By Don Finley
San Antonio Express News
November 5, 2011
Scientists here are rewriting an early chapter of the history books that describes how and when humans first set foot on North American soil, based on clues extracted from the blood of their San Antonio descendants.
Experts agree — based on genetic evidence, bones, artifacts and the study of languages — that the ancestors of American Indians came from Northeast Asia thousands of years ago, crossing the frozen Bering land bridge, probably in pursuit of wild game.
In recent years, some experts have speculated they stopped and settled on that land mass, known as Beringia, for perhaps 15,000 years — a long span of time when four distinct ancestral groups genetically drifted away from their relatives back in Asia. Some think their path to the Americas might have been blocked by glaciers until a great thaw.
The idea of that long pause was based both on genetic studies and archaeological finds, including evidence of human settlements on the Asian side dating to about 30,000 years ago.
But in a new paper, scientists at the Texas Biomedical Research Institute argue that new genetic evidence shows those ancient explorers didn’t camp out on the land bridge for 15,000 years as believed. Instead, it was less than 5,000 years — and it’s possible they barely stopped at all.
“If they stopped, it was very brief — less than 5,000 years,” agreed Satish Kumar, a geneticist at Texas Biomed and lead author of the paper. “Otherwise it is kind of a continuous movement to America. They were diverging from Siberian-Asian ancestors, coming to America.”
Their evidence comes from genetic material found within mitochondria — tiny bodies within cells that generate energy, like power plants.
Unlike regular DNA, mitochondrial DNA is inherited only from the mother and changes mainly through normal mutations from generation to generation. Scientists have calculated how often those mutations take place, making it possible to mark time from them like rings on a tree.
Kumar and Blangero didn’t set out to rewrite history books. Instead they were studying the mitochondrial DNA of hundreds of Mexican American volunteers in the San Antonio Family Heart Study to learn more about the genetic origins of diabetes and obesity. Many scientists have speculated that the high rate of diabetes in Mexican Americans is linked to their blood ties to American Indians, who have even higher diabetes rates.
Kumar identified American Indian portions of the DNA from 215 local volunteers and combined that information with records of DNA collected by other researchers from American Indians and Asians. From those he could create timelines, following each group through the years as new families branched off from old ones and spread into North, Central and South America.
Exactly when each of the four groups crossed into the Americas isn’t clear — probably between 16,000 and 20,000 years ago at the end of a period called the Last Glacial Maximum, the researchers said. But three estimates for mutation rates have been put forward by scientists, and Kumar ran all of them for this study — coming up with a range of dates. All of them showed a stop in Beringia of less than 5,000 years.
Kumar, an expert in mitochondrial DNA, had done similar work in years past, helping to trace the roots of Australian aboriginal tribes from India through southern Asia.
The paper appeared in BMC Evolutionary Biology. BMC Evol Biol. 2011 Oct 7;11(1):293.
Large scale mitochondrial sequencing in Mexican Americans suggests a reappraisal of Native American origins.