For example, humans. It would seem like, to me, if our brains and eyes, ears, etc. were situated in our stomach area they would be more protected from harm and more central than up at the top in and on a fragile skull. The same goes for other organisms. For example, horses have eyes on the sides of their heads; why wouldn't the eyes be near their stomach area? To me, it would seem like it would make it more safe and the horse could still see alot. I looked at this question, but it didn't fully answer my question.
We generally define a "head" as something that comes at the top/front of the body. I think that, we're likely to ignore examples of animals with nervous systems / sensory systems not at the top/front of their body as "animals without heads" rather than "animals whose heads are not at the top/front". For example, sponges and jellyfish.
Some animals who do, arguably, have heads, have their heads in the center of their bodies:
Image by james_michael_hill (CC BY-NC-ND 2.0)
- The evolutionary - developmental roots of head position (or having a "head" at all) go very deep (although despite this, some organisms, like the starfish above, shifted evolutionarily and shift developmentally from bilateral to radial symmetry).
$\begingroup$ Regarding your concluding line: evolutionary, radial symmetry came first? $\endgroup$ May 31, 2015 at 22:49
2$\begingroup$ @AliceD Developmentally, starfish and other echinoderms start out with a bilateral body plan as larvae and only become radial as adults. So they actually evolved from a bilateral animal and developed radial symmetry secondarily. However, some other early-branching animal lineages like cnidarians (jellyfish, sea anemones, coral) and ctenophores (comb jellies) are more or less radially symmetric. $\endgroup$ Jun 1, 2015 at 4:38
$\begingroup$ Point 2 is a little misleading, since the oral opening of a starfish is not really homologous to the head of a human or horse. $\endgroup$ Jun 1, 2015 at 4:39
$\begingroup$ @augurar - Thanks for explaining; I was talking evolutionary, you were talking developmentally. Glad that is sorted out:) However, removing "evolutionary" in point 2 may help to clarify. Overall, nice perspective on this question. And yes, I agree. To define the oral cavity in star fish as the head is tricky. $\endgroup$ Jun 1, 2015 at 4:47
Animals with a "front" and a "back" are known as Bilaterians. There is ongoing scientific debate about what the most recent common ancestor of the bilaterians, or "urbilaterian", was like. One proposed form is a simple wormlike animal with some form of eyelike structures, a segmented body with internal organs, and a centralized nervous system. This is contrast to animals like jellyfish which have radial symmetry with a central mouth and a network of nervous system structures distributed throughout the body.
Bilateral animals often exhibit directional motion with a defined "front" and "back" end. Eyes and other sensory structures are positioned at the front end so the animal can sense where it is going and what it encounters. The brain is located near the eyes because these structures require fast, intensive information processing. This process may also have been constrained by development, since evolution occurs by gradual modifications to the existing features of an organism. The Wikipedia article on cephalization has some more discussion of this topic.
Here is an example of two possible urbilaterians, taken from (Hejnol & Martindale 2008):
Image (a) shows a "traditional" model with segmentation, an anteriorly-located mouth (light blue), and a central nervous system (yellow). Image (b) shows an alternate model proposed by the authors of the paper with a mouth in the middle of the body and a decentralized network of neural tissue.
Hejnol, Andreas; Martindale, Mark Q (27 April 2008). "Acoel development supports a simple planula-like urbilaterian". Phil. Trans. R. Soc. B 363: 1493–1501. doi:10.1098/rstb.2007.2239.
