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Assuming a human had 4 legs and could perfectly use them for walking(they are a bit like centaurs,who have the upper body of a human and the lower body of a horse). Would they have any advantages over normal bipedal humans? What kind of disadvantages would they suffer from?

I would assume that they would be more stable than bipedal humans and that they would be able to move faster than humans. However they would probably be bigger than normal humans due to the 4 legs, bending down to pick up stuff on the floor would be a hell and they probably don't have feet(I can't imagine a four legged creature with feet) and instead have hooves.

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  • $\begingroup$ How does this 'human' differ from a (wo)man on horseback? E.g. horses can easily reach down and pick up things on the ground in their mouths, and could be trained to hand them to the rider. $\endgroup$ – jamesqf Sep 20 '16 at 18:27
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Possibly they also might have paws instead of hoofs, but the leg geometry would be much akin to any other quadrupedal animal.

The larger body would also mean, that the base caloric needs of the hexa-limbic being would be much larger: An average soldier is seen as "75 kg" and is assigned 3000 kcal per day to upkeep him under a moderate workload. That makes pretty nicely 40 kcal/kg. A site about horse feeding told me, that just to keep a horse alive, it would need 143 kcal/kg. So, just summing both up and rounding down a bit for you don't need to sustain two brains, I estimate something in the area of 180 kcal/kg is the need to sustain these beings under some light to moderate workload. Which is about 4.5 times what a human needs per weight, and these beings are much heavier in addition? Taking something like a middle grounds, 160 kcal/kg sounds like a reasonable number too.

The additional space in the 'lower torso' between the middle and hind legs, however, it could mean that the whole 'upper torso' between the front limbs and the middle limbs could be devoted to having incredibly large lungs while the lower torso contains the digestive system, heart and blood cleaning systems.

Because these hexa-limbic beings would have entirely different needs, housing would look unfamiliar to quadro-limbic humans: chairs are more akin to a bench on which one rests the belly (instead of sitting on them), Beds might be unheard of (or more akin to the chairs), pillows would be in high demand.

Another of the needs the hexa-limbic beings would likely have is space: A human with 4 limbs can operate in any place comfortably, that offers space in the shape of a tube of 1 meter diameter and 2 meters height, give or take some for the size of the human. The Hexa-limbic beings would occupy most likely a box with a 2 by 1 meters floor and 2 meters height - which would seriously impact the design of cities and how some industries have to be handled.

Possibly the hexa-limbic beings can't manipulate things close to the floor very well for they can't bend down that much easily and crouching is not possible at all. So to operate things on the floor or pick them up, they most likely have to lay down - or they need to have longer arms, that almost reach to the ground.

Procreation of said beings might be a bit complicated: most quadruped animals have positions in which they go for this that would be pretty much hindered by a second torso sticking out orthogonal from where their shoulders are.

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  • $\begingroup$ So they are much more expensive than normal humans $\endgroup$ – Skye Sep 17 '16 at 13:34
  • $\begingroup$ Adding the energy per mass of the two combining types makes no sense. At most, you would want to add the energy needs of each part; but I suspect this figures are telling you more about the efficiency of the respective digestive systems than anything else. $\endgroup$ – Diego Sánchez Sep 21 '16 at 10:25
  • $\begingroup$ @DiegoSánchez True, the figures tell some about the digestive system, but it is hard to estimate the effectiveness and need of such a being as there is no similar being on earth. $\endgroup$ – Trish Sep 22 '16 at 19:10
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There's a possibility that your four legged humans would be shorter, more of a pancake shape so that their center of gravity would remain stable even at high speeds.

And if we are talking about different kinds of feet(or foot), hooves would definitely allow the human to move faster across open plains, insect like feet could conceivably allow the person to climb up walls(I think you would need a lot of feet to allow that though), paws would give the humans a natural weapon.

A human with four legs would definitely be more mobile than normal humans, they would also need their lower body to be larger to accommodate the increased size of the organs. Which is answered well by trish

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    $\begingroup$ 'A human with four legs would definitely be more mobile than normal humans" - not really. Faster on plains, but far less mobile in woods, underground tunnels etc. And we need wood and coal and steel for civilization, so the kind of mobility they would gain wouldn't compensate for the kind they lost. $\endgroup$ – Mołot Sep 19 '16 at 13:57

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