Penile spines, which are still present in several modern animals, are usually small barbs of keratin—a type of hard tissue—that line the outside of the organ.

The prehistoric male enhancement existed in the common ancestor of chimpanzees and humans, which lived about six million years ago, according to the gene analysis.

But the "penile spine enhancer" code disappeared from human genes before our common ancestor split into modern humans and Neanderthals about 700,000 years ago, said study co-author Gill Bejerano, a developmental biologist at Stanford University in California.

(See pictures: "'Torture' Phalluses Give Beetles Breeding Boost.")

In total, the scientists found at least 510 DNA "deletions" that have occurred during human evolution by comparing the human