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.
"We're all effectively cousins, separated by 2,000 generations," he said.
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In his book, The Journey of Man: A Genetic Odyssey, Wells describes the exodus from Africa that began around 60,000 years ago, and the path we took to populate the world.
Following the southern coastline of Asia, the first early travelers crossed about 250 kilometers [155 miles] 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.
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.
He and his colleagues' paper, published in the September 2003 issue of the American Journal of Human Genetics, added evidence to the debate on the peopling of the Americas. The earliest archaeological evidence of human habitation in North America dates to 11,500 radiocarbon years old. Artifacts at a site in Monte Verde, Chile, are in the 12,500-year-old range. However, there are scientists who believe there is some linguistic and archaeological evidence suggesting the possibility that people may have arrived as much as 30,000 years ago.
Mutations in the Y-chromosome of populations in North and South America put an upper limit on human arrival at somewhere between 18,000 to 15,000 years ago. "I would put the number at closer to 15,000," said Wells.
Legacy of Genghis Khan
In another study that looked at blood samples collected over a period of ten years from more than 40 populations living in and around the former Mongol empire of Genghis Khan, Wells and colleagues found that nearly 8 percent of the men living in the region carry nearly identical Y-chromosomes. That translates to roughly 16 million descendants living today.
Archaeological finds and texts describing the history and culture of the region provide some insight into how this one lineage could be so successful, said Wells.
Khan's empire at the time of his death extended across Asia. His military conquests frequently entailed the wholesale slaughter of the vanquished, wiping out many populations. Contemporary documents report that after a conquest, looting, pillaging, and rape were the spoils of war for all soldiers, but that Khan got first pick of the beautiful women. In addition, his male descendants were markedly prolific. They extended the empire and maintained power in the region for several hundred years, in a culture in which harems and concubines were the norm. Khan's eldest son is reported to have had 40 sons. His grandson, Kublai Khan, had 22 legitimate sons, and was said to add 30 virgins to his harem each year.
"This is a clear example that culture plays a very big role in patterns of genetic variation and diversity in human populations," said Wells.
In addition to a general passion for history, Wells has an abiding interest in lost empires. One of his current projects involves trying to track down the Phoenicians, a civilization that extended from roughly 3000 B.C. to 332 B.C.
"There's remarkably little archaeological evidence of them, and yet they were the dominant culture of the time," said Wells. "Egyptian texts starting from about 1200 B.C. talk about marauding warriors arriving from the sea, yet we really have no idea who they were or where they came from and where they went."
The Phoenicians were a seafaring people who settled a number of independent city-states around the Mediterranean in modern-day countries Syria, Lebanon, Israel, Tunisia, Greece, and Spain. United by a common language, their alphabet became the prototype for the Greek and Roman alphabets; they were the first people to circumnavigate Africa, and may even have reached America.
But where did they come from and where did they go? For that we'll have to wait for the next installment of Spencer Wells' exploration.
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