By Jean Balchin 31/01/2018

When we think about our ancestry, we think about our grandmothers and grandfathers – and, if we have photo albums nearby, or a family member interested in genealogy, we might know a bit more about our great-grandmothers, great-grandfathers, and so on.

I remember traveling around Scotland a few years ago, and chancing upon a small churchyard. Every second gravestone, most of them crumbling and tilted over with age, had my mother’s maiden name on it. It was the strangest thing, looking at these weathered old stones and thinking of the bones that lay beneath them. I realised that I shared DNA with these people under the earth. They had shaped who I was today.

DNA sequencing technology is revealing how our modern human genome has been shaped by none other than our Stone Age ancestors. A review of 24 palaeolithic human genomes suggests in Eurasia between 35,000 and 45,000 years ago, at least four distinct populations were present. Shifts in climates and cultures led to interactions between these four population groups, resulting in the genetic similarities we now see among modern humans. Populations across Eurasia shared similarities by 7,500-14,000 years ago.

Decoding the modern genome. Chinese Academy of Sciences.

Until recently, very little was known about the genetic relationship between modern humans of the Upper Paleolithic age and today’s populations. The Paleolithic age was a prehistoric period of human history characterised by the development of the most primitive stone tools. It extends from the earliest known use of stone tools, probably by Homo habilis initially, 2.6 million years ago, to the end of the Pleistocene around 10,000 BP (Before Present). 

However with direct DNA sequencing, researchers are discovering unexpected genetic connections between individuals on opposing sides of Eurasia, suggesting a complex history that may represent early gene flow across Eurasia or an early population structure that eventually led to Europeans and Asians.

In a review published in the journal Trends in Genetics on January 25, scientists at the Chinese Academy of Sciences in Beijing discuss what we know about the genetics of ancient individuals from Eurasia (Europe and Western Asia) between 45,000-7,500 years ago. The authors summarised work investigating the genomes of more than 20 ancients in the Eurasian family tree, including the 45,000-year-old Ust’-Ishim individual from Central Siberia, for their paper. The Ust’-Ishim man fossil is notable in that it had intact DNA which permitted the complete sequencing of its genome, the oldest modern human genome to be so decoded.

“Aside from these individuals, it is a fact that sampling for the Eurasian region is sparse for all time periods except the present-day,” says co-author Qiaomei Fu, a paleogeneticist at the Chinese Academy of Sciences. “But with the information from the several individuals available for ancient DNA sequencing we do have hints at interesting population structure, migration and interaction in East Asia.”

This research has revealed that in Eurasia between 35,000 and 45,000 years ago, at least four distinct populations were present. These were early Asian and Europeans, as well as populations with ancestry hardly found or not at all in modern populations. By 15,000-34,000 years ago, however, DNA sequencing showed that modern humans in Eurasia are similar to either Europeans or to Asians, suggesting that a genetic Asian-European separation likely occurred prior to 40,000 years ago. By 7,500-14,000 years ago, the populations across Eurasia shared genetic similarities, suggesting greater interactions between geographically distant populations.

These analyses also revealed that two Neanderthal population mixing events occurred, one about 50,000-60,000 years ago and a second more than 37,000 years ago. This Neanderthal ancestry gradually declined in archaic ancestry in Europeans dating from ~14,000-37,000 years ago.

According to Fu:

“Genetic studies of ancient individuals have become more frequent in recent years because of technology. As a result, we can now see the presence of multiple distinct subpopulations in Europe and in Asia, and these in turn contribute different amounts of ancestry to more recent subpopulations.”

“Right now is a great time to study human evolutionary genetics because the development of sequencing technology and computing resources minimises destruction of samples and maximises data generation and storage,” Fu says. “With large present-day genomic datasets and increased international collaboration to handle the many newly sequenced ancient datasets, there is huge potential to understand the biology of human prehistory in a way that has never been accessible before.”

The researchers hope to extend this type of sequencing and analysis to learn more about the genetic prehistory of East Asia, Oceania, Africa, and the Americas.

“All of those areas have a rich human prehistory, particularly in Africa, so any ancient DNA from those continents will likely resolve some major questions on human migration,” Fu says.

You can read the paper here.