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A centaur as we all know is a horse with a human head (and torso).

I would like to consider a reverse centaur, which, for the purposes of this question, I will define to be a human with a horse's head as shown below:

enter image description here

Although the creature (Bojack Horseman) in the picture is completely fictional, let us suppose that a race similar to this evolved from humans on an isolated world.

Problem

A horse's head is excellent for horses; they have to eat from the ground and chew grass.

A human's digestive system is excellent for omnivorous humans but near to useless for digesting grass.

Question

How can we account for the evolution of reverse centaurs? What must we suppose about their diet and anatomy for this to make sense?

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    $\begingroup$ Do centaurs nurse from the human breasts of their parent or from the horse teats? $\endgroup$ – CaM Jan 18 '19 at 16:57
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    $\begingroup$ In traditional ancient Greek civilizations, these poor creatures are killed at birth. It is a tragic thing, but given how hard their lives would be, some say it's an act of charity. $\endgroup$ – CaM Jan 18 '19 at 16:57
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    $\begingroup$ The horse part should also have two front horse legs for this to be a truly reverse centaur. $\endgroup$ – Renan Jan 18 '19 at 17:01
  • $\begingroup$ @CaM - I don't know. This isn't a centaur - it's a reverse centaur. If you look at the picture, they don't have horse's teats - only human ones. $\endgroup$ – chasly from UK Jan 18 '19 at 17:03
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    $\begingroup$ I wonder why this was downvoted? It's a reasonable question. I recommend you make it formally a part of the Anatomically Correct series and be sure all the required rules are met. It'll be a good addition. (Frankly, it's along the lines of, "if horses evolved to become the dominant species....") $\endgroup$ – JBH Jan 18 '19 at 17:26
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Not naturally

The first problem is that the size of a human baby's head is one of the biggest issues in reproduction. Even though a horse's head is composed of 34 bones which are not rigidly connected initially, allowing some flexibility during birth, it is still so much larger than a human head that a reverse centaur baby could not be delivered naturally.

The second issue relates to a horse's eyes. Horses have really big eyes designed to give a wide field of monocular vision. These eyes have evolved because horses are:

  • prey animals that need to be able to detect threats over a wide field of view simultaneously; and
  • really fast runners that need to be able to see where they are going, obstacles in the way etc.

What these eyes are not good at is getting a good look at things close in front of them. They are also the equivalent of red-green colour-blind. The second quality is a relatively minor disadvantage, but the lack of good binocular vision of close objects to the front is an overwhelming disadvantage for tool users.

Finally, the large jaw and tooth structures are designed for an animal that needs to crop vegetation from the ground. These are an unnecessary biological cost to a tool user with opposable thumbs, that can use its hands to obtain food, prepare it and convey it to its mouth.

In short, this creature would not evolve - the horse characteristics would only be of benefit if the rest of the creature changed so that it was a much faster runner that could not use tools (ie a horse), but the "unable to give birth" characteristic makes it an evolutionary non-starter. Given sufficiently advanced technology it might be possible to genetically engineer such a creature in an artificial womb, but even if a breeding population were established they would not be able to give birth without technological assistance - and as mentioned, their rubbish short-range forward vision would make them poor surgeons when conducting the required C-sections.

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Not feasible, unless you handwave the problems away with magic. Centaurs are magical creatures after all.

Diet is not your main problem, although the head of an herbivorous horse isn't exactly well suited for an omnivorous human body. The other way around simply works better. But think about the sheer size of a horse's head.

A horse's head can weigh 15 kg easily. Even if the neck and shoulders can handle that, the rest of the body won't. The reverse centaur would simply keep falling over.

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    $\begingroup$ I disagree, weightlifters can handle significantly more weight that 15kg without falling over. Heck, students can have more than 15kg loaded in backpacks without falling over. $\endgroup$ – BKlassen Jan 18 '19 at 20:53
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    $\begingroup$ @BKlassen In a backpack, part of the time, isn't the same as carrying it continuously. Your entire life. From birth. Relatively speaking, it's already surprising a baby's neck can hold a human head up, let alone a horse's head. It's simply not happening. $\endgroup$ – Mast Jan 18 '19 at 21:11
  • $\begingroup$ luckily humans, being an intelligent species, the baby doesn't need to have complete control of its head at birth because the parent ensures that the baby is cared for until it grows enough to control its head movements. $\endgroup$ – BKlassen Jan 18 '19 at 21:22
  • $\begingroup$ @Mast - A little bit off topic but I don't agree that "The other way around simply works better." A human head simply isn't designed to provide sufficient grass to maintain a horse's body. It certainly can't efficiently chew hay. What's more it can't even reach the ground. $\endgroup$ – chasly from UK Jan 18 '19 at 23:45

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