Out of Africa, Not Once But Twice

[Egyptian King]

Out of Africa, Not Once But Twice
Jennifer Viegas, Discovery News 


Out of Africa 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."

[Egyptian King]

Out of Africa
A Neanderthal (foreground) and Homo sapiens skeleton are seen in this image. Modern humans left Africa in not one, but two, great waves, suggests new research. The earlier group -- which left the continent roughly 100,000 years ago -- bore genetic similarity to Neanderthals.
 

[Egyptian King]

Based on the findings, which have been accepted for publication in the Journal of Human Evolution, he concludes the first human group to have left Africa "may well have been a separate subspecies" of modern human.

Prior research could support that contention. At an Ethiopian village called Herto, archaeologists recently found fossils of individuals who were more robust than modern humans. They date to 154,000 to 160,000 years ago.

Erik Trinkaus, a professor of physical anthropology at Washington University in St. Louis, thinks the new paper "is an interesting analysis," but he told Discovery News that he hopes it will be redone with more fossils, "a better set of measurements and with the caveat that there is a huge (time) gap between his relevant samples."

Schillaci neglected one of the earliest known Southeast Asian humans in his study, noted Trinkaus.

This individual "predates the Australian fossils and is the only relevant fossil that we have between Israel and Indonesia for the relevant time period," he explained, adding that "we have no relevant fossils between 100,000 and 30,000 from the Levant [Near East] and Australia to sort out what might have been happening there."

Chris Stringer, a paleontologist at the Natural History Museum in London, however, expressed fewer reservations.

"This is a very interesting and important study that provides much food for thought," Stringer told Discovery News. "It revisits in more detail and with new approaches something which several researchers have previously noted -- certain early modern samples...seem closer to very early H. sapiens in Israel and Africa than to other early modern samples around the world."

Stinger isn't yet convinced that the Ethiopian fossils and early Australian/Indonesian individuals provide evidence of a new human subspecies. The rigors of dealing with prehistoric life might have simply resulted in sturdier bodies.