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Let us Now Praise Human Population Genetics

Editors’ NoteThe following blog post originally appeared on Oxford University Press’s
Academic Insights for the Thinking World blog.

Exactly who are we anyway? Over the last generation, population genetics has emerged as a science that has made the discovery of human origins, relatedness, and diversity knowable in a way that is simply not possible from studying texts, genealogies, or archeological remains. Viewed as the successor to a race science that promoted the superiority of some human groups over others and that provided a basis for prejudice, forced sterilization, and even extermination, population genetics is framed as a discipline that is based on discovery using the amazing content of fully-sequenced human genomes and novel computational methods. None of the recent discoveries would have been possible in the past. And what have we learned?

Humans descended from chimpanzees approximately 5 million years ago and that descent was not strictly linear. Other human-like species (“hominins’) emerged along the way, some fairly recently. Neanderthals emerged in Europe 300,000 years ago and Denisovans emerged 40,000 years ago. The latter group was identified only a few years ago when the DNA from a tooth in a Siberian cave was found to be that of a unique hominin species. Neither group kept strictly to itself. Rather, both mixed with humans who left Africa 50-60,000 years ago and left imprints in the genomes of contemporary Middle Easterners (Neanderthals) and Southeast Asians and New Guineans (Denisovans).

Humans originated in Africa 200,000 years ago. Although a Molecular Adam and Mitochondrial Eve have been inferred to have lived in Africa, they were not the first and they were not contemporaneous with each other. Instead, they were the successful pair among a number of early humans who were able to transmit the male-determining Y chromosome (Adam) and egg-transmitted mitochondria (Eve) to future generations. Differentiation occurred along the way. The click-speaking Khoisan split from other Africans 125,000 years ago and maintained this differentiation despite living in proximity with other groups. In fact, because humans spent most of their history in Africa, far greater genetic diversity accumulated among people on that continent compared to those in the rest of the world combined. This explains why contemporary Africans and African Americans have greater difficulty with finding compatible organ transplantation donors — there is simply a far greater range to choose from. It also renders the racist statements of the distant and not-so-distant past meaningless. Unlike Dr. Watson, we should not be “inherently gloomy about the prospect of Africa,” because the genetic diversity may give Africans a leg up on meeting the challenges of diverse environments.

Much of the population genetics of the Old World is local. Not only can the major continental groups be readily distinguished from one another, but within each of those continents, the residents of countries, provinces, and villages can be distinguished from their neighbors. A genetic map of Europe tends to overlay almost perfectly with the physical map. The same is true for the genetic maps of China, India, the Middle East, and North Africa. Some interesting exceptions exist. Religious groups who kept to themselves over the past two millennia, such as Jews and Gypsies, tend to resemble themselves genetically more than they resemble their neighbors — despite covering large geographical distances. People who speak Bantu-Niger-Kordofanian languages across equatorial, eastern, and southern Africa also tend to share greater genetic similarity than they do with their foreign language-speaking neighbors.

Much of the population genetics of the New World is not local. Because of the contact among Native Americans, Africans and Europeans, most of the people of the New World have hybrid genomes. Thus genetic maps do not tend to mimic geographical maps. Variation among the contributing populations — Native American tribe, African tribe, and European, Jewish, or North African ancestry — can all be identified. But as the result of geographic isolation and selective mating procedures among religious groups or social castes, a local population genetics has also developed in the Americas. This has occurred among the Plain People (Amish, Mennonites) of Pennsylvania as well as among the residents of the Central Valley of Costa Rica and other isolated valleys in North and South America. Oftentimes, the degree of relatedness of any two random people within these populations is what one would observe for first cousins once removed.

The content of human genomes has been shaped by exposure to diets and infectious agents through the process of natural selection — exactly as Darwin predicted in Origin of Species. The ability to digest milk sugar as an adult was selected by different genetic mechanisms both in Eurasia and in East Africa. Resistance to malaria, Lassa fever, HIV, and other infectious agents gave certain groups a leg up to exposure. In the Americas, not only were these agents brought by colonizing Europeans and enslaved Africans, but so too were the mechanisms of resistance.

Rather than promoting prejudice, population genetics should peel it away. Along with the study of history and cultures, population genetics should become a required part of curricula for understanding human origins, similarity and diversity.

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