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Evolution/more eyes?


QUESTION: HI Dr. Krempels I have a quick perhaps even silly question but I am curious from a professional point of view I am currently in high school and there was a discussion on how it would feel if the human had evolved an extra set of eyes behind our backs to view what's going on.. so basically would this have been an advantage for the human species to survive and if so how about now in modern times do you think it would actually have been a distraction since the modern human being has difficulties focusing with only 2 eyes? I am guessing there would be no need to turn our heads??
thanks for your time I really hope you can share your thoughts!

ANSWER: Dear Cindy

Yes, I suppose if we had evolved eyes on the backs (and fronts) of our heads, we wouldn't need to turn around.  And it might make it easier to avoid predators (and catch cheaters, if you're a teacher).  But yes, additional eyes would require different wiring of the brain so that the signals coming in from both directions could be synthesized and interpreted in a useful way.  (Pit vipers and other snakes that can sense infrared radiation via pits or heat-sensitive labial (lip) organs process the information in the visual cortex of the brain, and essentially form a heat "image" that way.  But they seem to do this without interference from the images being processed from their eyes.)

But remember:  Organisms do not evolve things because they would be helpful or because they need them.  Mutations are random.  Some are adaptive (help the organism survive), some are maladaptive (interfere with survival), and some are neutral (neither help nor hinder survival).  Mutations that are adaptive have a better chance of being passed to the next generation.  

Vertebrates haven't evolved eyes on the backs of their heads simply because there have been no (surviving) mutations that resulted in the evolution of that trait.  Some creatures have moved their eyes around (e.g., flounders and other bottom-dwelling fish), but vertebrates still have just two eyes to work with because that's what we inherited from our ancestors.  Structures evolve from pre-existing structures.  So for eyes to evolve on the backs of our heads, something *very* strange and unlikely would have to happen.

Hope that helps.


---------- FOLLOW-UP ----------

QUESTION: wow great explanation! Just an added thought so basically even if at one point in evolution..eyes would had evolved in the back of our heads would we most likely have no use for them now? do you think that mutation would have been adaptive to the "now" modern human? thanks for your time I am sure you are a real busy person I really appreciate your time!!!

Hi, Cindy

Well, I'm not saying that a set of eyes on the back of our heads would be maladaptive.  It's kind of hard to say, since we don't have them.  There could be drawbacks (more neural circuitry is energetically expensive to grow and maintain; more eyes to injure, etc.), but there could be advantages.  There are plenty of organisms with multiple eyes (or at least simple photoreceptors) that point in different directions (e.g., scallops, spiders, starfish, etc.).  But these are the exceptions, rather than the rule.

There's really no way that a modern human could suddenly mutate and have a fully functional set of eyes on the back of his/her head.  It's just not in our genome.  So while you might write a science fiction story about a human with an extra set of eyes on the back of the head and be able to cobble together a somewhat scientific-sounding explanation, it's not really possible for it to happen.  At least, it never has.  So it's pretty much impossible to say whether such a trait would be adaptive or not.



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Dana Krempels


I can answer questions about evolutionary mechanisms and theory, including genetic drift, mutation, natural selection, etc. I also can clear up misconceptions about evolution as it's sometimes talked about by those not well-versed in the subject (e.g., some politicians and many religious fundamentalists).


I have a Ph.D. in Biology, and presently teach Evolution and Biodiversity, Genetics, Botany, and Zoology at the University of Miami.

House Rabbit Society Beta Beta Beta Biological Honor Society

Exotic DVM Magazine (veterinary journal)

B.S. in Biology B.A. in English Ph.D. in Biology

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