When any cell divides, it first copies its entire genome. During this process, it can make errors. The cell usually fixes errors in the DNA. But when they aren't fixed, they become permanent changes called mutations, which are sometimes hurtful and sometimes helpful, though usually innocuous.
"There are approximately 30 genes that were selectively duplicated in humans," study researcher Franck Polleux, of The Scripps Research Institute in La Jolla, Calif., said in a statement. "These are some of our most recent genomic innovations."
An extra copy of a gene gives evolution something to work with: Like modeling clay, this gene isn't essential like the original copy, so changes can be made to it without damaging the resulting organism.