Under what conditions would humanoids evolve eyes in the backs of their heads?
Many creatures - humans among them - have binocular vision, where two eyes side by side allow good depth perception. There are quite a few other advantages over one eye, including a larger field of view, as well as redundancy (an injury leading to blindness in one eye won't cripple the creature's sight entirely).
I want to develop an Earth-like world where a group of humanoid creatures evolve with a sort of double binocular vision, with eyes in both the front and backs of their heads. Besides this, they would have relatively human-like cranial structure - only one nose, one mouth, one set of ears, and a fairly normal brain. The eyes in the back are perhaps slightly less developed than the front ones, but still quite functional.
I'm assuming that this sort of adaptation occurs in many other creatures in this world that would otherwise have normal binocular vision. An extra set of eyes requires a more complicated system of nerves and muscles, and it seems likely that this evolved quite slowly over time. Therefore, I'd bet that more than one species has evolved with this sort of property.
My question, then, is this: Under what sort of relatively Earth-like conditions would humanoids develop eyes in the backs of their heads? If you think there's no possibility for this to happen, then an answer supporting that would be, in my opinion, valid (and helpful!).
1 answer
You need to constantly threaten those humanoids from the back - for example big silent owls and manual tasks that require a lot of time
The eyes need to provide an advantage for these humanoids, so there needs to be something they excel at because of their eyes at the back. The first thing that comes to mind are very silent predators.
If your humanoids are gathering something, for example herbs, and this process takes quite a long time predators could attack them. Normally we have our ears to detect stuff like that, so the next thing would be to make sure that whatever is attacking is silent. Owls are exceptionally quiet:
feathers adapted for silent flight
Imagine your humanoids needing to farm something that is rooted quite deep in the ground. And it needs to provide a lot of nutrition, like Bonegrass seeds, so it's incredibly valuable to get that stuff. Now while your humanoids are concentrating on un-rooting those yummy seeds a bigger owl attacks them. These are adapted owls that normally attack in groups or are so large that they can kill one of your humanoids.
The humanoids won't hear them coming - but they can see them. And if their eyes are adapted well enough they might see them soon enough to be able to run away and survive the attack. That might mean that these eyes are better at detecting something in dim light or darkness - maybe they have far more rod cells, which work better in dim light, than we and far less cone cells, which are used for color vision, to make place for the rods. So the back-eyes are colorblind, but can detect something coming even at night when owls are more active.
If they are following the humanoids the eyes again show how they are better than anything else - something attacking from behind can be evaded, even if you can't hear it.
This might also have other impacts/implications/advantages on/for your humanoids. To quote a comment from Bellerophon:
These humanoids would possibly also be less social than humans. If you are a social creature then the solution would be to have a lookout while you collect the yummy seeds. These humanoids seem to be adapted to be able to work alone so are probably solitary or live in very small groups.
You can go searching for these seeds far easier alone if you already have someone to watch your back with you - your back-eyes. Without those you would need another human being.
Collecting seeds faster is a good way to outplay other humanoids that need to send at least two huamnoids where your species only needs to send one.
0 comment threads
0 comment threads