Researchers have delved back further than ever into the genetic history of humans, and found that the ancient population that gave rise to modern humans may have been nearly twice as genetically diverse than humans today, according a study published this week in Proceedings of the National Academy of Sciences.
While most studies on the genetics of ancient humans have focused on the last half million years, this study looks at particularly old areas of the genome, allowing the researchers to look at the more distant past, said molecular geneticist Prescott Deininger of the Tulane Cancer Center in New Orleans, LA, who was not involved in the research. “This [study] is a little window to look back a little bit further,” he said.
When examining genetic diversity, scientists often use a measure called the effective population size, which describes how big a population has to be to carry its level of genetic diversity. Modern humans have an effective population size of about 10,000 — a relatively low level of diversity. Chimps and gorillas, for example, both have effective population sizes of greater than 20,000. This estimate of 10,000 has been regarded as stable for about 200,000 to 400,000, maybe “as far back as a million” years, said population geneticist Chad Huff of the University of Utah. But looking deeper into human history, Huff and his colleagues determined that before about 1.2 million years ago, the effective population size of our ancestral populations was actually around 18,500.
The researchers gained their insight by looking at mobile elements — bits of DNA that can insert themselves into the genome — known as Alus. Occurring in an estimated 1 in 21 to 22 births, Alus, which are about 300 base pairs long and the most abundant mobile elements in the human genome, insert into the genome at a rate at least three orders of magnitude rarer than the single nucleotide mutation rate. Because of this rarity, any particular Alu is likely to be much older than an average mutation. Furthermore, because the DNA just outside of these Alu inserts is closely linked to the mobile element, that surrounding DNA is also likely to be relatively old. …