The Americas are a big place, but the Native American group that first settled it was small — just about 250 people, according to a new genetic study.

These people, known as a founding group because they "founded" the first population, migrated from Siberia to the Americas by about 15,000 years ago, said study co-lead researcher Nelson Fagundes, a professor in the Department of Genetics at Federal University of Rio Grande do Sul, in Brazil.

Figuring out the size of founding groups is key, because it determines the amount of genetic diversity that gets passed on to the group's descendants, Fagundes said. [In Photos: Human Skeleton Sheds Light on First Americans]

That, in turn, could alter how effectively natural selection weeds out bad genes, Fagundes said.

"Large populations have very efficient selection, while in small populations, mildly deleterious alleles [versions of genes] can spread, which may increase genetic susceptibility to some diseases," Fagundes told Live Science in an email.

To investigate the size of the original Native American founding group, Fagundes and his colleagues studied DNA samples from 10 Native American individuals scattered across Central and South America, 10 people from different Siberian groups and 15 people from China. (The Native American groups included the Aché of Paraguay; the Bribri, Guatuso and Guaymi of Costa Rica; the Lengua of Argentina; The Quechua of Peru; and the Arara, Waiwai, Xavante and Zoró of Brazil.) The researchers didn't include Native Americans from North America for the simple reason that many of them formed unions with people from later migrations, which would make the original founding group more challenging to pinpoint, Fagundes said.

Once they had the individuals' DNA, the researchers looked at nine regions, each containing about 10,000 base pairs, or letters, on each person's genome.

Researchers know that genetic variation within a sample (such as Native Americans) is directly related to population size, Fagundes said. That, combined with the fact that genetic divergence between two populations (such as the Native Americans and Siberians) increases with time, allowed the researchers to plug the DNA data into computer simulation models and work backward to figure out the original size of the founding group.

The models found that between 229 and 300 people were in the original group, which led to the final estimate of 250 people, the researchers said. This number is so small, it would have created a "genetic bottleneck," meaning there was little genetic variation associated with the first major migration wave into the Americas, Fagundes said.

However, so much time has passed since that original group arrived in the Americas, that Native Americans as a whole have had time to recover their genetic diversity through new genetic mutations, he noted. Moreover, some Native Americans in North America formed unions with people from later migrations, which also increased genetic diversity, Fagundes said.

Just a guess

It's important to note that the 250 number is just an estimate, Fagundes said.

"One must keep in mind that it is very hard (not to say impossible) to estimate how many real individuals correspond to this figure of about 250 effective individuals," Fagundes wrote in the email.

Even so, the estimate is similar to the findings of other studies. "This bottleneck probably involved less than 1,000 effective individuals, even though lower values (say between 150-700 effective individuals) seem more likely," Fagundes said. "There have been some even lower estimates around, but our data doesn't support them." [Genetics by the Numbers: 10 Tantalizing Tales]

Estimating the size of the genetic bottleneck is important because it helps scientists figure out how many genetic markers are needed to capture the genetic diversity of Native American populations in studies, as well as to evaluate how harmful or beneficial different versions of genes are in this population, the researchers said.

The genetic data illustrates how ancient migration unfolded in the Americas, said study co-researcher Michael Crawford, a professor of anthropology at Kansas University.

Native Americans would settle in a new place, and as the population — and thus, fertility — grew, people from one population would break off and form another population in a neighboring area, Crawford said. "After 15,000 years, you can put them all the way down in Argentina," Crawford said in a statement.

The study was published May 1 in the journal Genetics and Molecular Biology.

Original article on Live Science.