New DNA studies suggest that
all humans descended from a single African ancestor who lived some 60,000
years ago. To uncover the paths that lead from him to every
living human, the National Geographic Society launched the Geographic Project, headed by Spencer Wells. The scientist has travelled the world, more exact in 50 countries, collecting blood samples from people of far-flung cultures: Aborigines in Australia, the Chukchi tundra dwellers of Siberia, farmers in the hills of Afghanistan, and nomads in the deserts of Africa. By studying the DNA of modern humans, he seeks to learn who we are, where we travelled to populate the world, and how closely we are all related. "Each drop of blood is essentially a historical document," Wells said. "Our DNA tells the story of the journey of our species.”
The project will combine population genetics and molecular biology to trace the migration of humans from the time we first left Africa, 50,000 to 60,000 years ago, to the places where we live today.
"Our DNA tells a fascinating story of the human journey: how we are all related and how our ancestors got to where we are today," Wells said. "This project will show us some of the routes early humans followed to populate the globe and paint a picture of the
genetic tapestry that connects us all."
Tracing Human History "As often happens in science," said Wells, "technology has opened up a field to new ways of answering old questions—often providing startling answers."
One of the old questions that intrigued Wells was the question of human origins. Whether early humans evolved in Africa or elsewhere, when they began outward migration, and where they went, are issues that have been argued among archaeologists, anthropologists, and evolutionary biologists for decades.
Following the southern coastline of Asia, the first early travellers crossed about 250 kilometres of sea, and colonized Australia by around 50,000 years ago.
The Aborigines of Australia, Wells says, are the descendants of the first wave of migration out of Africa. A second wave left Africa around 45,000 years ago and settled in the Middle East, with smaller groups going off to India, northern China, and southern China. As the glaciers of the Ice Age began to retreat around 40,000 years ago and temperatures warmed up, humans moved into Central Asia and multiplied quickly.
"If Africa was the cradle of mankind, then Central Asia was its nursery," said Wells.
Small groups left Central Asia around 35,000 years ago for Europe. Around 20,000 years ago, another small group of Central Asians moved farther north, into Siberia and the Arctic Circle.
By analyzing genetic changes in the Y-chromosome of people in all regions of the world, Wells and colleagues concluded that
all humans alive today are descended from a single African man. This is an amazing discovery that dissolves the image which we all had in our heads about Adam, how he was portrayed through history.
Y-chromosomal Adam (how the scientists are calling him)) is the human most recent common ancestor, from whom all Y chromosomes in living humans are descended, from whom all mitochondrial DNA in living humans is descended.
The Y-chromosomal Adam for living humans probably lived between 60,000 and 90,000 years ago, judging from molecular clock and genetic marker studies.
"Genetic evidence tells us that Homo sapiens are of recent origin and arose in Africa," said S. Blair Hedges, a molecular biologist at Pennsylvania State University.
"African populations have the most ancient alleles
and the greatest genetic diversity, which means they're the oldest," Hedges explained. "Our species probably had arisen by 150,000 years ago, with a population of perhaps 10,000 individuals."
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