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Humans have evolved to look physically different from each other because our eyes play such an important role in social interactions, research suggests.

Most animal species use smell or sound to identify each other - but people rely primarily on sight to differentiate individuals.

Many animals are visually almost indistinguishable from each other, particularly those which are active at night.

Researchers from California compared DNA from around the world. They found genetic regions controlling facial features were more varied than those influencing other body parts. This indicated that facial variation in humans (left) has an evolutionary advantage, compared to other more identical-looking species (right)

But humans have developed to be physically unique, according to the new research, giving rise to a wide variety in hair colours, eye pigmentation, height and facial features.

Behavioural ecologist Dr Michael Sheehan, from the University of California at Berkeley, 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. 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 Professor Michael Nachman 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.

Facial traits were also independent of each other, unlike most body measures. People with longer arms tend to have longer legs, according to the research. But individuals with wide noses or widely spaced eyes do not have longer noses, for example. These charts show the correlation between width and height of hands and noses

The scientists also used 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 Prof 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.'