Scientists say that the most variable facial traits lie within the triangle of the eyes, mouth and nose.

They may be attractive or ugly, but there is a reason why human faces all look so different, research suggests.

Scientists believe we evolved individual features because of the importance of our eyes in social interactions.

Many animals use smell or sounds to identify each other, making distinctive faces unnecessary. This is especially true for animals that are active at night, say the researchers.

Behavioural ecologist Michael Sheehan, from the University of California at Berkeley, US, said: “Humans are phenomenally good at recognising faces; there is a part of the brain specialised for that.

“Our study now shows that humans have been selected to be unique and easily recognisable.

Humans are phenomenally good at recognising faces; there is a part of the brain specialised for that

“It is clearly beneficial for me to recognise others, but also beneficial for me to be recognisable. Otherwise we would all look more similar.”

The scientists, whose findings are reported in the journal Nature Communications, showed that facial traits are much more variable than body features such as the length of the hand.

Facial traits were also independent of each other, unlike most body measures.

People with longer arms tend to have longer legs.

But individuals with wide noses or widely spaced eyes do not have longer noses, for example.

Comparing DNA from around the world showed that genetic regions controlling facial characteristics were more varied than those influencing other parts of the body.

This was a clear sign that facial variation had an evolutionary advantage.

Co-author Michael Nachman, also from the University of California at Berkeley, said: “Lots of regions of the genome contribute to facial features, so you would expect the genetic variation to be subtle, and it is. But it is consistent and statistically significant.”

The most variable facial traits lie within the triangle of the eyes, mouth and nose, the researchers found.

They assessed human facial variability with the help of a 1988 US Army database of personnel body measurements used to design and size everything from uniforms to vehicles.

The scientists also had access to data from the 1000 Genome Project, which has catalogued nearly 40 million genetic variations among humans worldwide.

“Genetic variation tends to be weeded out by natural selection in the case of traits that are essential to survival,” said Nachman. “Here it is the opposite; selection is maintaining variation. All of this is consistent with the idea that there has been selection for variation to facilitate recognition of individuals.”

Facial differences appear to date back a long way, before the main human lineages split. A comparison of human genomes with those of Neanderthals and Denisovans – two extinct human sub-species – showed that all had similar levels of facial variation.