terça-feira, 5 de agosto de 2008

The Migration History of Humans

July 2008
By Gary Stix

The Migration History of Humans: DNA Study Traces Human Origins Across the Continents

DNA furnishes an ever clearer picture of the multimillennial trek from Africa all the way to the tip of South America

Key Concepts

  • Scientists trace the path of human migrations by using bones, artifacts and DNA. Ancient objects, however, are hard to find.
  • DNA from contemporary humans can be compared to determine how long an indigenous population has lived in a region.
  • The latest studies survey swathes of entire genomes and produce maps of human movements across much of the world. They also describe how people’s genes have adapted to changes in diet, climate and disease.

A development company controlled by Osama bin Laden’s half brother revealed last year that it wants to build a bridge that will span the Bab el Mandeb, the outlet of the Red Sea to the Indian Ocean. If this ambitious project is ever realized, the throngs of African pilgrims who traverse one of the longest bridges in the world on a journey to Mecca would pass hundreds of feet above the probable route of the most memorable journey in human history. Fifty or sixty thousand years ago a small band of Africans—a few hundred or even several thousand—crossed the strait in tiny boats, never to return.

The reason they left their homeland in eastern Africa is not completely understood. Perhaps the climate changed, or once abundant shellfish stocks vanished. But some things are fairly certain. Those first trekkers out of Africa brought with them the physical and behavioral traits—the large brains and the capacity for language—that characterize fully modern humans. From their bivouac on the Asian continent in what is now Yemen, they set out on a decamillennial journey that spanned continents and land bridges and reached all the way to Tierra del Fuego, at the bottom of South America.

Scientists, of course, have gained insight into these wanderings because of the fossilized bones or spearheads laboriously uncovered and stored in collections. But ancestral hand-me-downs are often too scant to provide a complete picture of this remote history. In the past 20 years population geneticists have begun to fill in gaps in the paleoanthropological record by fashioning a genetic bread-crumb trail of the earliest migrations by modern humans.

Almost all our DNA—99.9 percent of the three billion “letters,” or nucleotides, that make up the human genome—is the same from person to person. But interwoven in that last 0.1 percent are telltale differences. A comparison among, say, East Africans and Native Americans can yield vital clues to human ancestry and to the inexorable progression of colonizations from continent to continent. Until recent years, DNA passed down only from fathers to sons or from mothers to their children has served as the equivalent of fossilized footprints for geneticists. The newest research lets scientists adjust their focus, widening the field of view beyond a few isolated stretches of DNA to inspect hundreds of thousands of nucleotides scattered throughout the whole genome.

Scanning broadly has produced global migratory maps of unprecedented resolution, some of which have been published only during recent months. The research provides an endorsement of modern human origins in Africa and shows how that continent served as a reservoir of genetic diversity that trickled out to the rest of the world. A genetic family tree that begins with the San people of Africa at its root ends with South American Indians and Pacific Islanders on its youngest-growing branches.

The study of human genetic variation—a kind of historical Global Positioning System—goes back to World War I, when two physicians working in the Greek city of Thessaloníki found that soldiers garrisoned there had a differing incidence of a given blood group depending on their nationality. Beginning in the 1950s, Luigi Luca Cavalli-Sforza started formalizing the study of genetic differences among populations by examining distinct blood group proteins. Variations in proteins reflect differences in the genes that encode them.

Then, in 1987, Rebecca L. Cann and Allan C. Wilson of the University of California, Berkeley, published a groundbreaking paper based on analyzing the DNA of mitochondria, the cell’s energy-producing organelles, which are passed down through the maternal line. They reported that humans from different populations all descended from a single female in Africa who lived about 200,000 years ago—a finding that immediately made headlines trumpeting the discovery of the “Mitochondrial Eve.” (Despite the Biblical allusion, this Eve was not the first woman: her lineage, though, is all that has survived.)

All about Eve
The fast, relatively predictable rate of “neutral” mitochondrial mutations—ones that are neither beneficial nor harmful—lets the organelles operate as molecular clocks. Counting the differences in the number of mutations (ticks of the clock) between two groups, or lineages, allows a researcher to construct a genetic tree that tracks back to a common ancestor—Mitochondrial Eve or another woman who founded a new lineage. Comparison of the ages of the lineages from different regions permits the building of a timeline of human migrations.

Since 1987 the data bank on human diversity has broadened to encompass the Y chromosome—the sex chromosome passed down only by males to their sons. The male-transmitted DNA carries many more nucleotides than mitochondrial DNA does (tens of millions, as opposed to just 16,000), enhancing investigators’ ability to distinguish one population from another. Analyzing mitochondrial and Y chromosome DNA from human populations has turned up hundreds of genetic markers (DNA sites having identifiable mutations specific to particular lineages).

The route humans took from Africa to the Americas over the course of tens of thousands of years can now be tracked on the map as if the travelers were moving, albeit extremely slowly, on a series of interconnected superhighways. Alphanumeric route signs, such as I-95, can be recast as alphanumeric genetic markers. In the case of the Y chromosome, for instance, cross the Bab el Mandeb on highway (genetic marker) M168, which becomes M89 when heading north through the Arabian Peninsula. Make a right at M9 and set out toward Mesopotamia and beyond. Once reaching an area north of the Hindu Kush, turn left onto M45. In Siberia, go right and follow M242 until it eventually traverses the land bridge to Alaska. Pick up M3 and proceed to South America.

