Shattering the "tropical ceiling" of ancient DNA research
Last February, my colleagues and I published in Nature the largest ancient DNA study to date on ancient African foragers—people who lived by hunting, gathering, and fishing. Humans have been foragers for the vast majority of the approximately 300,000 years our species, Homo sapiens, has been on the planet. In fact, everyone living today is descended from these populations. Only in the last roughly 12,000 years did various forms of food production, like herding and farming, begin to spread around the world, bringing about major demographic change as people moved around and mixed in new ways. In Africa, this shift largely erased more ancient patterns of human variation and diversity, making it difficult to study our species’ deep history using the DNA of living people.
One of the best ways to access our deeper past is to study the people who were there—that is, extract DNA from ancient human remains. However, doing so is challenging in sub-Saharan Africa, where hot, humid conditions tend to destroy the DNA in old bones and teeth. Although scientists have sequenced several hundred thousand-year-old hominins (or ancient human-like species) in cooler places like Europe, most DNA from sub-Saharan Africa is younger than 5,000 years. As a result of poor preservation, along with other factors, only about 1 percent of all ancient human genomes sequenced worldwide come from Africa.
In our paper, we shattered the DNA “tropical ceiling” by sequencing African foragers who lived before the sweeping demographic changes of the past several thousand years. We reported six new genomes, including the oldest African ancient DNA to date—from a woman who lived about 18,000–20,000 years ago in Tanzania. We compared these individuals with 28 previously published ancient forager sequences from sites extending from Ethiopia to South Africa. For some of these individuals, we were even able to use new methods to generate higher-quality data.
Because every person carries the genetic legacy of their ancestors in their DNA, we were able to use this information to model how ancient foragers in Africa interacted as far back as 50,000–80,000 years ago. What we learned is that people were moving and mixing during the Pleistocene ice ages far more than we thought. These patterns help explain enigmatic changes we see in the African archaeological record around 50,000 years ago, or what archaeologists refer to as the Middle to Later Stone Age transition.
Co-leading this study was an exciting and meaningful experience. As a bioarchaeologist (someone who studies archaeological skeletons), I recovered some of these individuals, including the woman who yielded what is now the oldest DNA sequence from Africa. When I excavated her in 2010, studying her DNA seemed impossible. Even though the first fully sequenced ancient human genome was published that same year, kicking off the “ancient DNA revolution,” many scientists were skeptical we would ever be able to apply such methods in tropical places like Africa. But then, in 2015, the first ancient African genome was published, followed by over 100 more, forever changing our understanding of human evolution and population history. These breakthroughs are a humbling reminder that science is always advancing, and that research that seems impossible today may end up making headlines sooner than you’d expect.
To learn more, check out this piece in The Conversation about our study's significance, or explore the original open-access paper in Nature.
If you're curious what other research news attracted interest in 2022, browse the National Science Foundation's top-10 most-read list.
