The authors gathered evidence from other studies that these three genes are actively transcribed and translated into proteins, but furthermore, they needed to show that the corresponding DNA sequences in other primates are inactive.

They found that these DNA sequences in several species of apes and monkeys contained differences that would likely disable a protein-coding gene, suggesting that these genes were inactive in the ancestral primate.

The authors also note that because of the strict set of filters employed, only about 20 percent of human genes were amenable to analysis.

Therefore they estimate there may be approximately 18 human-specific genes that have arisen from non-coding DNA during human evolution, said a Trinity College release.

What are the proteins encoded by these genes doing? 'They are unlike any other human genes and have the potential to have a profound impact,' McLysaght noted.

While these genes have not been characterised yet and their functions remain unknown, McLysaght added that it is tempting to speculate that human-specific genes are important for human-specific traits.

The study was published online in Genome Research.