Eli5: why do the vast majority of mammals walk on 4 legs, while humans only walk on 2?
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Way back in the day, our ancestors evolved away from the ground, to live in the trees. You’ll notice that your hand is almost perfectly suited to grasping long, thin objects from the top, with four fingers curling downwards to create a hook that clings on to branches and an opposable thumb making a hook in the other direction to provide a “lock” we can open or close at will so we don’t slip off of a high branch. This method of locomotion via grasping from branch to branch to branch in a treed area is called “brachiation”, and many of our relatives among the great apes and monkeys still utilize this way of getting around.
With such a heavily specialized set of forelimbs, using them as part of your method of getting around on the ground becomes impractical, but there are still times when moving around on the ground might be necessary- not every tree is close enough to every other tree to jump from one to the other, naturally. This is usually accomplished by our relatives by a knuckle-walking, loping gait made possible by their relatively long forelimbs and short hindlimbs, but a lot of our relatives can display bipedalism in short bursts or when grabbing something, just due to the way their body is set up, in a way animals like dogs and cats really can’t.
We aren’t really exactly sure why the lineage that led to humans became SO bipedal. A common working theory is that we were subject to a severe biome shift due to climate change in our formative evolution around 7 mya, when significant bipedalism starts showing up in our ancestors in the fossil records. This would have led to a much less treed habitat, and forced our ancestors to reckon with traversing a lot more flat ground. Since there also would likely have been evolutionary pressure to keep our grasping appendages around for very primitive tool use (think throwing a stick or a rock) evolution simply used what it had and made us fully bipedal.
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