PARIS – Some defining human traits -- a penis bereft of the stiff sensory hairs common to many male mammals, a bulging brain -- come less from new genes than genetic material lost through evolution, according to a study published Thursday.

The findings suggest a new way of thinking about what sets Homo sapiens sapiens, or modern man, apart from our close evolutionary cousins, especially the chimpanzee, whose DNA overlaps with ours by 97 percent, the authors said.

Most research on this question has looked for what is genetically novel in humans, and focused on the genes themselves rather than the regulatory mechanisms that drive them.

"But we asked, 'are there functional, highly-conserved genetic elements in the chimpanzee genome that are completely missing in humans?'," said Gill Bejerano, an assistant professor at Stanford University School of Medicine and co-leader of the study.

More than 500 these deeply-rooted clutches of DNA found in chimps and their evolutionary forebear, going back millions of years, are entirely absent from human genome, the researchers found.

Most are also missing in Neanderthals, which means they dropped out the pathway leading to our species at least 500,000 years ago, before the evolutionary split with our doomed, cave-dwelling cousins.

Losing bits of regulatory DNA -- but not the genes they control -- means that the related changes in anatomy and behaviour are likely to be subtle.

But these blank spots can eventually lead to new traits, and even new species, Bejerano said.

"The current study not only identifies an intriguing list of deletions in humans, it also linked them with specific anatomical changes that are unique to the human lineage," he explained in a statement.

The study uncovered two main categories of transformation. The first affects how brain cells signal the presence of steroid hormones such as testosterone.

Which brings us to the human penis.

One bit of the missing bits of DNA drives a sex hormone responsible for the growth of sensory hairs, called vibrissae, and surface spines found on the penises of many mammals, including big apes.

The loss of these structures in humans decreases tactile sensitivity, but increases the duration of intercourse, arguably a trade-off when it comes to its impact on gratification.

The other transformative category influences development of the brain. A genetic snippet found in chimps and other early primates but, again, absent in humans activates a gene that hampered neural growth, including tumours.

It's suppression, scientists speculate, might have boosted development in our brains, especially in areas giving rise to uniquely human traits.

The resulting changes may have also contributed to monogamous bonding and the complex social structure required to raise our species' relatively helpless infants, they said.