By Zoë Corbyn

Sex would be a very different proposition for humans if -- like some animals including chimpanzees, macaques and mice -- men had penises studded with small, hard spines.

Now researchers at Stanford University in California have found a molecular mechanism for how the human penis could have evolved to be so distinctly spine-free. They have pinpointed it as the loss of a particular chunk of non-coding DNA that influences the expression of the androgen receptor gene involved in hormone signalling.

"It is a small but fascinating part of a bigger picture about the evolution of human-specific traits," said Gill Bejerano, a developmental biologist at Stanford who led the work along with colleague David Kingsley. "We add a molecular perspective to a discussion that has been going on for several decades at least."

Published in Nature today, the research also suggests a molecular mechanism for how we evolved bigger brains than chimpanzees and lost the small sensory whiskers that the apes -- who are amongst our closest relatives and with whom it has been estimated we share 96% of our DNA -- have on their face.

Monogamous strategy

It has long been believed that humans evolved smooth penises as a result of adopting a more monogamous reproductive strategy than their early human ancestors. Those ancestors may have used penile spines to remove the sperm of competitors when they mated with females. However, exactly how this change came about is not known.

The researchers did not set out to study penile spines. Rather, they were looking for chunks of DNA that had been lost from the human genome but not the chimp genome, so they could then try to pinpoint what those chunks did.

The approach differs from that in most studies, explain Bejerano and Kingsley, in looking at what has been deleted from the human genome rather than what is present. "In the case of our study, had you started from the human genome, there would be nothing there to see," says Bejerano.

They first systematically identified 510 DNA sequences missing in humans and present in chimps, finding that those sequences were almost exclusively from the non-coding regions of the genome, between genes. They then homed in on two sequences whose absence in humans they thought might be interesting -- one from near the androgen receptor (AR) gene and one from near a gene involved in tumour suppression (GADD45G).

Inserting the chimpanzee sequences into mouse embryos revealed that the former sequence produced both the hard penile spines and sensory whiskers present in some animals. The latter sequence acted as a kind of brake on the growth of specific brain regions -- with the removal of its function appearing to have paved the way for the evolution of the larger human brain.

"The goal of the project was to find molecular lesions [losses] that underlie human evolutionary traits, with the examples illustrating different aspects of the principle," says Kingsley.

"Until we looked at where the DNA was expressed, we had no idea which switch -- if any -- it would actually control," adds Bejerano.

Other molecular biologists praised the work for its clever approach and said it would open up new avenues of inquiry, particularly for those working on the evolution of the human brain.

"As so often with very good ideas, it seems almost obvious in hindsight," said Svante Pääbo, who directs the genetics department of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and was part of the team that recently sequenced the Neanderthal genome. "Since two of the almost 500 deleted sequences they identified turn out to be interesting, I am sure that several other ones on their list will turn out to be interesting too," he added. The researchers are continuing to analyse the remaining 508 DNA sequences."It is detective work and a great reminder that, in the course of evolution, information is both gained and lost," said Sean Carroll, an expert in animal genetics and evolution at the University of Wisconsin, Madison.

David Haussler, who studies the molecular evolution of the human genome at the University of California, Santa Cruz, added that our ancestors' loss of penile spines is our gain today."Couples everywhere can be thankful that this particular piece of DNA was ditched," he says.