Seeking the Parents of Us All
We are an orphan species seeking the mother and father we never knew.
To discover our evolutionary parentage, researchers are sifting the genes all people share for telltale variations that show how modern humans first came into the world. Those mutations can also reveal any kinship with the other primitive human species that our forebears encountered as they migrated around the globe.
Along with lessons about manhood and unsolicited advice, every father passes to his sons a biochemical archive contained in the paternal Y chromosome, which determines maleness. In the same way, every mother passes to all her children a record of maternal descent preserved in the genes of small cell structures called mitochondria.
Subtle mutations in these maternal and paternal genes encode a chemical map of human migrations and diversity.
By reading the genetic genealogies preserved in a single drop of blood or a scraping of skin cells, researchers are trying to settle a lingering, bitter scientific dispute over the geographic origins of contemporary humanity.
In what is perhaps the most extensive paternity test ever conducted, researchers in China and the United States recently determined that the men of East Asia could all trace their ancestry to forefathers who lived in Africa 35,000 to 89,000 years ago. They analyzed Y-chromosome DNA from 12,000 men, all of whom turned out to share distinctive genetic variations that indicated their prehistoric African origin.
An equally sweeping survey of male DNA drawn from Europe, Australia, Asia and the Americas also suggests that the founding fathers of all humankind today emerged relatively recently from Africa. Based on paternal DNA data, a team of researchers from Stanford University constructed a human family tree that roots in a single Y-chromosomal African Adam and splits into 10 main branches as it spreads around the world.
Taken together, the findings are the strongest evidence yet that all modern humans are descended from a single group of people who migrated from Africa and replaced all other existing indigenous human species without interbreeding.
Earlier studies of maternal DNA have strongly supported the theory that modern humans emerged from Africa and elbowed aside the more primitive peoples they encountered in Asia and Europe with no intermarriage or inbreeding. When scientists recently analyzed genetic material extracted from a 29,000-year-old rib bone of a Neanderthal child, they found substantial differences from modern Europeans, suggesting that the ancient child belonged to a species that did not contribute to the modern human genome.
Even so, some scholars still argue that modern humans are descended from several archaic populations, including a species called Homo erectus that roamed Asia, and the Neanderthals, who lived in Europe. Some anthropologists have suggested that vestiges of traits from these other species might account for the physical differences that today set some groups of people apart from others.
These anthropologists too have marshaled genetic evidence to buttress their side of the debate.
The apparent antiquity of the gene responsible for red hair among Celtic peoples suggests that it may have arisen through interbreeding between Neanderthals and Homo sapiens, a genetics team in Oxford, England, suggested last month. Other researchers recently reported that maternal DNA recovered from 62,000-year-old human remains in Australia was not related to other anatomically modern humans.
To help settle the debate, a team of 22 researchers from four countries examined 12,000 men to analyze the DNA in the Y chromosome that determines the male gender. The men belonged to 163 population groups in such places as China, Iran, Siberia and New Guinea, but all of them owed their prehistoric parentage to Africa, the researchers demonstrated.
The team was led by population geneticist Li Jin at Fudan University in Shanghai and the University of Texas at Houston. Their work was published earlier this month in Science.
To draw a more comprehensive family tree based on the Y chromosome, Stanford University molecular biologist Peter Underhill and his colleagues analyzed 1,062 men selected from 21 population groups around the world. Their work was made public earlier this year in the Annals of Human Genetics.
“The Y chromosome really makes the argument bulletproof,” Underhill said. “This is compelling evidence that there is no survivorship of archaic lineages that made it down into the contemporary gene pool.”
With the tools of molecular biology, researchers are using the Y chromosome to peel back the layers of prehistory in the human genome, reconstructing the more recent movements of many population groups.
By examining significant variations in the DNA passed from father to son, researchers in recent months have:
* Traced the origins of India’s high-ranking castes to Europeans and those of lower castes to Asians, according to research by University of Utah geneticist Michael Bamshad published this month in Genome Research. The paternal genetic links show the impact of ancient Western migrations on people in India. It suggests that most of those early migrants were men who found themselves a high-ranking place in the fledgling Hindu caste system.
* Found that the ancient ancestors of people in Hawaii and other Polynesian islands may have migrated to their present-day homes from Southeast Asia and Indonesia rather than from Taiwan or Melanesia as many scholars previously thought. Bing Su and his colleagues at the University of Texas in Houston reached that conclusion after studying the Y chromosomes of 551 men from across the Pacific and Southeast Asia.
* Revealed the imprint of 19th century whaling crews in the present-day population of Micronesia, according to research presented recently by molecular geneticist Jeffrey Friedman at Rockefeller University in New York. Ten generations after European sailors first visited the islands, up to half the men born there today stem from those Caucasian lineages.
* Discovered in the men of Connaught, Ireland’s westernmost province, the genetic signature of a prehistoric people who migrated across Europe to become that island’s earliest settlers 4,000 years ago, according to research by geneticist Daniel Bradley and his colleagues at Trinity College in Dublin. Their work was published in Nature. The same variant occurs in about half the men of France and only 1.8% of the men in Turkey, suggesting that it is a trace left by the world’s first farmers as they moved west from Asia Minor, where agriculture developed, beginning about 10,000 years ago.
* Gleaned from the Y chromosome of men living in the Orkneys, a small island chain off the northeastern coast of Scotland, the genetic signature of invading Vikings as well as the prehistoric farmers who inhabited Europe in Paleolithic times, according to research by geneticists at University College in London published in the Proceedings of the National Academy of Sciences. The genetic signature of these farmers is found nowhere else in Europe today except among the Basque people of northern Spain, who many believe are descended from the continent’s first human inhabitants.
“The Y chromosome is nice because it does not recombine,” said geneticist Mark Shriver at Pennsylvania State University. “A lot more evolutionary information is available than is found in [maternal] mitochondrial DNA.”
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Taking the Genetic Measure of Man
By analyzing variations in the male Y chromosome, researchers have peeled away layers of prehistory in the human genome to reconstruct the ancient movements of many population groups. In male DNA, researchers have found the strongest evidence yet that all modern humans are descended from a single group of people who migrated from Africa and replaced all other human species without interbreeding.
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A Competing Theory
Some scholars have theorized that anatomically modern Homo sapiens instead arose through interbreeding among several regional groups, including a more primitive species, Homo erectus, in Asia and Neanderthals in Europe.
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Source: Stanford University, “Science, Genes, People and Languages,” by Luigi Luca Cavalli-Sforza; The History and Geography of Human Genes, Princeton University Press; The Origin of Modern Humans, Scientific American Books, Discover Magazine