Mitochondrial DNA and the Y chromosome remain powerful analytical instruments. The National Geographic Society, IBM and the Waitt Family Foundation have joined in a privately funded $40-million collaboration through 2010, research that is primarily devoted to using these tools. With the help of 10 regional academic institutions, the so-called Genographic Project is gathering DNA from up to 100,000 indigenous people worldwide. “What we’re focusing on is the details of how people made the journeys,” says Spencer Wells, who heads the project. In a recent report its researchers found that the Khoisan people of southern Africa remained genetically separate from other Africans for 100,000 years. In another study, they demonstrated that some of the gene pool of Lebanese men can be traced to Christian Crusaders and Muslims from the Arabian Peninsula.

Power Tools
Genetic researchers have sampled the DNA of many people living along the migratory routes they have discovered. Yet the seeming certainty of the data sometimes deceives. Scientists who study human origins still would prefer a fossil they can hold in their hands over a genealogical tree. DNA differs from the radioactive isotopes used to date fossils. The rate of mutation can fluctuate from one stretch of DNA to another.

But paleoanthropologists are in a fix. Fossil remains are rare and too often incomplete. The earliest migration from Africa to Australia shows up in mitochondrial and Y genetic material (thanks to Andaman Islanders, among others), but the physical artifacts are largely missing along the route.

The answer to the absence of stones and bones: more DNA, from wherever. To bolster the case for genetics, researchers have looked to microbes that have hitched a ride on humans, inspecting their genes to look for similar patterns of migration. Freeloaders include bacteria, viruses and even lice. Besides microorganisms, the Human Genome Project and related efforts to look across the expanse of whole genomes have yielded a set of power tools that are helping to compensate for deficiencies in genetic methods. “You can look at so many different places in the genome from many individuals and in many populations to achieve more statistical power in testing different hypotheses,” says Tim Weaver, a professor of anthropology at the University of California, Davis.

During this decade, researchers have made dramatic discoveries by simultaneously comparing a multitude of variable, or polymorphic, sites interspersed throughout the genome’s three billion nucleotides. The first whole-genome studies earlier in this decade looked at differences among populations in short repetitive stretches of DNA known as microsatellites. More recently, the scope afforded by whole-genome scans has widened further. In February two papers, one in Science, the other in Nature, reported the largest surveys to date of human diversity. Both examined more than 500,000 single nucleotide polymorphisms (SNPs)—swaps of one nucleotide for another at a particular spot in the DNA—from the Human Genome Diversity Panel. These cell lines were drawn from about 1,000 individuals from 51 populations worldwide and are maintained by the Center for the Study of Human Polymorphisms in Paris.

The two research teams analyzed the wealth of data in various ways. They compared SNPs directly among distinct populations. They also looked at haplotypes, blocks of DNA containing numerous SNPs that are inherited intact through many generations. The group that wrote the Nature paper also explored a new technique for surveying human variation by comparing repetitions or deletions of DNA stretches of up to 1,000,000 nucleotides long (copy number variations) throughout a person’s genome, consistent with the larger trend to mine the genome for ever more markers of variation. “Any one piece of the genome will have a history that doesn’t necessarily reflect the ancestry of the genome as a whole,” says Noah A. Rosenberg of the University of Michigan at Ann Arbor and lead author of the Nature paper. But looking at many areas at once, he explains, can overcome that problem: “With thousands of markers, it’s possible to determine the overall story of human migrations.”

Looking at hundreds of thousands of SNPs allowed the researchers to resolve the identities of individual populations—and to see how genetically close relations spread far and wide. Native South American ancestry was tracked back to Siberians and some other Asians. The Han people, China’s principle ethnic group, has distinct northern and southern populations. Bedouins are related to groups from Europe and Pakistan as well as the Middle East.

The findings, which jibed with previous research from anthropology, archaeology, linguistics and biology (including previous mitochondrial and Y DNA studies), also provided a broader statistical foundation for the out-of-Africa hypothesis, supporting the idea that a small population of humans moved out of the continent, then grew in size in a new home until another subgroup of “founders” broke off and moved away—a process that repeated itself until the entire world was settled. These wayfarers edged out archaic human populations—Homo neanderthalensis and Homo erectus—with little or no interbreeding when they met. The new DNA work indicates that each time a smaller group split off, it carried only a subset of the genetic diversity originally present in the African population. So as distance (and time) removed from Africa lengthens, diversity diminishes, providing a means to follow population movements. Native Americans, sojourners on the last major continental migrations, have much less variety in their genomes than Africans do.

Many scientists believe that the weight of evidence, now backed by large statistical analyses such as the ones in Science and Nature, gives the out-of-Africa proponents a clear edge in a long-running debate over human origins. The multiregional hypothesis—a competitor to the out-of-Africa one—argues that populations that descended from archaics, such as H. erectus, evolved over the past 1.8 million years in Africa, Europe and Asia, and gradually emerged as Homo sapiens. Occasional interbreeding ensured that the groups did not split off into separate species.

Source: http://www.sciam.com

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