jueves, 4 de junio de 2009
Ancient Argentinian skeletons
Ancient Argentinian skeletons may help resolve a raging anthropological debate: whether or not early Americans came from a single original population.
“We don’t know how people got to the New World, when, or who they were,” said anthropologist Judith Habicht-Mauche at the University of California, Santa Cruz. “Those questions are very much up for grabs right now and very controversial.”
The controversy centers around two conflicting sets of data. Studies of skull shapes noted that people in South America 14,000 years ago looked different from the people that were there 8,000 years ago and from modern Native Americans. Some anthropologists think that means there were at least two migrations to South America. The first group, Paleoamericans, had long narrow skulls and small eye sockets and was closely related to Northeast Asians. The second, Amerindians, had short broad faces, larger eye sockets, and was related to Southeast Asians.
But the molecular data disagrees. Studying modern people’s mitochondrial DNA, which is inherited from the mother, suggests all the Native Americans in South America split off from a single Northeast Asian group that migrated over about 15,000 years ago.
Now for the first time, anthropologists have put the same bones to both tests. The verdict: The DNA is right. There was a single ancestor, at least for the part of Argentina they studied. The work, done by a group from the Universidad Nacional de La Plata in Buenos Aires, Argentina, was published in PLoS ONE Friday.
The team studied 8,000-year-old bones from an archaeological site in east central Argentina called Arroyo Seco 2. They also used bones from three different historical periods found in other nearby sites to see how skull shape and mitochondrial DNA changed over time.
They ran the skulls through standard statistical analyses to compare their shapes to each other and to modern humans. They focused mostly on facial features, which are thought to change less through the generations than other parts of the body.
“Facial structure is not as influenced by new environmental things, like cold or diet,” said anthropologist David Smith of the UC Davis. “With facial data, you’re more on solid ground.”
To extract the DNA, they subjected the bones to some rough treatment. To keep them from being contaminated with modern genetic material, the researchers soaked bones and teeth in hydrochloric acid, irradiated them with ultraviolet light, sand-blasted them, and powdered them in liquid nitrogen. These are mostly standard procedures for DNA extraction, but very few researchers had used them on such old bones.
“It’s really hard to extract mitochondrial DNA from old skeletons like that, and we have very few of them,” Haibcht-Mauche said. “You probably couldn’t do this in the United States. People aren’t going to let you do destructive stuff on the oldest bones in North America.”
They found that, even though older skulls and newer skulls still looked different, they shared the same genetic markers. This lends support to the idea that these ancient Argentinians had a single common ancestor.
“What that does is, it drops out an idea. These guys who have different cranial features, these very earliest guys, they’re not coming from a separate genetic pool,” Haibcht-Mauche said. “Paleoamericans were the original guys.”
The group discussed several explanations for the difference in face shapes, including evolution in response to changes in climate and diet. If the original population that entered South America was small before fanning out across the continent, the resulting groups of people could look very different while still being genetically related.
“Morphology is much more responsive to environmental pressure and selective pressure,” Smith said. “With selection driving many different genes that affect the same feature, you can get very very rapid morphological change.”
“It always surprised me how when anthropologists measure the physical shape of these skulls and compare them to DNA and find they’re different, they tend to ignore the possibility of evolution happening,” said Nate Dominy, an anthropologist at UC Santa Cruz. “This paper basically calls attention to this and says ‘Yeah, people are going to evolve to fit their own environmental circumstances.’”
But anthropologists who study skull shapes caution against seeing DNA as a smoking gun.
“A misconception people have is that DNA will give you the truth, and anything else will give you an approximation of the truth,” said Christopher Stojanowski of Arizona State University. “But different types of DNA might not give you the same answer.” For instance, mitochondrial DNA and Y-chromosome DNA each only carry genetic information from one parent. Studying only one of them only tells half the story.
“It really is important to continue with both types of research,” Stojanowski said. “It’s important to embrace disparities that arise when you do different types of data analyses, rather than assume that this indicates necessarily that one is wrong.