Discussion of everything related to the Theory of Evolution.
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Hello! I am involved in academia professionally, but my knowledge of evolutionary biology is pretty rudimentary. I sometimes read about animals in my spare time, particularly arthropods, which have interested me since I was young. Anyhow, I was looking at photos of spiders today and was suddenly struck by the following question:
What is the cause of the interesting commonality of head morphology seen across different phyla? Comparing, say, arthropods with mammals, the locations of the mouth and sensory organs are more or less analogous. From a subjective standpoint I can even say that they have identifiable "faces," that as a human I - consciously or not - ascribe emotions and meaning to.
Is it simply a matter of convergent evolution, that most animals have an anterior and a posterior end, with most of the sensory apparatus arranged around the former in a more or less predictable pattern? If someone knows something more of this topic, I'd be very interested to learn about it.
Imagine a very simple creature: Something with a blind gut (one opening is mouth and anus), like a jellyfish. After the evolution of a true gut, with mouth and anus, animals gain a front and rear end (movement now is not random, but mouth-first). The most usefull and logical place for sensory organs like eyes and noses now is at the front end, close to the mouth (to see/smell what the animal is eating). So, the development of a head is a logical step. This is called cephalisation, and happens convergently in many different animal groups.
Why is it convergent and not evolved once? Well, arthropods and mammals are very very different: remember the jellyfish? what happens after that stage is crucial: mammals are deuterostoma (mouth second) and insects protostoma (mouth first): when the embryo develops from a solid ball into a blastula (a hollow sphere) into gastrula a hollow sphere with an opening), the first opening becomes the mouth in protostoma, but the anus in deuterostoma. So after the true gut evolved from the blind gut in those different lineages, which opening was to become mouth or anus was kind of random (thus the same opening became mouth in one group, anus in the other), but once it was fixed, both lineages developed eyes and noses etc to go with the mouth.
This is a nice picture for that http://en.wikipedia.org/wiki/File:Proto ... stomes.svg
Thanks for your response! So, the initial arrangement of sensory organs mostly has to do with eating? I am a little fuzzy on the selective nature of this... is it that animals with better-placed sensory organs are more likely to obtain food, or higher-quality food?
The generally-accepted connection is not with feeding, but with symmetry and movement - bilateral animals, with propulsion driven equally from 2 sides, wind up with a directionality and a front end. Packing sense receptors and processors there, to deal with first-contact issues, makes sense (and putting the mouth there, but that's considered more secondary). You'll also see SOME concentrations on the posterior end, to deal with things coming up behind.
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