Humans may be evolving a third as slowly as commonly thought, according to an investigation into genetic changes in two generations of families. 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 changes in these nucleotides. But the new study says that far fewer changes occur. Each parent hands on 30 on average. \"In principle, evolution is happening a third as slowly as previous thought,\" said Philip Awadalla of the University of Montreal, who led the study by the CARTaGENE group. The discovery came from a painstaking look at the genomes of two families, each comprising a mother, a father and their child. The study 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. The study also challenged thinking about whether DNA changes are more likely to be handed on by the father or by the mother. The mainstream notion is that DNA changes -- known in scientific terms as mutations -- are likelier to be transmitted by the man. 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 percent of the changes were derived from the father. But in the other family, only 36 percent of the mutations came from the paternal side. \"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.