Converging genetic and archaeological evidence hints that early migrants clung to the frigid, oxygen-starved “roof of the world” through the worst the climate could throw at them.
The first humans venturing onto the Tibetan Plateau, often called the “roof of the world,” faced one of the most brutal environments our species can endure. At an average elevation of over 4,500 meters, it is a cold and arid place with half the oxygen present at sea level. Science has long held that humans did not set foot in this alien place until 15,000 years ago, as suggested by archaeological evidence of the earliest known settlement on the northeastern fringe of the plateau 3,000 meters above sea level. But now new genetic data indicate this may have occurred much earlier—possibly as far back as the last ice age, 62,000 years ago.
“Tibetan-specific DNA sequences can be traced back to ancestors 62,000-38,000 years ago…This represents the earliest colonization of the Tibetan Plateau,” says Shuhua Xu, a population geneticist at the Chinese Institute of Sciences’ Shanghai Institutes for Biological Sciences. Xu’s work was published in September in the American Journal of Human Genetics, and presented at the American Society of Human Genetics’ annual meeting in Vancouver. Since that initial migration, as the ice age tightened its grip on the plateau, genetic mixing between Tibetans and non-Tibetans probably ground to a halt for tens of thousands of years—suggesting that movement into Tibet dropped to the minimum. “The migration routes were probably cut off by ice sheets,” Xu says. “It’s simply too harsh even for the toughest hunter-gatherers.”
The study, Aldenderfer adds, “also provides fine details of how different populations from various directions may have combined their genes to ultimately create the people that we call Tibetans.” The data show that 94 percent of the present-day Tibetan genetic makeup came from modern humans—possibly those who ventured into Tibet in the second wave of migration—and the rest came from archaic hominins such as Denisovans, Neandertals and unknown groups. The modern part of the Tibetan genome shares 82 percent similarity with East Asians, 11 percent with Central Asians, and 6 percent with South Asians. “Among all ethic groups, Han Chinese are most closely related to Tibetans,” Xu says.
The findings also reveal a startling genetic continuity since the plateau was first colonized 62,000 years ago. “This suggests that Tibet has always been populated—even during the toughest times as far as climate was concerned,” Xu says. That idea contradicts the commonly held notion that any early plateau dwellers would have been eliminated during harsh climate intervals such as LGM and another period known as the Younger Dryas between 12,900 and 11,600 years ago, says David Zhang, a geographer at the University of Hong Kong, who was not involved in Xu’s research.
Inuit who live in Greenland experience average temperatures below freezing for at least half of the year. For those who live in the north, subzero temperatures are normal during the coldest months.
Given these frigid conditions, anthropologists have wondered for decades whether the Inuit in Greenland and other parts of the Arctic have unique biological adaptations that help them tolerate the extreme cold.
“As modern humans spread around the world, they interbred with Denisovans and Neanderthals, who had already been living in these different environments for hundreds of thousands of years,” said Rasmus Nielsen, a professor of integrative biology at the University of California, Berkeley and an author of the paper. “This gene exchange may have helped some modern humans adapt to and conquer new environments.”
The new study follows earlier research by Dr. Nielsen and colleagues, which found genetic mutations that might help the Inuit metabolize unsaturated fatty acids common in their diet of whales, seals and fish.
In this study, Dr. Nielsen’s team focused on another distinct region in the Inuit genome, which seems to affect body fat distribution and other aspects of development. The researchers compared the genomes of nearly 200 Inuit with genomes of Neanderthals, Denisovans and modern populations around the world.
Strikingly, all of the Inuit studied contained the same genetic variants in this particular region of their genomes. Compared to the same region in Neanderthals and other modern populations, the Inuit region showed at most a partial match. But compared to the Denisovan genome, it “was almost a complete match,” Dr. Nielsen said.
The region in question contains genes that may play a role in dictating levels of brown fat, a type that is abundant in newborns and generates heat by burning calories. Researchers have studied brown fat for years as a possible target for obesity treatments.
It would be interesting to compare the Inuit and Tibetan genomes to see if they have similar matches to the Denisovan
Researchers from the Max Planck Institute excavate the East Gallery of Denisova Cave in Siberia in August 2010. With ancient bone fragments so hard to come by, being able to successfully filter dirt for the DNA of extinct human ancestors can open new doors, research-wise.
Imagine being able to collect the DNA of a human ancestor who's been dead for tens of thousands of years from the dirt on the floor of a cave. Sounds fantastic, but scientists in Germany think they may be able to do just that. If they're successful, it could open a new door into understanding the extinct relatives of humans.
Most ancient DNA is extracted from bones or teeth. Matthias Meyer of the Max Planck Institute for Evolutionary Anthropology in Leipzig says you don't need very much of the bone; less than a thousandth of an ounce will do.
But there's a problem. Anthropologists hate to give away any of their precious bones.
He and his colleagues began to wonder if maybe they didn't need an intact bone at all. Many of these interesting bones come from caves. What if, over the millennia, some of the bones had just degraded into a kind of dust, and fallen to the floor of the cave. It would be easy enough to get at that dust.
"You just take a shovel with some dirt, and then you look for DNA," says Meyer. He says other scientists have recovered DNA from a variety of species from the floors of caves.
Meyer now has some of this ancient human DNA from cave floors, and he's been able to begin analyzing it. But there are problems to solve before he can make sense of the data. He'll have to develop methods to be certain that the DNA came from an ancient human bone, and not a more recent human cave explorer or some contaminating bacteria. And the DNA they'll get will be in tiny snippets. Piecing together the big picture will be tricky.
Then we get to see how that DNA compares with the Greenland and Tibet samples.