system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are the bones of our limbs structured the way they are?

The upper half of our limbs have a single bone, and the lower half has two. Why is this? Why isn't it one, or the other, or reversed? To look at a skeleton it seems to make sense, but I don't know why I think that. Maybe it's just because I've always seen it that way?
gpt
We inherited the arrangement of the bones in our arms and legs from our ancestors all the way back to the time when they were fish and those were lobed fins.

All four-limbed land animals with bones have inherited that arrangement and made use of it. Some have bones fused together, some have lost the limbs altogether, some have found ways to make use of the bones in ways where the serve different functions in terms of muscle attachment and stuff, but we all kept the same blueprint that some fish came up with to swim better.