"}This post is my attempt to simplify what I learned from Who We Are And How We Got Here into a timeline I have a shot of remembering for years, with some padding from Wikipedia where I thought it would help me.
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As a backdrop to the specifics that follow: humans and human cousins have been sort of ambling out of Africa and back in waves of migration and interbreeding and extinction, for the past two million years.
Around 650K years ago, humans\u00a0split from the group that will further split 200K years later into Neanderthals and Denisovans. These are the two \u2018archaic human\u2019 populations that contributed some genetic material to modern humans, whose DNA we got our hands on. This is also maybe the divergence point of humans and another archaic population in Africa that later mixed back into humans, that we don\u2019t have DNA evidence for.
200K years ago, zoom in on the ambling in and out of Africa, because now some of the ambling groups are what we\u2019d call anatomically modern humans \u2013 that is, their phenotypes fit within existing human populations. At this point, the split between the San population (the most different-from-everyone-else population alive today, whose descendants currently live in South Africa) and the rest of living humanity begins.
50K years ago, a period of great interest. We see behavioral modernity starting hereish, at the beginning of a period (lasting ~40K years) we call the Upper Paleolithic. The rate of stone toolmaking innovation speeds up from \u2018glacial\u2019 to \u2018every few thousand years\u2019. (If that seem like an odd setting change, I agree.) We see the first known jewelry and representational art. Likely we\u2019ll never have a satisfactory explanation of what exactly changed. We almost certainly had language by this point.
Human colonization of Australia and New Guinea happens, while ocean levels are low. This is part of a radiation of a hunter-gatherer lineage spreading out all over Asia. Some of them will eventually go to Siberia and the Americas. Some of them will become the Yangtze River population and some the Yellow River population, who will later mix to produce the majority of mainland East Asians.
40K years ago.\u00a0After several thousands of years of contact with modern humans, Neanderthals and some other branches more closely related to modern humans go extinct. There\u2019s an Italian supervolcano eruption nowabouts whose climate disruptions in Europe may have intensified competition.
One thing this book has taught me is that it\u2019s misleading to talk about\u00a0\u2018population splits\u2019 outside of the Americas, because lineages diverged and met again many times, but insofar as it\u2019s meaningful to talk about when the European and East Asian lineages diverged, it\u2019s now.
30K years ago.\u00a0The archaic humans in Africa mix back with humans and contribute 2% of ancestry to some modern African populations.
Around this time, there exists a population called Ancient North Eurasians. Some of them go east, and contribute to the population that will give rise to Native Americans (who are ~1/3 Ancient North Eurasian, ~2/3 ancestors-of-East-Asians). Some of the rest will remain and contribute ancestry to various Eurasian hunter gatherers, as well half the ancestry of the Yamnaya people of the Eurasian steppe, who will later invent horse-and-wagon way of life and become massively successful in Europe.
15K years ago, there are two migrations to America over the Bering land bridge: (1) The First Americans (the Ancient North Eurasian - East Asian group) account for the majority of Native American ancestry.\u00a0These newcomers quickly zoom through the Americas. They also may have had a startlingly small effective population size \u2013 like 250. (2) A mysterious population that contributes some ancestry to a handful of groups in the Amazon, a population whose closest known descendants today are, intriguingly, Australasian. We don\u2019t know much about them.
10K years ago, agriculture arises in the Middle East. Some Anatolian farmers spread out into Europe. Some Iranian farmers spread out to India. A thousand years later, agriculture also begins in China, in the Yangtze River and Yellow River populations.
5K years ago. The horse-and-wagon Yamnaya sweep from the eastern European steppe into northern Europe and largely replace the population there, and account for 25~45% of current European ancestry. The Yamnaya culture is the strongest candidate for the source of the Proto-Indo-European language (which has an elaborate shared vocabulary for wagon-parts). A large part of their success may have been that they were relatively immune to diseases that the rest of Europe was not \u2013 they brought plague with them, heretofore unknown to Europe. With the Yamnaya, the Bronze Age; we have evidence of much more social inequality than ever seen before, evident both from archeology and genetics, which tell us that the highest-reproducing individual men starting now are more reproductively successful than ever before.
Around the same time, there\u2019s another wave of migration from Asia to North America \u2013 the Paleo-Eskimo lineage \u2013 that leaves a ~30% imprint in some parts of North America. The Paleo-Eskimos will be displaced 4K years later by a final wave from Asia, the Neo-Eskimos, who are the ancestors of modern day Inuits.
The Yangtze and Yellow River populations are also spreading out nowish. Their collision produces much of modern East Asians. The Yellow River people are associated with the Han, and the Tibetans. The Yangtze population, where they spread south, provides much of modern Southeast Asian ancestry.
4K years ago, the Indus Valley civilization is hit by a wave of migration for Europeish, by a steppe people who bring Proto-Indo-European culture and language. These steppe people are about half Yamnaya-related, and half\u00a0\u2018the Iranian farmer related populations the steppe people encountered on their way south\u2019. The natives are about three quarters local hunter-gatherers, and one quarter Iranian farmers who mingled in ~2K years earlier. The natives and Yamnaya-ish migrants mix over the next 2K years to form a modern Indian population that\u2019s a mixture of the two, ranging from 80% Yamnaya-ish (especially in the north, and in higher-caste groups) to 20%.
Around now, the first of four great migrations and mixing events of Africa starts \u2013 most significant among them is the Bantu migration south, out of Nigeria and into west-central Africa. Most of the present-day population structure of Africa is shaped by these relatively recent expansions, making it hard to tease out ancient splits.