Humans have a very annoying quality: pride. And they like to praise themselves as being the most evolved species.

But a new research has found that since the human-chimp split about 4 million years ago, the chimpanzee genome evolved at higher pace that the human one. This challenges that our large brains, cognitive performances and bipedalism are the result of an intensive genetic selection.

A team at the University of Michigan has investigated DNA strings from nearly 14,000 genes common to both chimps and humans. The researchers looked for differences in each genes and if these differences induced modifications in the protein they encoded.

Genes encode an organism's proteins and different variants (mutants) of the same gene are named alleles.

Changes in DNA that make a gene affect a protein's structure are considered functional changes, while "silent" changes do not shift the protein's morphology. "If we see an excess of functional changes (compared to silent changes) the inference is these functional changes occurred because they were positively selected, because they were useful in some way to the organism," said co-researcher Margaret Bakewell.

The team discovered that much more genes in chimps evolved through the selection of the beneficial mutations than was the case with the human genes, a fact blamed on a bottleneck situation experienced by humans in their evolution: at a given moment the human population was extremely small compared to that of the chimps. "Although there are now many more humans than chimps, in the past, human populations were much smaller, and may have been fragmented into even smaller groups," said Bakewell.

In small populations, random events are more important than selection, a fact called genetic drift: a fortuitous break for one or two alleles can inflict a disproportionately higher effect on the overall genes of dwarf populations compared to vast ones.

Scientists also discovered more neutral variants in the chimp genome. "There are possibly a lot of differences between human and chimps that we don't know about, [perhaps] because there are differences in chimps that nobody has studied; a lot of studies tend to focus on humans," said Bakewell.