Researchers from Stanford University found hundreds of sequences still present in chimpanzees and other species, but absent in the human genome.

Credit: Gill Bejerano

The penis of a Bean weevil, Callosobruchus analis, is spiked, and researchers have found that each act of mating scars the female beetle's reproductive tract. Humans, on the other hand, appear to have lost their penile spines due to a crucial deletion of genes.

Credit: Johanna Rönn, Uppsala University

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CAMBRIDGE: Two fundamental aspects of being human - a spineless penis and an expanded brain - may result from genes dropped during evolution, researchers suggest.

Traditional developmental studies have largely focused on genetic gain in human evolution. The study by two researchers at Stanford University in California describes how the deletion of genes allowed human-specific traits to flourish, emphasising evolutionary loss to be a fundamental component of evolutionary gain.

“We're beginning to understand some of the steps that underlie the evolution of human traits,” said David Kingsley, a developmental biologist at Stanford and co-author of the paper published today in Nature. “We're discovering the molecular basis of what makes us human.”

Genetic deletions in human history

Humans differ from other animals in many biological aspects, yet the genetic basis of most human-specific traits is still a mystery. We know that, for instance, the increased size of the cerebral cortex is unique to humans, but we don't know why.

The advent of genome sequencing has made comparison with other species possible. In particular, it has been speculated that comparing alterations to regulatory genes in humans - as compared to, say, chimps - might produce interesting and novel results.

This is exactly what Gill Bejerano, David Kingsley and colleagues sought to discover. They used a computational, comparative approach to identify human-specific deletions of functional DNA.

The duo found hundreds of sequences still present in chimpanzees and other species, but absent in the human genome. Many of these deleted sequences were 'regulatory' - in other words, they inhibited the expression of nearby genes.

Penile spines and monogamy

“We tried to pick the genomic deletions that we suspected to play along in recent human evolution,” said Bejerano, co-author of the paper and a developmental biologist at Stanford University, near San Francisco. “We found each one of these deletions to have their own fascinating story.”

The researchers found two human-specific deletions to be the most fascinating: one removed a neural regulator, a loss correlated with the expansion of certain brain regions. The other eliminated the keratin 'spine' - the structural component of hair and nails - in the human penis.

Interestingly, the lack of penile spines tends to be associated with monogamous behaviour in primates, as it increases the duration and consistency of copulation behaviour, possibly due to decreased penile sensitivity.

This suggests that our ancestors may have evolved physiological - not simply behavioural - characteristics associated with monogamy and increased parental care.