Lost DNA led to spineless penis, big brains

Big brains and a softer penis are the result of the things we have lost on the evolutionary road, U.S. researchers reveal.

In a paper published Thursday in Nature, the Stanford University researchers show the differences between humans and our nearest relative, the chimpanzee, are the result of DNA sequences lost since our divergence from a common ancestor about 6 million years ago.

Women of the world will be particularly grateful that among the ditched DNA is the penile spine enhancer, which gives chimpanzees spiky-spined penises.

Co-author Professor David Kingsley, of the Howard Hughes Medical Institute at Stanford University, says the loss of the "intimidating" penile spines could be a factor in humans developing monogamous relationships.

It is believed in chimpanzees and other mammals penile spines are used, among other things, to sweep away competing sperm.

"Organisms that have penile spines generally have a competitive mating system," says Kingsley.

"Females advertise when they are fertile and females are only sexually receptive around ovulation.

"In humans ovulation is hidden in females and the female in our species is sexually receptive for an extended period of time. [The loss of the penile spines] is part of a whole suite of changes related to longer-term bonding and not brief encounters."

Chimps have it, humans don't

The findings are based on a comparison between the genomes of modern humans and our nearest relative, the chimpanzee, and the macaque.

The researchers identified 510 DNA segments that were missing in humans, but "highly conserved" in the chimp and macaque.

All but one of these was found in the non-coding regions of the genome — between genes — which help control and co-ordinate gene activity.

"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 alteration that's favoured when a new trait is evolving."

The researchers then used a computer analysis to identify deletions that were clustered around particular genes.

"We saw more changes than you would expect near genes involved in steroid hormone signalling," Kingsley says.

Penile spines and brain growth

A number of deletions also appeared near genes involved in neural development, their analysis revealed.

The team then used experiments with mice to find where two DNA sequences were expressed that were associated with the human androgen receptor gene and a gene called GADD45g that suppresses cell growth. They found in mice, the first gene was associated with genitalia and sensory whiskers while the latter restricted brain growth.

"I think one of the cool things about the paper is when you look at molecular loss, less can be less and you can lose an anatomical structure," says Kingsley.

"But less can also be more if you lose a switch that is the molecular difference to the expanded production of a trait."

Kingsley says the loss of the gene that kept brain cell growth in check may have contributed to the growth of bigger brains.

But he says the loss of DNA structure would be only one of the factors in evolving new traits.

Humans same as Neanderthals

Kingsley says they are continuing to work through the lost sequences to identify links with other anatomical differences such as sweat glands, walking upright, s-shaped spines and skin changes.

He says the two sequences in DNA highlighted in the paper were lost sometime between when humans and chimps split from each other 6 million years ago and before humans and Neanderthals diverged about 600,000 years ago

"Both of the losses are also lost in the Neanderthal," says Kingsley.

"The Neanderthal brain was already big and there is a lot of interest in whether Neanderthal and humans interbred - the loss of the penile spine was the anatomical change that would allow interbreeding."

Kingsley says although the modern genome is evolving, our understanding of evolution will also change the way the genome continues to evolve.

"I think humans have evolved a series of cultural adaptations that mean we are not just at the mercy of evolution - we wear glasses, we have health care, we intervene in ways that overturns the survival of the fittest.

"Evolution is now an interplay between things."