1491_ New Revelations of the Americas Before Columbus - Charles C. Mann [79]
All human beings have two genomes. The first is the genome of the DNA in chromosomes, the genome of the famous human genome project, which proclaimed its success with great fanfare in 2000. The second and much smaller genome is of the DNA in mitochondria; it was mapped, to little public notice, in 1981. Mitochondria are minute, bean-shaped objects, hundreds of which bob about like so much flotsam in the warm, salty envelope of the cell. The body’s chemical plants, they gulp in oxygen and release the energy-rich molecules that power life. Mitochondria are widely believed to descend from bacteria that long ago somehow became incorporated into one of our evolutionary ancestors. They replicate themselves independently of the rest of the cell, without using its DNA. To accomplish this, they have their own genome, a tiny thing with fewer than fifty genes, left over from their former existence as free-floating bacteria. Because sperm cells are basically devoid of mitochondria, almost all of an embryo’s mitochondria come from the egg. Children’s mitochondria are thus in essence identical to their mother’s.*14
More than that, every woman’s mitochondrial DNA is identical not only to her mother’s mitochondrial DNA, but to that of her mother’s mother’s mitochondrial DNA, and her mother’s mother’s mother’s mitochondrial DNA, and so on down the line for many generations. The same is not true for men. Because fathers don’t contribute mitochondrial DNA to the embryo, the succession occurs only through the female line.
In the late 1970s several scientists realized that an ethnic group’s mitochondrial DNA could provide clues to its ancestry. Their reasoning was complex in detail, but simple in principle. People with similar mitochondria have, in the jargon, the same “haplogroup.” If two ethnic groups share the same haplogroup, it is molecular proof that the two groups are related; their members belong to the same female line. In 1990 a team led by Douglas C. Wallace, now at the University of California at Irvine, discovered that just four mitochondrial haplogroups account for 96.9 percent of Native Americans—another example of Indians’ genetic homogeneity, but one without any known negative (or positive) consequences. Three of the four Indian haplogroups are common in southern Siberia. Given the inheritance rules for mitochondrial DNA, the conclusion that Indians and Siberians share common ancestry seems, to geneticists, inescapable.
Wallace’s research gave Pena a target to shoot at. Even as the Brazilian government was wiping out the Botocudos, some Brazilian men of European descent were marrying Botocudo women. Generations later, the female descendants of those unions should still have mitochondria identical to the mitochondria of their female Botocudo ancestors. In other words, Pena might be able to find ancient American DNA hidden in Brazil’s European population.
Pena had blood samples from people who believed their grandparents or great-grandparents were Indians and who had lived in Botocudo territory. “I’m looking for, possibly, a very odd haplogroup,” he told me. “One that is not clearly indigenous or clearly European.” If such a haplogroup turned up in Pena’s assays, it could write a new chapter in the early history of Native Americans. He expected to be searching for a while, and anything he found would need careful confirmation.
Since the sixteenth century, the origins of Native Americans have been an intellectual