How does evolution rule out the possibility of humans or others fragile herbivores from having 2 pairs of eyes, one at the front and the other at the back of their heads? Why didn’t that ever happen?
Asked
Active
Viewed 1,090 times
1
-
I’m sorry but I’m not a biology student; currently pursuing mechanical engineering. So, I don’t have any knowledgeable viewpoint over the same. The idea was that you people could hopefully provide some intellectual insight. – RedHelmet Oct 01 '17 at 13:31
-
“Given limited resources” - what does that mean ? – RedHelmet Oct 01 '17 at 13:42
-
@user115962 A limited resource is any element an agent utilizes, in which the quantity, or frequency of use, of the element can not be fully assumed throughout a completely defined process. The most general form of any limited resource is free energy. – Oct 01 '17 at 13:58
-
On top of the question marked as duplicate, you might want to have a look at Why don't mammals have more than 4 limbs?. – Remi.b Oct 01 '17 at 16:20
-
First, humans aren't fragile herbivores, they're omnivores, and quite successful hunters even pre-technology :-) Now that that's out of the way, the basic vertebrate body plan - four limbs, two eyes, &c - was fixed hundreds of millions of year. Herbivores, such as horses, have the eyes located at the sides of their head, and have nearly a 360 degree visual field. Predators tend to have front-facing eyes, the better to focus on potential prey. – jamesqf Oct 01 '17 at 17:34
1 Answers
-6
Vertebrates have ears which sense 360 degrees for a range of many meters, and they have social eyes, where they group many eyes and ears into one social group, it's easier to listen and live in groups, than grow eyes in the back of the head.
2 eyes exists in nearly all bilateral animals, arthropods, flatworms, sea-slugs, crabs, crickets, squids, flies. Arthropods have compound eyes and it would be easier for them to have 4 eyes, and only spiders and horseshoe crabs have more than 2.
It would be easier for a vertebrates to develop 4 nostrils and ears than 4 eyes because they are simpler than the orbs of the eye.
Same reason as 5 fingers has been the optimal numbers for all vertebrates since early lizards. in short, it's very easy to lose extra organs and to modify old ones, but it's very difficult to evolve new organs, and to relocate ones around the nerves and veins... centrally connected organs travel around the body and the circulation moves in front of them, i.e. if you did get 4 eyes and two of them moved to the back and sides, the nerves for your face expressions that are next to your ear would have to go in front, around, and through the moving eye pathways.
To simplify, we stopped evolving our symmetrical archetypes when we were related to starfish, ammonites and ediacaran fauna, and the fastest most versatile animals all had 2 sided symmetries, with entirely symmetrical nervous systems, and forwards moving at that point in time, using binocular compound vision, which did have 180, and that binocular compound vision afterwards became our 2 eyes. We share the kingdom Animalia with starfish, our common ancestor experimented with shape during the Ediacaran era.
