D: Yaël, if we want to understand how aging evolved in human beings then we need to know about the aging process in our closest relatives, the great apes.  Scientists know that female humans and chimpanzees have their last offspring at about the same age, but do we know anything else?

Y: Another part of aging is decline in eyesight. As humans get older, the lenses of our eyes get stiffer, and the muscles that change the shape of the lens for focusing get weaker.  People lose the ability to focus on nearby objects and need to get reading glasses.

D: Yes, I know, Don.  I wear reading glasses myself.  It's called farsightedness.

Y: In 2016 an international team of primatologists published a study of farsightedness in one of our closest ape relatives, the bonobo. They studied a colony of wild bonobos in Congo.

D: I suppose they trained the apes to take an eye test.

Y: Actually, they found an easier way.  Bonobos groom one another as a social activity.  They carefully examine the fur of their partner in order to spot and remove parasites, insects, and dirt.  The distance the bonobo holds its eye from its partner's fur is a measure of how good its close vision is.

D: That's brilliant. 

Y: The scientists photographed fourteen bonobos, whose ages they knew, while they were grooming.  By measuring the photos, they found that young bonobos, under thirty years old, held their eyes about four inches from their partners fur, but by age forty the distance increased to eight inches and, by forty five, to sixteen inches.

D: So, then, the eyesight of bonobos ages in about the same way as that of humans.