$\begingroup$ The brain is located near the eyes because these structures require fast, intensive information processing. The visual system is among the slowest in the body. The argument may hold for the ears though. $\endgroup$ Jun 1, 2015 at 5:18
$\begingroup$ @AliceD It's the slowest because the information being processed is so complex. If the brain were farther away it would be even slower. But of course this too is speculative. $\endgroup$ Jun 1, 2015 at 5:19
$\begingroup$ Eyes and other sensory structures are positioned at the front end so the animal can sense where it is going: In the case of insects they often have 360-degree vision. So the eyes are rostral, yes, but not because they need to see where they are going. $\endgroup$ Jun 1, 2015 at 5:20
$\begingroup$ Small correction: bilaterian doesn't actually mean an organism with an anteroposterior/dorsiventral axis. It primarily means that these organisms exhibit bilateral symmetry i.e. symmetry across left-right axis. $\endgroup$– WYSIWYGJun 1, 2015 at 14:51
$\begingroup$ @WYSIWYG True, I was being somewhat general. Having a front/back is not at all a synapomorphy of Bilateria, just a common feature within the clade. $\endgroup$ Jun 1, 2015 at 16:34
Here too is a speculative answer, but evolutionary why questions are many times speculation, as it cannot be directly scientifically tested (there are no animals with a head in their tummies), only inferred in retrospect. I will sum up a few reasons to build the eyes and ears into a solid, rotational structure and not in the torso:
- The eyes are hardwired to see depth. The most important depth cue is retinal disparity, which is defined by a slight difference in the positioning of the field of view on both the retinas. This retinal disparity varies between people because it is dependent on the exact distance between the eyes. Hence, babies must learn to judge depth, as it is subjective perception deducted from a flat retinal surface. Hence, you want the two eyes to be contained in a solid, unmoldable structure and not in a bodily structure that is subject to change due to adipose tissue (belly) or breathing (rib cage).
- The ears are hardwired to sense direction. The auditory nervous system does this by measuring microsecond-delays between the sound arriving in both ears to sense the azimuth. Again, along the same reasoning as for the eyes, the ears must be contained in a solid structure.
- Directional hearing is less well-developed in the vertical plane, and when a sound comes from behind it may be hard to figure out exactly where it comes from. Also, pure-tone sounds are hard to distinguish in terms of elevation. Simply turning your head helps to re-locate the source to the side, or simply being able to see it. If you have to move the whole body, you lose time and effort. In case of a T. rex running in attack modus from behind, it means the difference of life and death.
- Eating needs a mouth. Consider a bat using echolocation. If the bat targets a fly using echolocation, it wants to devour it. If the ears are in the belly, it still couldn't catch its prey. The same goes for vision (birds of prey) etc.
$\begingroup$ 1. In horses and many other prey species, the eyes are on the sides of the head and have a largely non-overlapping field of view with reduced depth perception. $\endgroup$ Jun 1, 2015 at 5:08
$\begingroup$ @augurar, yes but the hearing arguments still count. They are examples, not fixed arguments. $\endgroup$ Jun 1, 2015 at 5:10
$\begingroup$ 2. Many animals do not have external ears and sense vibration in a variety of ways. In mammals, the ear bones evolved from former jaw bones, which evolved from the gill arches in fish. So the positioning of ears on the head is more happenstance than an inevitable outcome of the demands of hearing. $\endgroup$ Jun 1, 2015 at 5:10
$\begingroup$ @augurar - evolution is happenstance, but a fixed position is what counts here. And hearing is sensing vibration. $\endgroup$ Jun 1, 2015 at 5:12
$\begingroup$ Right, but in this case mammalian ears are probably positioned on the head simply because that's where the structures they came from were positioned, not necessarily because that was the best place for them. $\endgroup$ Jun 1, 2015 at 5:14
I think part of the reason is that our distant ancestors were aquatic - fish, for us vertebrates*. Living in the water, fish need to be streamlined, so a long, narrow body plan works well. Given this, and adding in the often limited visibility underwater, it makes sense to put the mouth towards the front, and sense organs near them, since they get to the food first.
This body plan got inherited by the descendants of that first fish to crawl out on land. It would require a major re-organization of the genes to get a radically different plan, and there was no great evolutionary pressure towards some different one. In many cases the mouth & eyes in the front works best - for predators, of course, but also think of a giraffe trying to browse on trees if its eyes were near its stomach. For most land vertebrates, food goes directly to the mouth. Only a few types - humans, elephants, and so on - have anything like hands to get food at a distance. (And if you look at an elephant, they do seem partway to a mouth near the stomach plan.)
If you think about creatures like the starfish, that have a central mouth, "brain", and sense organs, their food tends to be sessile. The starfish commonly crawls on top of its prey, and everts its stomach to digest it, so their the radially symmetric body plan works well.
*I don't know that much about arthropod evolution, but am guessing similar logic applies.