Researchers at the Harvard Medical School have discovered new genetic evidence supporting the theory that the migration of modern humans to Asia occurred in multiple waves.

The results of the study—a joint effort between scientists at HMS and the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany—contradict previous studies arguing that the dispersion of human populations to Asia occurred in a single wave.

The researchers identified separate migration waves to east Asia, southeast Asia, and Oceania by looking for traces of genetic material from Denisovans—a previously unknown group of ancient humans that occupied southeast Asia, among other areas—in 33 populations in the regions studied.

“The major challenge was analytical, coming up with a method that could detect even small amounts of Denisovan genetic material,” said Max Planck Institute evolutionary professor Mark Stoneking, one of the lead authors of the study.

Denisovan genetic material was identified in populations in southeast Asia and Oceania—including the Australian aborigine, New Guinean, and the Philippine Negrito—suggesting ancestral interbreeding with Denisovans that did not occur in populations found further west and northwest, such as the Asian mainland.

HMS Professor David E. Reich, the other lead author, argued that these results fit the patterns of multiple migration waves, rather than one single wave.

“This leads to two implications,” he said. “First, Denisovans interbred with the population very early in southeast Asia ... Second, there had to be two waves of migration in southeast Asia.”

The initial wave led to the ancestors of the human populations with traces of the Denisovan genome in southeast Asia and Oceania.

Other subsequent waves of humans—likely ancestors of modern-day Chinese and Indonesian populations—later displaced the existing occupants of East Asia and Southeast Asia, including some of the Denisovan-human groups, according to Reich.

The sequencing of the Denisovan genome—an essential precursor to the present study—was carried out last year by another group at the Max Planck Institute from a fossilized finger tip bone found in the Denisova Cave in Siberia. The scientists found that Denisovans were descended from similar ancestors as early humans and Neanderthals.