This chimpanzee has an obvious mutation that all people but almost no other apes have: the whites of its eyes. In all her years living around chimpanzees, Jane Goodall saw only 3 such chimpanzees, 2 of which were brothers.
The apes below have normal primate eyes. The photos are from James Mollison’s magnificent series. He would distract his subjects by grooming them & pretending to pick fleas from their arms. When their attention was focussed he’d snap the picture. His web site seems to be broken; see here instead; here‘s a NYT article about another of his photo series of flamboyant variations within a single primate species.
As you of course know, the part of your eyes surrounding your pupils is called the sclerae. White sclerae make the direction of your gaze obvious to your companions, including the most faithful companion of all: dogs. Dogs are adept at following human gaze, unlike my good-for-nothing cat, who couldn’t follow directions if they were written in strips of tuna, & who won’t take out a spider for me unless I physically carry her & point her head at it. Wait, what was I talking about?
Dogs. Dogs & humans are experts at picking up on what direction someone’s looking at. Apes don’t do that so much – they focus on the direction the head is turned, instead of the eyes, presumably because they can’t easily tell which way their friend’s eyes are pointed.
So a newish theory has it that people with white sclerae, & the dogs that loved them, had a powerful advantage that lead to the spread of the white sclerae mutation throughout our population. Hunters stalking prey could silently, even unconsciously, direct their dogs. Dogs know where to rush in to flush or trap the deer, humans finish them off with spears & knives – it’s a win/win for everybody except Bambi.
Dogs & humans have been hunting cooperatively for a very long time: 30,000 or maybe even 45,000 years. It’s mutually beneficial – Miskito people of Nicaragua hunt with dogs that help capture more than 20 kilograms of meat each month, sometimes much more. In a study of Finnish moose hunting – a proxy for mammoth hunting back in the good old days – dogs increased the take by 56%. Also, dogs found 6 times as many delicious armadillos as people alone.
I’ll remember that the next time I roll my eyes at my Cat Lassie.
American Scientist explains all this in a great 4 page article by Pat Shipman called Do the Eyes Have It?