Why hasn’t evolution given us eyes in the back of our head or rear-view mirrors? (continued)
Eric Anderson, Wickford, Rhode Island, US
Advertisement
While the previous answers to this question were good, they missed something.
Many vertebrates have a third or “parietal” eye at the top of their head, including some lizards, frogs, salamanders and sharks. This eye doesn’t form images, but it is sensitive to light and dark. The parietal eye may even have developed from a more functional eye or pair of eyes that would have allowed true sight overhead or even to the rear.
“Some swallowtail butterflies have rear vision. The male can sense light with its penis to help find the right mating position”
The human pineal gland, which produces melatonin in response to light signals from our eyes, is thought to have originated from the parietal eyes of the non-mammalian ancestor that we evolved from.
Depending on how an eye is defined, it could be that scorpion fluorescence serves as a basic eye. It seems that a scorpion’s glow is a by-product of its exoskeleton’s ability to sense light – or “see” – hinting that it can tell whether it has left any part of itself visible to predators when it takes cover during the day.
Peter Jacobsen, Davis, California, US
One species of swallowtail butterfly has rear vision. The male has photoreceptors on its penis that help it position itself for mating. The female can also sense light with , probably to help her lay eggs in the right place.
Mike Follows, Sutton Coldfield, West Midlands, UK
Humans have few predators and can use our hearing and the eyes of fellow hunter-gatherers to detect many of them, so there is little evolutionary advantage to expending energy in order to have eyes in the backs of our heads.
Evolution doesn’t always find the perfect fit to the environment. But if rearward vision were advantageous, then I think it would be more likely that we would have adapted to turn our heads 180 degrees like owls.
To answer this question – or ask a new one – email lastword@newscientist.com.
Questions should be scientific enquiries about everyday phenomena, and both questions and answers should be concise. We reserve the right to edit items for clarity and style. Please include a postal address, daytime telephone number and email address.
91ɫƬ retains total editorial control over the published content and reserves all rights to reuse question and answer material that has been submitted by readers in any medium or in any format.
You can also submit answers by post to: The Last Word, New Scientist, 25 Bedford Street, London WC2E 9ES.