Who We Are and How We Got Here

Author: David Reich

My first entry into the world of genetics was from the book “Neanderthal Man: In Search of Lost Genomes”, by Svaante Paabo. In it, he chronicles the challenge and excitement of deciphering the first Neanderthal genome, and it got me totally hooked on the science of using DNA to theorize how our early ancestors roamed the world and laid down their roots all over the globe.

David Reich has spent his life analyzing the DNA of both past and present humans, looking for genome-wide patterns of mutations to gain a better understanding of how humans migrated and mixed among populations.

Humans probably lived as hunter-gatherers for over 35,000 years in Europe. Around 8500 years back, Anatolian farmers brought farming to Europe and around 5000 years ago the Yamnaya culture moved into Europe from the Caspian Steppe. The Yamnaya people are credited with the spread — if not the invention — of the wheel, using it for transportation in wagons to making pots with the pottery wheel. By hitching a horse to the wagon, they could carry water and supplies to far off lands and inhabit places that may have been inaccessible until then. As a result of this, the Yamnaya culture most likely enjoyed a much higher level of productivity than their peer cultures at the time.

The Indus Valley Civilization flourished in the border of what is currently India and Pakistan, around 5000 years ago. One of the great unsolved mysteries is what caused their relatively quick decline. Could they have been vanquished by warriors from the Yamnaya culture riding horses and chariots? The archaeological evidence from the Indus Valley does not show much by way of domesticated horses.

David spends a good chunk of the book dissecting the Indian population. At a broad level he breaks us down into Ancestral North Indian (ANI) and Ancestral South Indian (ASI) which roughly corresponds to the people of North Indian culture and South Indian culture respectively. By studying the genetic makeup of different groups of Indians he makes some interesting observations.

1. Indian people have very diverse physical characteristics that is most likely the result of a lot of mixing of gene pools
2. His studies conclude that the West-European mixture in India can range from a high of 80% to a low of 20%, and almost every group has some common genealogy with Western Europe.
3. Groups speaking Indo-European languages have more ANI history than others
4. Higher social status is correlated with ANI ancestry
5. Most of the ANI genetic input came from Europe to India from males. The female ancestry is mostly restricted to India and most likely came from ASI.
6. There was lot of mixing between ANI and ASI in the periods between four thousand and two thousand years ago.
7. This implies that the population structure of India four thousand plus years ago was dramatically different than what it is today
8. Indians are notorious for endogamy, which is the practice of marrying people whose culture is very similar to yours. This has ended up creating population bottlenecks that are a great vehicle for studying genetics and ancestry

One of the conclusions is that India is composed of a large number of small populations. The book is truly insightful and is a fascinating read.