A new study demonstrates that specific traits that distinguish humans from their closest living relatives — chimpanzees, with whom we share 96 percent of our DNA — can be attributed to the loss of chunks of DNA that control when and where certain genes are turned on. The finding mirrors accumulating evidence from other species that changes to regulatory regions of DNA — rather than to the genes themselves — underlie many of the new features that organisms acquire through evolution.
Seeking specific genetic changes that might be responsible for the evolution of uniquely human traits, Howard Hughes Medical Institute investigator David Kingsley and colleagues at Stanford University scanned the human genome for features that set us apart from other mammals. The team found 510 segments that are present in chimps and other animals but missing from the human genome. Only one of the missing segments would actually disrupt a gene; the remaining 509 affect the DNA that surrounds genes, where regulatory sequences lie.
Careful analysis of a handful of these segments demonstrated that loss of regulatory DNA could explain how humans developed some features not found in other animals — such as big brains — as well as how they lost features common in other species, such as sensory whiskers and spiny penises. Their findings are published in the March 10, 2011, issue of the journal Nature.
Genes — segments of DNA that carry the blueprints for proteins — make up less than two percent of the human genome. Hidden within the remainder of our more than three billion base pairs of DNA are regulatory sequences that control when and where genes are expressed. Direct alterations to a gene can have dramatic effects, sometimes killing an organism or rendering it sterile.
“In contrast, if you alter the way a gene turns on or off at a particular place in development, that can have a very large effect on a particular structure, but still preserve the other functions of the gene,” Kingsley says. “That tends to be the sort of alternation that’s favoured when a new trait is evolving.”