10 things about human evolution (genetics) you should know

In 2011 I had dinner with a friend of mine from college. He’s a smart guy. Ph.D. in chemistry form M.I.T and all that. I mentioned offhand how it was rather proven to good degree of certainty there was Neanderthal gene flow into modern humans (our lineage)[1]​. He was somewhat surprised by this information, and I was aghast that he didn’t know. 1It was one of the biggest science stories of the year. Right?

What that brought home to me is that something that seems revolutionary near to your heart or field of occupational interest may not be so visible to those who are not similarly situated. My friend is a well educated person with a science Ph.D., but it was just not on his radar. Similarly, I had very smart friends in college who were evangelical Christians who were surprised by the high degree of identity between the chimpanzee and human in regards to our DNA sequence (they were Creationists, and skeptical of any close kinship).

Here’s a final example that might interest readers. I had a long conversation with a relatively prominent journalist a month or so ago. Someone who writes in biology, and in particular genetics (who is not Carl Zimmer). I mentioned offhand to them that the work of various labs utilizing ancient DNA is showing rather conclusively that the vast majority of human populations are relatively recent admixtures between highly diverged lineages. To put it in plainer language: we are all mestizos! This journalist was totally surprised by this fact.

This indicates to me again that facts which are “known” in the “in-group” may be surprising to those who are not as hooked in. It isn’t a matter of being educated, smart, or interested. It’s a matter of narrowly constrained social channels.

Here are 10 facts that we’ve recently discovered about human evolution, with a focus on genetics since that’s what I know best, which you should probably know. Or might find interesting.

1) The expansion/development of modern humans occurred within Africa for tens of thousands of before their expansion “Out of Africa.” Most of the ancestors of non-African humanity seem to have started expanding rapidly from a small founder group of 100-1,000 50-75 thousand years years ago. African humanity has a different and more complex historical pattern, with lineages which began diverging as early as 200,000 years before the present, and then mixing back with each other.

2) Related to #1, we’re one species, so rather than an expanding “tree” from common ancestors, it’s better to think of a mesh which keeps coming back together, as some branches are pruned, and the whole pulses out periodically. All major human populations seem to be the product of relatively recent fusions between diverged branches. Africa was the source of modern humanity, but clearly there has been “back migration” from Eurasia.

3) Many of the phenotypes we define as characteristic of various human populations are relatively recent. E.g., the depigmented look of Northern Europeans, or the thick straight hair of East Asians.

4) The Denisovan version of EPAS1 which is found in Tibetans illustrates a general trend: we have adaptations from other very diverged human lineages through low levels of gene flow[2]​.

5) The transition to agriculture and complex “civilization” seems correlated to pulses of highly fecund male paternal lineages. Many of the common Y chromosomes today exhibit a pattern of diversification indicative of explosive population growth.

6) It seems unlike there is one singular genetic change which makes us sui generis or distinctive in relation to our hominin cousins. This is less certain than 1-5, but I’m pretty sure that this is so. Researchers have been looking for years, and not finding anything definitive, and I think there’s a reason. There isn’t anything definitive. Many genetic changes come together to make our lineage distinctive.

7) A lot of adaptation occurs through reemergence of old variation which is floating around in the human population. For example, the lightening of skin across parts of Eurasia co-opt common mammalian pigmentation pathways.

8) Cultural flexibility does not negate biological evolution. On the contrary, strong shifts in cultural norms seem to drive biology. Lactase persistence is a clear case, but even something like malaria adaptation is ultimately due to anthropogenic environmental changes.

9) We are all equally descended from common ancestors. There are no “most ancient” human lineages. We’re all equally recent by definition.

10) There are evolutionary genetic events in our history which are hinted at in the most recent data, so there are major lacunae in our knowledge. The picture is well formed, but not complete. E.g., there is evidence of pulses out of Africa ~100,000 years ago into Eurasia in both genetic and fossil data. These lineages may have gone extinct, or, their contribution may be difficult to detect with current data sets. But there is clearly more to be told in this story.