Out of Africa, Not Once But Twice

March 14, 2008 -- Modern humans are known to have left Africa in a wave of migration around 50,000 years ago, but another, smaller group -- possibly a different subspecies -- left the continent 50,000 years earlier, suggests a new study.

While all humans today are related to the second "out of Africa" group, it's likely that some populations native to Israel, Jordan, Lebanon, Syria, Australia, New Zealand and Indonesia retain genetic vestiges of the earlier migrants, according to the paper's author, Michael Schillaci.

Schillaci, an assistant professor in the Department of Social Sciences at the University of Toronto, also found the earlier group of emigrants had some genetic similarity to Neanderthals, a hominid that left Africa much earlier, settling in Europe and parts of western and central Asia.

"This could be the byproduct of limited [interbreeding] with Neanderthals, or a shared more recent common ancestry with Neanderthals," he told Discovery News. "Humans and Neanderthals share a common Homo ancestor in Africa at around 500,000 years ago. However, Neanderthals evolved in Europe, while modern humans evolved in Africa."

For the study, he calculated genetic similarity by comparing measurements of the cranium, the part of the skull that encloses the brain. In addition to actual DNA testing, researchers often use such skull measurements to establish relationships between ancient human groups.

Schillaci examined fossils representing at least 28 modern and prehistoric human populations.

The earliest known individuals from the Near East, he found, were genetically similar to the earliest individuals from Australia, New Zealand and Indonesia. All modern-day humans are more similar to Europeans who lived between 40,000 and 10,000 years ago -- after the second wave from Africa.

"The most likely explanation...is that the expansion out of Africa that was ancestral to the early Australasians occurred before the well-accepted expansion at around 50,000 years ago that led to the colonization of Europe," he said, adding that the first populations out of Africa were later "swamped genetically by the subsequent larger expansion."