Huge data set.

The debate about the origin of modern humans has been lively. Genetic studies from around the world have consistently found evidence of a single African ancestor. However, analyzing skull dimensions provided mixed results, with some studies arguing for a single origin and others supporting multiple origins of modern humans. So which one is it?

The debate can finally be put to rest. A new paper looked at a large database of skull measurements from around the world and also looked at large genetic databases of samples collected from around the world. The results are very clear; quoting the abstract:

We find evidence for an African origin, placed somewhere in the central/southern part of the continent, which harbours the highest intra-population diversity in phenotypic measurements. We failed to find evidence for a second origin, and we confirm these results on a large genetic data set.

The technical result is this: genetic diversity is mirrored by phenotypic variability. Let's try to explain this in simpler terms. Genetic diversity of a species is a measure of how similar or different the genetic makeup of its individuals are. An example of a species that has high genetic diversity is any wild animal population that's not about to go extinct, like lions or monkeys, or zebras. The diversity is good because as the individual mate, the offspring will have random combinations of genes - the more diverse the genes are, the more random the combinations. This genetic randomness is very important, and is the subject of another post.

If a population loses members, say through an earthquake or over-hunting by humans or a disease, then the genetic variation decreases significantly. This is because you lose potential combinations of genes. The same effects can happen in artificial in-breeding. The net effect is that the genetic makeup of individuals in a population will start becoming more and more similar, a fact we can trace using advanced genetic analysis. This is one side of the paper.

The other side of the paper is phenotypic variation. A phenotype is the observed effect of a genetic makeup. An example: being tall (i.e., a phenotype of height) is the effect of having a certain set of genes (the genetic makeup, also called the genotype). You can guess that what we observe in the phenotype side should have a basis in the genetic side.

This is where the today's paper comes in. All genetic studies said one thing, namely that modern humans started in Africa and spread out to the rest of world. However, phenotype studies gave mixed results, suggesting multiple human origins. Today's paper fixes the phenotype observations: by looking at a huge data set, the researchers found that the phenotype changes (the skull measurements) mirrored the genetic changes. This nicely fixes the errors in the previous data.

It gets better: The farther away the samples were from the original starting point, the less genetically diverse and phenotypically variable they were. Reversing the argument, the samples with the highest genetic diversity and the highest phenotypic variation are the earliest (most historic) samples. Nicely, these diverse samples all came from somewhere in the central/southern part of Africa. As researcher Dr Francois Balloux explains, "To test the alternative theory for the origin of modern humans we tried to find an additional, non-African origin. We found this just did not work. Our findings show that humans originated in a single area in Sub-Saharan Africa."

And that's how we know that modern humanity started in Africa. I know this post is a bit more technical than what I usually blog about, but it had to be done. Any questions please ask in the comments!

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