. This means that there is a clear geographic stratification of the genome variation. So this geographic stratification is what we have to understand in terms of population history.