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WHEN the ancestors of modern humans spread from Africa, they didn’t extinguish or replace existing populations. Instead, the emigrants from Africa interbred extensively with groups who had arrived in Europe before them, a new genetic analysis suggests.

Most anthropologists agree that low-browed Homo erectus evolved in Africa and then spread into southern Eurasia around 1.7 million years ago. Anatomically modern humans also appeared first in Africa, before spreading through most of Eurasia around 100,000 years ago. But what happened next?

Most geneticists argue that the high-browed, strong-chinned colonisers supplanted the regional populations derived from Homo erectus pioneers. But others believe they have interbred, allowing genetic and anatomical features of both lineages to survive.

Based on DNA evidence from people alive today, Alan Templeton of Washington University in St Louis, Missouri, has come down on the side of the second group. He performed a new kind of statistical analysis of 10 different genetic markers spread across the genome. The DNA samples came from nearly 6000 people from around the world.

Templeton’s method is called nested clade analysis and involves putting people with related genetic variations into groups called clades, then looking for geographical differences in where they come from. His results suggest that a modern population from Africa didn’t simply wipe out all other groups. Instead he says there were at least two major waves of expansion from Africa after Homo erectus, which both swapped genes extensively with locals.

The first wave, between 840,000 and 420,000 years ago, would have spread the Acheulean culture, typified by complex stone tools, through much of Eurasia. A second wave, 115,000 to 80,000 years ago, carried modern anatomical traits, including high rounded skulls, small brow ridges and vertical foreheads.

The analysis also shows signs of genetic mixing between Africa and Eurasia dating back at least 600,000 years. “Most, but not all, of our current genetic legacy traces back to Africa,” says Templeton. “Over a long time, there was sufficient genetic exchange to ensure that all humanity evolved as a single species.”

David Wolpoff, an anthropologist at the University of Michigan, agrees. “The key thing is that the later expansions didn’t wipe out the evidence of the earlier ones,” he says. “It shows how important Africa has been, but it totally takes out of the picture the replacement model.”

But Rebecca Cann, a geneticist at the University of Hawaii still supports the replacement theory. She says Templeton’s technique needs to be tested further before she’ll trust its conclusions about human prehistory.