In response to my earlier post some people suggested that the language was impenetrable. Nathan Taylor offered to make it more plain spoken, so here is this go at it. I think it’s pretty good.
(text below is from Nathan)
1) Modern humans stayed in Africa for tens of thousands years of before expanding beyond it. Most of the ancestors of non-African humanity seem to have started expanding rapidly from a small founder group of less than 1,000 people, starting around 50-75 thousand years years ago. African humanity has a more complex pattern. Some groups diverged as early as 200,000 years ago, then mixed back together.
2) Don’t think of humanity as a branching tree. Rather think of humanity as groups of streams. Some streams end, many fold back on one another, and some suddenly have massive expansions. Surprisingly, all major human population groups we know today seem to be the product of relatively recent fusions. Even Africa, the source of modern humanity, has seen streams flow back from Eurasia.
3) Many of the characteristics people focus on today are of recent origin. At least as measured in thousands or tens of thousands of years. 8,000 years ago parts of Europe were populated by brown skinned hunter gatherers with blue eyes. Whiter skin is a (relatively) recent development. And the thick straight hair now common among East Asians is recent as well.
4) The genetic variant which helps Tibetans tolerate very high altitudes comes from a human lineage as divergent from modern humans as Neanderthals are. The Denisovans. This illustrates a general trend: we have adaptations from other very diverged human lineages in our genes today. Even if the genetic percentage is small.
5) The transition to agriculture and complex civilization seems correlated with the explosive growth of a few select male Y chromosomes. Think Genghis Khan.
6) It seems unlikely there is one genetic change which made humans humans. This is less certain than 1-5, but I’m pretty sure it’s true. Researchers have been looking for this for years and haven’t turned up anything definitive. I think the reason is simple enough: many genetic changes came together to make us distinctive.
7) A lot of variation between human groups is not due to novel genes. Rather it comes from increasing the popularity of pre-existing genetic variants. For example, the lightening of skin across parts of Eurasia is due to an increase of genetic variants which are common to many human populations. Height is another example.
8) Cultural flexibility does not means humans are not evolving. On the contrary, strong shifts in cultural norms seem to drive human evolution. Lactase persistence (the ability to drink milk as an adult) is a clear case. But even genetic tolerance to malaria was ultimately driven by human created environmental changes.
9) There are no “most ancient” human group. By definition, we are all equally separated in time from our common ancestors.
10) There are hints of possible new discoveries coming from ancient human DNA. For example there is evidence of humans leaving Africa ~100,000 years ago into Eurasia in both genetic and fossil data. These earlier humans may have been overrun by a later group. But this is hard to determine with the current data. The DNA of current and ancient humans still has many stories to tell.