One family, for example, is essential for our sense of smell. These genes encode 390 different kinds of proteins produced in our noses, called olfactory receptors. Each olfactory receptor has a slightly different structure, allowing it to capture a different set of molecules.

Over long periods of evolutionary time, some copied genes change drastically — so drastically, in fact, that they take on entirely new tasks.

Consider hemoglobin, which stores oxygen in red blood cells for delivery throughout the body. Scientists have found that it belongs to a family of genes that do many different things with oxygen and recent studies suggest that it evolved from proteins that grabbed extra oxygen molecules inside cells before they could do harm.

The case for gene duplications became so strong that many scientists grew convinced that it was the source of all new genes. They speculated that when life originally emerged billions of years ago, the first primordial microbes had a tiny set of genes. Those genes then duplicated over and over again to give rise to all the genes on Earth today.

But when scientists gained the ability to sequence entire genomes, there was a surprise waiting for them. They started to find genes that existed in the genome of just one species. According to the duplication theory, these solitary genes shouldn’t exist; they would have to have been copied from earlier genes in other organisms.

“They looked like perfectly normal genes, except they were only found in one species,” said Anne-Ruxandra Carvunis, an evolutionary biologist at the University of California, San Diego. “There was no explanation for how a gene could be in one species and not in other ones.”