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I was wondering if it's an evolutionary advantage to have many sensory systems in a small place of the body, the head. This applies to mammals, reptiles, synapsid, dinosaurs... and many more.

My theories:

  • The distance to the brain is short and thus smaller time delay to respond faster to the outside world,have more reflexes, hunt faster prey and fight more accurately.
  • The short distance makes the "wire" (signal carrying) shorter and thus less vulnerable to be cut from a injury, because for a organism losing an eye reduces possibilities of survival dramatically.
  • In humans at least the head is the upper part, then vision organs see more with less obstacles such as vegetation or terrain irregularities, or anything that obstacles vision, normally higher means seeing more things. This I'm not sure applies so strongly to smell. To taste, certainly not. To hearing, possibly a little, thought nor much as vision.

We evolved from a common ancestor that was like that but that doesn't explain why it's not common to get out of that coincidence.

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  • $\begingroup$ The existence of a common ancestor is part of the answer to the question of why the layout of vertebrate sensory systems are so consistent (it's included in both answers). The identity of the common ancestor is a separate question and I think it's off-topic for your main question so it would be best posed as a separate question may with a link to this one. $\endgroup$
    – Michael_A
    Commented Oct 20, 2016 at 20:25
  • $\begingroup$ It's called "design" $\endgroup$
    – Ben
    Commented Oct 22, 2016 at 2:05
  • $\begingroup$ @BenPiper Who or what designed what? Do someone designed the organs to be all in the head, that's what you are saying? $\endgroup$
    – Santropedro
    Commented Oct 22, 2016 at 2:15

2 Answers 2

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No it's not a coincidence that eyes, nose, mouth, and ears are all near each other in the head. As you note the layout is inherited from an ancestor, that's your answer. enter image description here

A study of vertebrate head sensory systems shows that the

The olfactory, vestibular auditory, and lateral line sensory organs displayed a strong link with the urochordate-vertebrate ancestor.

So this paper highlights that these structures go back earlier than the animals in your list. The development of vertebrates is very complicated so major departures from this developmental program which are successful are going to be rare events. This is reflected by blind cave dwelling animals that have vestigial non-functional eyes. The lack of selection pressure means the eyes don't work but they are still there because building an eye is part of a deep developmental programme. The image below shows (in orange) the key drivers of differentiation for various cell types. It makes it obvious that the program that produces olfaction also produces vision (otic) so modifications to the developmental program will have wide-ranging effects.

enter image description here

The theories that you've listed are interesting and there is work going into developing new senses. Udo Wachter wore a belt that vibrated to north 24/7, "north" became part of his dreams, he always knew where home was, and he mourned the loss of the the sense. So proximity to the brain is not a critical factor for the senses.

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It's not a coincidence. To understand why, you have to know what is anterior and posterior in zoology.

The first bilateral animals arose in the precambrian. Those free swimming animals had a polarity: the extremity of the body pointing to the direction of movement is the anterior region (in the anterior-posterior axis). Therefore, with the bilaterians the very concept of a head appeared.

Over time, sense organs started to develop in this anterior region (the head), because this region is the first one to explore the environment. Almost all the bilateral animals that exist today (as you and me) inherited this pattern.

But this is not mandatory. Animals have sensorial structures all around the body, and sometimes somatic exteroceptors are located in "strange" positions, as the chemoreceptors in the legs of insects.

PS: just to be precise in the definitions, you have hypothesis, not theories.

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  • $\begingroup$ the question is about vertebrate sensory organs so the sensory systems of insects seem off-topic to me. Can you update the last paragraph with a vertebrate example? I can't think of any but I'd be interested to know if there were some. $\endgroup$
    – Michael_A
    Commented Oct 20, 2016 at 19:54
  • $\begingroup$ @Michael_A Are you sure the question is about vertebrates? $\endgroup$
    – user24284
    Commented Oct 21, 2016 at 3:22
  • $\begingroup$ The question hasn't been asked precisely but I think it is implicit as the OP lists only vertebrates ("this applies to mammals, reptiles, synapsid, dinosaurs... and many more") while also mentioning a "common ancestor". Does the question remain valuable if we include taxa that frequently diverge from the central tenet, that sensory organs are always in/near the head? $\endgroup$
    – Michael_A
    Commented Oct 21, 2016 at 4:09
  • $\begingroup$ The answer of this question, even if it was (but it isn't) explicitly about Vertebrata or Craniata, lies in the origin of the Bilateria. $\endgroup$
    – user24284
    Commented Oct 21, 2016 at 4:11
  • $\begingroup$ I disagree. This is the crux "We evolved from a common ancestor that was like that but that doesn't explain why it's not common to get out of that coincidence." So it's not about the origin of the inheritance but the difficulty of diverging from the inherited path. Anyway, I doubt we'll convince one another so thanks for kicking about these ideas. $\endgroup$
    – Michael_A
    Commented Oct 21, 2016 at 4:56

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