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Israeli study shows early humans neared extinction
Posted By Judy Siegel-Itzkovich On May 25, 2008 @ 1:09 pm In | No Comments
DNA samples from people alive today were used by the researchers to determine how the human race evolved.For a large part of its history the early human race was split into two separate species, and was so small that it likely verged on extinction, an Israeli-led genetic study has shown.
The research shows that two separate populations may have formed in the East and South of Africa, and perhaps dropped to as few as 2,000.
The study also demonstrates that DNA samples from people alive today – and not only studies of bones, archeological evidence and wall drawings – can be used to determine how the human race evolved.
The analysis – just published in the American Journal of Human Genetics and led by Dr. Doron Behar at Rambam Medical Center’s laboratory of molecular medicine in Haifa and Dr. Saharon Rosset of IBM’s T.J. Watson Research Center in New York and Tel Aviv University – is said to be the most extensive survey to date of African mitochondrial DNA handed down by mothers since the beginning of the human race.
“The migrations 60,000 years ago that led modern humans on their epic journeys to populate the world have been the primary focus of anthropological genetic research, but relatively little is known about the demographic history of our species over the previous 140,000 years in Africa,” Behar said. “The new study returned the focus to Africa, and in doing so refines our understanding of early modern Homo sapiens history.”
“This study illustrates the extraordinary power of genetics to reveal insights into some of the key events in our species’ history. Tiny bands of early humans, forced apart by harsh environmental conditions, coming back from the brink to reunite and populate the world. It was truly an epic drama, written in our DNA,” said Dr. Spencer Wells, a National Geographic explorer-in-residence and director of the Genographic Project.
For the article called “The Dawn of Human Matrilineal Diversity,” the researchers used blood or cheek epithelial cell samples stored in labs around the world – that included mitochondrial DNA – from some 600 people who originated in Africa. The researchers put a special focus on the Himla Soodyall indigenous people, who number in the thousands, to tens of thousands, and whose native language consists of clicks.
Various studies in other disciplines showed these people had many things in common, including their features and the fact that they were hunter-gatherers rather than farmers.
The 600 unrelated participants, including 38 Himla Soodyall and people from all of North Africa – including Chad, Egypt, Guinea-Bissau, Cameroon, Morocco, Tunisia and even Israeli Jews, Bedouin and Druse – gave their informed consent for taking part in the research.
The researchers’ analyses of the extensive genotyped data provided surprising insights into the early demographic history of human populations before they moved out of Africa. Research shows that early human populations were small and isolated both geographically and socially from each other for tens of thousands of years.
“Our aim was to document if these people descend from the oldest in the human species. It seems that their ancestors split off from other Africans between 90,000 and 150,000 years ago and were isolated due to topographical and other reasons,” Behar said. “Small groups of hunter-gatherers looked for a good place to live in a difficult environment.”
Recent paleoclimatological data suggest that eastern Africa went through a series of massive droughts between 90,000 and 135,000 years ago. It is possible, the researchers suggest, that this climatological shift contributed to the population splits. What is surprising is the length of time the populations were separate – as much as half of our entire history as a species.
The genetic split in Africa resulted in distinct populations that lived in isolation for as much as 100,000 years, the scientists say. At the time of the split – some 150,000 years ago – our species, Homo sapiens, was still confined to the African continent.
On one side of this divide were the mitochondrial lineages now found predominantly in East and West Africa, and all maternal lineages found outside Africa.
On the other side of the divide are lineages predominantly found in the Khoi and San (Khoisan) hunter-gatherer people of southern Africa.
This could have been caused by arid conditions driving a wedge between humans in eastern and southern Africa.
It would be the longest period for which modern human populations have been isolated from one another.
The isolated African groups started to meet up again with each other about 40,000 years ago, during the African Late Stone Age, and then grew in numbers and moved to an expanding area. Many archeologists believe this era heralded the beginning of fully modern human behavior, including abstract thought and complex spoken language.
Other researchers have hypothesized that during the initial isolation, about 70,000 years ago, Homo sapiens ,/I>was almost wiped out, with the world population numbering as low as 2,000 people. But they banded together and survived.
The researchers were members of the Genographic Project launched in 2005 by National Geographic and IBM, with field research supported by the Waitt Family Foundation and laboratory research supported by Applied Biosystems. The aim was to use genetics as a tool to address anthropological questions on a global scale. A consortium of 10 regional scientific teams took DNA samples and analyzed them in their respective regions.
Mitochondrial DNA, inherited down the maternal line, was used to discover the age of the famous “mitochondrial Eve” in 1987, said Behar. This work has since been extended to show unequivocally that the most recent common female ancestor of everyone alive today was an African woman who lived in the past 200,000 years. Paleontology provides corroborating evidence that our species originated in Africa approximately 200,000 years ago.
“We see strong evidence of ancient population splits beginning as early as 150,000 years ago, probably giving rise to separate populations localized to eastern and southern Africa,” Behar said.
While human populations had been quite small prior to the Late Stone Age, the expansion after this time led to the occupation of many previously uninhabited areas, including the world beyond Africa.
“It was only around 40,000 years ago that they became part of a single pan-African population, reunited after as much as 100,000 years apart.”
Rosset said that “the analysis of such a massive dataset presents statistical and computational challenges as well as great opportunities for discovery of the events that shaped our history and genetic landscape. For example, we can see evidence of a population expansion period starting around 70,000 years ago, perhaps leading to the out-of-Africa dispersal shortly afterward.”
Paleontologist Meave Leakey, a Genographic advisory board member, National Geographic explorer-in-residence and research professor at Stony Brook University in New York, said: “Who would have thought that as recently as 70,000 years ago, extremes of climate had reduced our population to such small numbers that we were on the very edge of extinction?”
Printed by courtesy of The Jerusalem Post.
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