An investigation into genetic changes in two generations of families has found humans are evolving slower than previously thought.

Credit: Brett Eloff / AFP

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PARIS: Humans may be evolving a third as slowly as commonly thought, according to a discovery that revolutionises the timescale we use to calculate the number of generations separating us from other species.

The genetic code comprises six billion nucleotides, or building blocks of DNA, half of which come from each parent. Until now, the conventional theory among scientists was that parents each contribute between 100 and 200 mutations in these nucleotides. But the new study says far fewer mutations occur and each parent hands on an average of 30.

“Our genetic study, the first of its kind, shows that actually much fewer mistakes – or mutations – are made,” said co-lead author Philip Awadalla of the University of Montreal's Faculty of Medicine and Director of CARTaGENE. "In principle, evolution is happening a third as slowly as previously thought.”

Affects chronology of evolution

As mutations play a key role in the evolutionary process, geneticists will now revise the number of generations separating us from genetic relatives such as apes.

The discovery came from a painstaking look at the genomes of two families, each comprising a mother, a father and their child. The study, published in Nature Genetics, breaks new ground although its sample size is very small.

If confirmed on a wider scale, it will have a bearing on the chronology of evolution. It would change the way we calculate the number of generations that separate Homo sapiens from a primate forebear who is also the ancestor of the apes.

Who contributes more mutations?

In another first, the findings enabled the team to determine whether men contribute more mutations to their offspring than women.

The theory is that because mutations are made during cell division and DNA replication, and males produces many millions more gametes (sperm) than women (eggs), more mutations would come from men compared to women. In one of their families, men contributed six times as many genetic errors to their children.

This is because mutations occur during cell division and DNA replication, and thus are much likelier to happen in sperm, for which many millions are made, than in eggs.

In one of the families, 92% of the changes were derived from the father. But in the other family, only 36% of the mutations came from the paternal side.

Rethink on inherited disease

"The mutation rate is extremely variable from individual to individual or (...) some people have mechanisms that reduce the likelihood of mutations," concluded Awadalla.

This variability could prompt a rethink on predicting the risk of inherited disease, caused by flawed genes bequeathed by one or both parents.

Some individuals might be at risk of misdiagnosis of a genetic disease if they have a higher natural mutation rate than the benchmark rate, he suggested.