Evolutionary Sprint Made Us Human
Many more genes separate humans from chimpanzees than scientists believed. A new study shows that what sets us apart from our closest primate cousin is the accelerated rate at which we acquire new genes and ditch unnecessary ones.
It's often said that there's only 1% to 2% difference between the genomes of chimps and humans, two species that had their most recent common ancestor about 5 million years ago. But that percentage refers to the nucleotide differences in shared genes. Evolution can do more than just tinker with gene sequences; the number of copies of a gene can also vary from one species to the next, even when the gene itself stays the same. Sometimes genes are gained, and sometimes they are lost. Quantifying this turnover has been difficult, however, because it requires the complete genome sequences of many species.
Now, with several mammals sequenced and a suite of new statistical methods available, Matthew Hahn and colleagues at Indiana University, Bloomington, have taken a closer look. They measured how quickly genes were duplicated or lost across six mammalian genomes. By looking at about 120,000 genes in 10,000 gene families, they discovered that gene turnover was faster in primates than in dogs or in rodents, and even faster in humans, who swapped genes 1.6 times faster than monkeys and 2.8 times quicker than nonprimates. Thanks to this rapid change, 6.4% of the 22,000-odd human genes aren't present in chimps, making the gap between the two suddenly seem much wider.
"You can think of the genome as a revolving door--genes keep coming and going," says Hahn, who published the findings online 18 October in the journal Genetics. He argues that the turnover provides fuel for natural selection to act upon; gene families that rapidly expanded also showed the signatures of adaptive changes in their DNA. And one gene family that stood out in particular was a group of brain genes, which more than doubled in size in humans.
The study highlights "the important role that gene turnover plays in mammalian evolution," says genome biologist James Sikela of the University of Colorado, Denver. But he cautions that the recently "finished" genome sequences that the researchers used may have so-called assembly errors in them that could skew the results. It's difficult to prove the complete absence of a gene, he warns; in addition, extra copies of genes can be missed. Although these errors could affect the rate estimates, Hahn says he tested for their effects, and he doubts that they explain the rapid acceleration seen in humans.