Sometimes, catching a disease is more complicated than you'd think.

Penn State scientists have tracked several conditions in humans from different areas of the world to try to pinpoint how and when humans, who may have a single common ancestor, became distinct populations, said Mark Shriver, associate professor of anthropology and genetics and head researcher of the team.

The researchers used the theory of natural selection to find several links between populations, Shriver said.

Natural selection is the passing down to offspring of beneficial genes that help ensure survival of the species. Naturally, organisms that are best adapted to their environment will survive and reproduce, while the least adapted are eliminated over time.

The researchers examined the genetic coding of people from different areas of the world through admixture mapping, a process that gathers data from samples of DNA, identifies genes for certain conditions between populations, and then determines the frequency of those conditions' occurrence between the populations, Shriver said.

"If natural selection is happening, we expect natural selection to be happening across the genome," which is the map of genetic information for an entire population, Shriver said.

The Duffy gene, an extreme case of natural selection among African populations, codes for immunity to malaria, Shriver said. This gene may have been naturally selected many years ago to combat the heavy presence of malaria in Africa, he said.

The average frequency for one gene is 10 percent in a population, but the Duffy gene has a 100 percent frequency in African populations, he said.

Abigail Bigham (graduate-anthropology) said she collects samples from populations in high altitudes -- the Tibetan Plateau and Andean Altiplano -- and those at sea level -- the Han Chinese and the Nahua -- and tracks genes coding for erythrocytosis, a condition marked by an abnormal increase in the number of circulating red blood cells.

Erythrocytosis makes up for the lack of oxygen in higher elevations, a trait favorable to survival for people living in those areas, she said.

"I'm looking at high-altitude populations to understand if natural selection has affected their genomes," Bigham said.

Heather Norton (graduate-anthropology) works with human skin pigmentation. Pigmentation is caused by the presence of melanin, which blocks harmful rays from the sun -- the more melanin in your skin, the darker it is, Norton said.

Populations that exist under extreme tropical conditions are naturally selected to have higher levels of melanin, which also gives them a darker skin color, she said.

Although there have been many adaptations to different environments, they are only minor differences when compared with the entire human genome.

"There is only a small amount of variation in physical and genetic differences," Shriver said. "Biologically, there is no justification for racism."

Students can sign up for a free ancestry test conducted by the Department of Anthropology.

Largely funded by Penn State, the research also has received grants from the National Human Genome Research Institute, a branch of the National Institutes of Health, and donations from Penn State alumni.