Science Takes Another Step Toward Understanding Human Evolution

In a previous post I highlighted one of the great questions facing science today: how did we evolve and what specific genes make us different from our cousins in the animal kingdom?

In a new study reported in this month’s issue of PLoS Genetics, Carolin Kosiol and colleagues have demonstrated the most complete analysis of the human, chimpanzee, macaque, mouse, rat, and dog genomes to date, highlighting many genes and pathways that have contributed to our own evolution as mammals and primates.

Evolution fundamentally occurs at the gene level. If a gene becomes mutated, thus making an organism (or population) more likely to pass on that gene, that gene can be said to have undergone “positive selection.” The environment has positively selected that gene to become more prevalent.

Just to give you a very quick primer on gene evolution, one thing necessary to understand is that all mammals (and indeed all vertebrates) contain a large number of genes that we share in common. For instance Tbx20, a gene involved in heart development (which I used to study), exists in all organisms from flies to humans. The function of this gene is the same or similar in these organisms, though there are many specific differences between them as well.

It is these genes that we share with the other organisms that these researchers compared. What the authors of this study have done is to look at the differences in the sequences of these mammalian genes to determine which sets of genes have changed the most – i.e. which genes have undergone positive selection during evolution. They highlighted several pathways that have undergone the “strongest” positive selection, such as defense/immunity, chemosensory perception, reproduction and taste perception.

Surprisingly, to me, they did not find pathways and processes in the brain that have a high number of positively selected genes. It seems to me that this can be explained by a few different possibilities: 1) only a few specific genes have evolved strongly, but these few genes resulted in huge changes in the brain, 2) new genes have arisen (which were not looked at in this study – again, only genes that we share were compared), or 3) the brain genes that changed weren’t exclusively part of “brain processes” (for example, the gene I mentioned above, Tbx20, is involved in both heart and brain development).

Regardless, this is a very interesting study, and it brings us one small step closer to understanding what exactly makes us who we are as humans, as primates, and as mammals. And it opens us to new questions of how these specific genetic changes evolved in the first place.