When scientific research is published, the authors often confess that they wish they’d collected more data. Critical reviews of research studies often say the same thing. Indeed, if there’s anything scientists love, it’s more data.
Which is why the members of an international team of genetic anthropologists led by Sarah Tishkoff of the University of Pennsylvania are probably quite pleased with themselves. In a new study published this week in Science, the team took the concept of “more is more” to heart by collecting and analyzing the DNA of thousands of people, mostly from Africa, so that they might uncover more clues to not only the genetic make-up of modern Africans, but also the genetic history of Africans and non-Africans alike.
The scientists’ first step was to collect DNA from a diverse set of Africans. Africa is the most culturally and linguistically diverse place on Earth, so it was important to take a wide sample of individuals from all corners of the continent. In total, they collected 2,432 DNA samples from 113 diverse and distinct groups of people from across the African continent as well as 60 non-African groups. They sampled everyone from the Mozabite Berbers of Morocco to the hunter-gatherer San of the Kalahari Desert, and many in between.
But the hard work didn’t stop there. The scientists then examined 1,327 genetic markers across the human genome for each individual studied. While many studies focus on a particular part of the genome such the mitochondrial DNA or the Y chromosome, this study took a comprehensive approach. Finally, the researchers used sophisticated statistical techniques, piecing together how these populations from Africa and around the world were the same, and how they were different.
The results confirmed that Africa has the highest genetic diversity of any continent, as many scientists have proposed. In fact, the authors found genetic diversity to decrease the further one traveled away from Africa. Genetic diversity is often used as a measure of how long ago humans inhabited a region — conventional wisdom places the earliest humans in East Africa, which had exceptionally high genetic diversity according to this study, though an analysis by the researchers put the origin of the human expansion farther south near the border of Namibia and Angola.
The study also shed light on the incredible genetic diversity among African populations, said Roy King, a professor of psychiatry and anthropological geneticist from Stanford University:
Not only did farming and pastoral communities differ from hunter-gatherers, but within the broad range of agricultural populations of West and West-Central Africa — from which many African Americans derive their ancestry — the authors also found some genetic diversity. For example, the Dogon of Mali, although geographically near the Mandinka of Senegal, cluster with North African Berber populations. Thus, this study supports the notion that not only is Africa varied in culture — art, music, religion and language — but also harbors a rich genetic diversity across its multitude of ethnic groups.
The authors also found a loose connection between the genetics of a population and its language. However, there were a few exceptions, most often the result of a population adopting a new language within the last few thousand years.
The sheer size and diversity of the DNA samples collected allowed the researchers to construct a human family tree based on their analyses. Not unexpectedly, the tree they constructed fits well with current theories on the genetic relationship between Africans and non-Africans; namely that all non-Africans are descended from a particular group or groups of people who were the first humans to migrate out of Africa tens of thousands of years ago.
This study is important for a multitude of reasons. It has been able to confirm theories from the archaeological, cultural, and linguistic records on the origins and movements of Africans and non-Africans.
“It fits nicely with earlier genetic studies, while subverting the early 20th century colonialist idea of sub-Saharan Africa as constituting a homogeneous genetic an cultural unit,” King said.
It also creates a new resource that historians, linguists, archaeologists and scientists from a range of other disciplines can use in their own work. If we are lucky, this study will bring forth a flurry of activity surrounding the origins and history of the African continent, and the people who live there.