To understand the difficulty of adding new eyes later, you should learn why nearly all vertebrates have 5 fingers, except some frogs which have 4. it
bandybabboon
- 10,397
- 22
- 39
-
-
-
2 eyes and 5 fingers has been the optimal numbers for all vertebratesthis is fantastically wrong (might want to have a look at Why don't mammals have more than 4 limbs?).we stopped evolving our symmetrical radiationis wrong.when we were starfishsounds like you mean that the common starfish is our ancestor which is wrong. You seem to equate symmetry with binocular vision which is also wrong. Your only two links are google search! – Remi.b Oct 01 '17 at 16:19 -
We share the kingdom Animalia with starfish, our common ancestors experimented with symmetries during the Ediacaran era resulting in worms, corals, urchins, bivalves. Terrestrial vertebrates have 5 fingers.... you send me to a page called "mammals have 4 limbs" I send you to 4 pages called "why nearly all vertebrates have 5 fingers" .... bi-focal vision is a common theme in planar symmetry or in radial? PLANAR symmetry. – bandybabboon Oct 02 '17 at 04:34
-
"Optimal number" is wrong. It's much more of an accidental number: the long-ago last common ancestor just happened to have 4 limbs, 5 digits per limb, 2 eyes (and ears &c), and those are buried so deep in the genes that any changes make the organism non-viable. See e.g. Hox genes. – jamesqf Oct 02 '17 at 06:33
-
Quadruped 5 digit number = "The number chosen by natural selection of fitness" was "optimal" because there were 7, then 6, then 5 digits on Both limbs of archetype quadrupeds. Neither 7 nor 6 digits could have been optimal, because they were both lost by selection against ray fin derived Hox Genes for 6/7/4 digits. Eyes: Planar symmetry arthropods can all develop multiple eyes, and they dont. Even all Flatworms and seaslugs have two eyes, which is certainly the chosen number of eyes for planar symmetries, for the same reason that we have 2 ears, it's genetically efficient. – bandybabboon Oct 02 '17 at 10:10
-
1I downvoted too. This is the very worst kind of answer for StackExchange; it's superficially plausible and written with great confidence and authority, so a naive reader will probably think that the author is deeply knowledgeable; yet the answer is simply bluster, mistaken stereotypes, and sweeping claims based on ignorance. If you don't actually understand the field, you shouldn't answer questions. The fact that you think you understand this is simply Dunning-Kruger in action. – iayork Oct 02 '17 at 11:59
-
@comprehensible Even the expression
natural selection of fitnessis non-sense. – Remi.b Oct 02 '17 at 15:53 -
You don't know the difference between a limb and a digit, Remi.B. Also, learn to speak english. natural selectrion of fitness for purpose. https://www.google.fr/search?q=%22natural+selection+of+fitness%22&oq=%22natural+selection+of+fitness%22&aqs=chrome..69i57j0l5.6993j0j7&sourceid=chrome&ie=UTF-8 <--- check that, selection according to fitness of a phenotype and morphology... iaYork you yourself are like tim-nice-but-dim. WHY? hyper beginner question = beginner answer, at least i bothered. It just shows that you don't have basic knowledge of evolution, you reply without facts/content, vaguely – bandybabboon Oct 02 '17 at 16:29
-
Remi.b, you must feel like an idiot telling me "that doesnt even exist' and then reading it www.genetics.org Ph.D texts ... and confusing 5 digits (polydactylism) with 4 legs? you are truly the one talking nonsense. – bandybabboon Oct 02 '17 at 16:33
-
@comprehensible: Then by your argument, 5 digits aren't optimal either, since many vertebrate lineages are evolving away from that. E.g. snakes with no digits (or limbs), dogs with four (plus a largely vestigal dewclaw), cows and such with two, horses with one... – jamesqf Oct 02 '17 at 17:29
-
That's a fair comment, snakes are not quadrupeds, there are 4 species of dogs (40 species of cats) and 350 species of primates including humans, 5000 lizard, 200 species of squirrels, 5000 frogs, all 5 digits, a dog and a horse can never evolve back into a forest environment. losing all grabbing ability is not an optimal design unless it gets wings/trunk/aquatic. all land vertebrates that survived after the dinosaurs had 5 digits except snakes? all non grasping species didn't survive, and ungulates would all vanish when there is another apocalyptic winter. – bandybabboon Oct 02 '17 at 18:27
-
@comprehensible: If you think that having single-digit hooves means the critter loses all grabbing ability, I can deduce that you've never spent much time around horses. And hooves seem to work pretty well if your niche in life is as a grazing herbivore :-) – jamesqf Oct 03 '17 at 06:02
-
ok, so the dominant big species of african ecosysters have soft pads and hooves. That knocks over my theory. perhaps hooves are not digits, so when 1 finger vanishes the limb ceases to have a hand a very fast becomes a clawed pad and a hoof. no rodents, marsupials, lizards and salamanders have 4 finger hands, and pandas, koalas, opossums and some frogs have opposable thumbs. It's a theory I read on a Polydactyly biology page, I only copied it. – bandybabboon Oct 03 '17 at 07:57