Look at the picture above.
And now the one below. It is low quality because it was taken from an old video. At first it may seem unremarkable, but look a little closer at the gorilla's eyes.
That's right, like us they have white around their pupils.
The white around our eyes is called the sclera. Just like you and I, this gorilla (Nadia) has no pigmentation in her eyes, which makes her sclera white.
Astonishingly the gorilla in the very top picture, taken from a photo library, has some white in its eyes too. It took me about ten minutes to find.
Nadia might be more than just an evolutionary freak
Despite how easy this picture was to find, it was firmly believed that we were the only primates with white around our eyes.
We now know that is no longer true. Juan-Carlos Gomez of St Andrews University UK, first noticed that Nadia, the gorilla he had been working with for some time, had white around her eyes.
At first he thought nothing of it, but then in 2001 two researchers, Hiromi Kobayashi and Shiro Kohshima at the Tokyo Institute of Technology in Japan, published a study looking at what makes the human eye unique (an expansion of an earlier paper on the same topic).
They compared human eyes to those of almost half all known primate species and concluded that white sclera were unique to us. However, their study only looked at four gorillas assuming they were representative of them all.
One idea is that the whiteness of our eyes is important for our social world
Gomez and his PhD student Jessica Mayhew decided to investigate further. Looking at footage from zoos and at pictures online, they quickly realised that Nadia might be more than just an evolutionary freak.
They looked at a sample of 85 gorillas from two species. Out of the 60 western lowland gorillas they considered, only 30% had completely dark scleras. The remaining 70% had some degree of white in their eyes. Of these, a small sample of 7% had all white, human-like sclera, just like Nadia and Bana, below.
This trait does not seem to offer the gorillas any advantage or disavantage, Gomez says.
For humans on the other hand, one idea is that the whiteness of our eyes is important for our social world. "This is why it was selected to become a robust trait common to all humans."
The theory, called the cooperative eye hypothesis, proposes that the advantage of having white sclera helps us to follow the gaze of others more easily. White is a more conspicuous colour than black, so when we fix our gaze to something, our friends can easily do so too.
As we evolved, this helped us develop an almost instant understanding of each others' intentions, without speaking a word.
When I spoke to Michael Tomasello, of the Max Planck institute for Evolutionary Anthropology in Leipzig, Germany, earlier this year, he told me that's why we have terrible poker faces. Our eyes can give our intentions away.
"It's to my advantage that you know where I'm looking so we can coordinate better. That the eyes are advertising where I'm looking is a fundamentally cooperative adaptation," he says.
Only recently, another study looking into the importance of our eyes in our social world found that humans have an instinctive preference for white eyes, even if those eyes are found on stuffed toys. This work once again emphasised that white sclera are unique to us.
We can now add this to yet another trait that we share with animals, as BBC Earth has extensively featured previously.
Our white eyes cannot therefore be the most important change that helped us to cooperate better, Gomez says. Instead, he believes it has to do with the shape of our eyes. We have horizontally elongated eyes, which are less rounded than ape eyes. That's what makes the human eye better designed to show where we are looking, Gomez believes.
It's known that most, if not all, primates use their eyes to communicate. "Many can use eye contact not only in aggressive interactions, but also in play or friendly interactions," Gomez says.
But humans have gone several steps further. Not only do we use our eyes to communicate in much of our social interaction, we even use emoticons to communicate virtually.
White sclera may be found in other primates too. Mayhew discovered other apes including chimpanzees, bonobos and orangutans also show a differing degree of whiteness in their sclera.
Though not as white as ours, this shows that they, and our common ancestor, may have had a variation of the gene needed for white eyes. The change to the all-white scleras that humans have, must therefore have been gradual rather than sudden, Gomez says.
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