Razib Khan One-stop-shopping for all of my content

June 27, 2018

Genetics of uniqueness

Filed under: Genetics,indigenous-people,science — Razib Khan @ 1:54 pm
Ati woman from the Philippines
Hui Chinese Muslim man

True genetic isolation is hard to pull off. Human populations tend to mix when they are in close proximity.

Consider the Hui people. These are Muslims who live across China and speak the local Chinese dialect of their locale. The Hui claim descent from Central Asians and Persians who arrived in China around 1,000 years ago. But the vast majority of their genomes are no different from the Han Chinese. Physically they are impossible to distinguish from Han Chinese unless you take note of their attire.

How can that be when they are so culturally different? For example, as Muslims the Hui do not eat pork and consider it unclean. In contrast, for the majority Han pork is dietary staple.

Imagine that the Central Asian ancestors of the Hui arrive 1,000 years ago to China. The historical record suggests that is roughly correct. Each generation is 25 years long, so that’s 40 generations. Since the population of Muslims is small in comparison to the native Chinese, we can ignore the latter, while focusing on the former. If on average 1 out of 20 marriages was between a Han Chinese and a Muslim within the Hui community per generation, after 1,000 years 88% of the ancestry within the Muslim community would be traceable to Han Chinese ancestors. Even though in each generation the overwhelming majority of marriages were within the Muslim community, over time the genetic distinctiveness of the Muslims would diminish.

The lesson is that even a small degree of intermarriage can even out the differences between groups. Similarly, in population genetics one individual moving between two groups per generation is enough to prevent them from becoming distinct. In small populations, which diverge fast, one individual is a substantial proportion of a population. In large populations the divergence is going to be much slower, so even one individual is enough.

An Andaman Islander

So how do populations remain genetically distinct if mixing and homogenization is so easy? The simplest way is simply geography. Consider the Andamanese. These slim and dark-skinned people are the natives of the Andaman Islands, in the middle of the Bay of Bengal. To the knowledge of archaeologists and historians these people have been hunter-gatherers since time immemorial. The only verified continuous such tradition in all of Asia.

The Andamanese likely arrived in the islands during the Pleistocene, when sea levels were lower, and the Andaman Islands were much more accessible from the Southeast Asian mainland. But over the past 10,000 years, as much of the world adopted agriculture, and population turnover occurred in South Asia and Southeast Asia, the Andaman Islands remained relatively untouched due to their isolation.

But it wasn’t just geography. Over the past 2,000 years the Indian ocean has become a major thoroughfare of trade and travel. The Andaman Islands were on a route between India and Southeast Asia. Because of this fact they were often a convenient stopping point to refresh water supplies. But these traders never settled the islands. The local people had a habit of attacking any vessel which tarried too long in their waters.

Pygmies from Central Africa

Unlike many animals humans have complex and evolving cultural practices. The Andaman Islanders discouraged contact with outsiders by maintaining a savage and hostile reputation.

But other groups have remained genetically distinct through symbiosis rather than separation.

The Pygmy peoples of Central Africa are distinguished from their neighbors by their small stature, and hunter-gatherer lifestyle. But they invariably speak the languages of their neighbors. Anthropologists have observed that Pygmies and the farmers who they live nearest to seem to exist in some form of interdependence. Hunter-gatherers can obtain resources from the deep rainforest inaccessible to famers, while the farmers offer the Pygmy people goods which they themselves could not produce.

African farmers and hunter-gatherers have lived in close proximity for over 2,000 years, and yet the Pygmies remain different physically and genetically from their neighbors. Some mixing has occurred, but the Pygmies are as much a separate caste as a different people. Their lifestyle is so different that farmers and Pygmies view each other as profoundly alien and peculiar, despite speaking the same language and occupying nearby geographical space.

Roma in Romania

Isolation then can be both a physical and psychological phenomenon. Some groups, such as the Andamanese, are physically separated from other humans. They add cultural adaptations which reinforce this separation. Others, such as the Pygmies, or the Roma of Europe, are culturally very distinct, and occupy a specific role in the social ecology of their region. In both cases the isolation is strong enough to result in genetic differences between populations of the majority and the isolate.

In many cases these populations are not so isolated in the modern age. In the Andaman Islands most of the tribes now interact with settlers from the Indian mainland. Only the people of North Sentinel island remained truly isolated and cut off from the rest of the world. Meanwhile, the Pygmy people of Central Africa have been caught up in the massive civil wars that have wracked that region of the world since the 1990s. In other cases, as with the indigenous Negrito people of the Philippines, their biological and cultural assimilation into the dominant Austronesian mainstream is proceeding to such an extent that they may no longer being a distinctive people by the end of the 21st century.

For many peoples the 21st century will be the twilight of their solitude, as they merge into the world.

Interested in learning where your ancestors came from? Check out Regional Ancestry by Insitome to discover various regional migration stories and more!

Genetics of uniqueness was originally published in Insitome on Medium, where people are continuing the conversation by highlighting and responding to this story.

Powered by WordPress