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I can assume that the hands used to be the same as legs. And they got weaken, and changed direction when human started to walk on two? Also why do the hand palms facing the body and not facing down like the feet?

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    $\begingroup$ What research have you done on this so far? $\endgroup$ – James Jun 3 '16 at 1:49
  • $\begingroup$ I just found an article talking about god creation, this wasn't what I was looking for. And I still need to read about why human walk on two. $\endgroup$ – Ronen Festinger Jun 3 '16 at 13:31
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    $\begingroup$ Do you want to know evolutionarily why or developmentally why knees and elbows point in opposite directions? The latter question is more easily answered. $\endgroup$ – kmm Jun 4 '16 at 22:43
  • $\begingroup$ Your assumption is wrong, hands were never the same as legs. $\endgroup$ – John Aug 25 at 16:48
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I am no expert on comparative anatomy, but I will give this a shot. Please edit if you know more about this subject!

The configuration of opposing elbow and knee joints is a feature we humans share with large group of mammals. For example, below is an image of a shrew skeleton.

enter image description here

Opposing joints are course a major feature of mammalian anatomy, and there are many studies on its functions and advantages. Among other things, this configuration allows for characteristic gaits of mammals such as the gallop of a horse, and it allows mammals to rest with legs protected under their body. See this page for some basic information.

Neil Shubin's book Your Inner Fish on human evolution also discusses the opposing joints arrangement, noting that this is a "key feature that gives us the capacity to walk, one we share with other mammals". A preview of the book is here.

A quick literature search turns up many technical articles on this subject, such as this review series. I'm afraid the details are beyond me (again, experts please edit), but I think it's fair to say that there is plenty of evidence for advantages of this joint arrangement in mammals.

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    $\begingroup$ Good answer+1 Would be great to have answers to the following question: Do we have an ancestor having knees and elbows bending in the same direction? (2) Did joint on forelimbs and hindlimbs evolved simultaneously? Do they use similar metabolic pathway? (Might be worth eventually opening a new post for these) $\endgroup$ – Remi.b Jun 5 '16 at 3:21
  • $\begingroup$ @Remi.b I'd add any other mammal species, not only direct ancestors. $\endgroup$ – Rodrigo Mar 9 '18 at 3:19
  • $\begingroup$ you can add any other terrestrial vertebrate, not just mammals. $\endgroup$ – John Aug 25 at 16:49
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Unfortunately, your question isn't clear. "Why" could mean different things. What course of events led to it? What benefits does it provide? We could just say, it's not disadvantageous this way, so why not. So perhaps you could clarify.

Evolutionarily, through the stages as our ancestors evolved from small mammal to human, the changes that occurred were sufficiently suitable for the species. The joints are oriented well enough for walking, running, and holding things, and there doesn't need to be more reason. As for the hands, they point away from the arms like they do in other great apes. The difference is that in humans, the feet have a hard time pointing down since they are never used as hands, and hands have a harder time pivoting to be perpendicular to the arm because they are never used as feet anymore. Because of these behavioral differences from other great apes, variations that helped feet be used as hands and hands be used as feet were not selected for, but variations that reinforced walking and running ability and that reinforced object manipulation were selected for.

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  • $\begingroup$ Good answer. What i meant was that i expcted the 4 limbs to be identical since they all were used for walking. But I got already other good answers here that explained it. $\endgroup$ – Ronen Festinger Jun 6 '16 at 6:18
  • $\begingroup$ @RonenFestinger Okay well then I guess you have your answer. You might also be interested in asking an engineering or mechanics question about it, as tetrapods walking robots often have front and back joints that are mirror opposites of each other. You might also be interested in asking a question of why other animals have opposite front and back joints. What you should keep in mind is that although front and back legs are all used for walking, they don't have identical functions. Back legs might spring more, front legs might pull more and grab prey more. $\endgroup$ – A L Jun 6 '16 at 18:31
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It's an amazing question, really. And you might be right about your assumption. In the womb, our legs and arms bent in the same direction. But as the foetus continued to develop, the legs and arms rotated to bend in opposite direction (to each other). As of now, there is no rational explanation as to why this happens.

So I guess all we can do is be grateful to the way our bodies have come into being. If, for instance, our knees bent backward (just like our elbows), our legs would bend forward like our arms and we wouldn't be able to walk the way we do. Apply the same to the arms and we'd be lifting things in an entirely different way, and backwards too! Also, we won't be possessing the ability to write.

You can check out this page for more info:https://answersingenesis.org/human-body/your-legs-are-on-backwards/

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  • $\begingroup$ Thanks. That's what I found too before asking this question. But I was looking for a different answer than god created it this way. $\endgroup$ – Ronen Festinger Jun 3 '16 at 13:40
  • $\begingroup$ Well I'm sorry I couldn't be of much help, because there is no scientific theory to explain this as of now. You can always do research on it, though. $\endgroup$ – Irena Jun 4 '16 at 8:39
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    $\begingroup$ "there is no scientific theory to explain this" --- this is clearly not true. See my answer for some basic information and pointers. $\endgroup$ – Roland Jun 4 '16 at 22:06
  • $\begingroup$ When exactly are they bent the same way? parents.com/pregnancy/what-my-baby-looks-like And even if they would change orientation in development, that would mean nothing about God, it would mean something about evolution. For example, turtles in development start out with rib cages, like the reptiles they evolved from, and the rib cages fuse into a shell. This is actually evidence for evolution, because if animals were created separately, it would make no sense to make a rib cage and change it later. Please do yourself a favor and un-brainwash yourself with actual science websites. $\endgroup$ – A L Jun 5 '16 at 21:56
  • $\begingroup$ I arrived here four days ago. And not to parade my belief in God (I don't see life that way), but to get better, more informative answers of scientific questions. So if you think I blindly swallow whatever's fed to me, I'm sorry. I don't. Or else I wouldn't be here. This was my first answer on this site, and now that I'm here, I look forward to add substance to it. And ironically, your link above doesn't happen to be an actual science website either, so kindly practice what you preach. (I don't mean any offense.) $\endgroup$ – Irena Jun 6 '16 at 9:24

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