Two big genetic studies confirm theories that modern humans evolved in Africa and then migrated through Europe and Asia to reach the Pacific and Americas.
The two studies also show that Africans have the most diverse DNA, and the fewest potentially harmful genetic mutations.
One of the studies shows European-Americans have more small mutations, while the others show Native Americans, Polynesians and others who populated Australia and Oceania have more big genetic changes.
The studies, published in the journal Nature on Wednesday, paint a picture of a population of humans migrating off the African continent, and then shrinking at some point because of unknown adversity.
Later populations grew and spread from this smaller genetic pool of founder ancestors — a phenomenon known as a bottleneck.
Populations that remained in Africa kept their genetic diversity — something seen in many other studies.
"The one thing that I think we cannot say from this study is that any one person's genome is any healthier or evolutionarily fit than another person's genome," said Carlos Bustamante of Cornell University in New York, who worked on one study.
"You have to think of this at the population level," Bustamante said in a telephone interview.
Fit or extinct
Bustamante's team has been looking at the DNA sequences of 15 African-Americans and 20 European-Americans, examining tiny one-letter changes in the DNA code called single-nucleotide polymorphisms or SNPs (pronounced "snips").
They tested these changes to qualify them as benign, or potentially affecting genes, amino acids and eventually proteins in a way that could damage health or make people less "fit" — in evolutionary terms, less likely to survive and reproduce.
"Like every other study ... the African-American panel as a whole showed more variation than the European-American panel," Bustamante said.
Then his team did a computer simulation of a bottleneck, and found it predicted this pattern.
Bustamante said it is possible some of the SNPs are beneficial, and he said his team and others should compare the genetic changes they found to known genetic changes linked with diseases.
"I wish we had done that (already)," he said.
In the other study, Noah Rosenberg and colleagues at the University of Michigan and the National Institute on Aging analyzed DNA from 485 people around the world.
They looked for three types of genetic variation, including SNPs and larger changes that involve duplications, deletions and repetitions of large segments of DNA.
The patterns they found produced what they call the highest-resolution map yet of human genetic variation.
They also reinforce the idea that humans originated in Africa, then spread into the Middle East, followed by Europe and Asia, the Pacific Islands and finally to the Americas.
"Diversity has been eroded through the migration process," Rosenberg said in a statement.
People of African descent are the most genetically diverse, followed by people from the Middle East, and then Asians and Europeans. Native Americans resemble one another the most on a DNA level.
The study also found it is sometimes possible to trace a person's ancestry to a small group within a geographic region.