A new study has revealed that humans living today have DNA that was passed down from a mysterious, unknown ancestor. Using a new algorithm to analyze genomes, the experts found that early humans and related species crossbred multiple times.
The research, which is published by PLOS, confirms previously reported cases of gene flow between ancient humans and their relatives, and also points to new instances of interbreeding.
The study was conducted by Melissa Hubisz and Amy Williams of Cornell University and Adam Siepel of Cold Spring Harbor Laboratory.
About 50,000 years ago, a group of humans migrated out of Africa and crossbred with Neanderthals in Eurasia. Recent genomic studies have shown that this was not the only time that our ancient human ancestors and their relatives shared DNA.
Genetic research involving the Neanderthals and the Denisovans has shed new light on interbreeding events, as well as the movement of our oldest ancestors.
For the current investigation, the researchers developed an algorithm for analyzing genomes to identify segments of DNA that came from other species. The algorithm can pinpoint gene flow even if it occurred thousands of years ago and involved an unknown source.
The team used the algorithm to analyze genomes from two Neanderthals, a Denisovan, and two African humans.
The analysis showed that three percent of the Neanderthal genome came from ancient humans, and the interbreeding occurred between 200,000 and 300,000 years ago.
The experts also determined that one percent of the Denisovan genome came from an unknown and more distant relative, possibly Homo erectus. The team estimates that about 15 percent of these “super-archaic” regions may have been passed down to modern humans who are alive today.
The researchers explained that given the number of these events, genetic exchange was likely whenever two groups of our human ancestors overlapped in time and space. The new algorithm may prove to be useful for studying gene flow in other species where interbreeding occurred, such as in wolves and dogs.
“What I think is exciting about this work is that it demonstrates what you can learn about deep human history by jointly reconstructing the full evolutionary history of a collection of sequences from both modern humans and archaic hominins,” said Siepel.
“This new algorithm that Melissa has developed, ARGweaver-D, is able to reach back further in time than any other computational method I’ve seen. It seems to be especially powerful for detecting ancient introgression.”
The study is published in the journal PLOS Genetics.