system
Given the following post, respond as if you're commenting on Reddit:
human
Why have most animals evolved with four legs instead of three or five?

A tripod has three legs because that's the minimum number for stability.  It seems like biology would have also realized this, and that we'd see animals with three legs.  But that seems rare -- I can't think of any offhand.

So the argument might go:  Four legs were evolved because an animal with only three legs would be screwed if it breaks one.  But in that case, why not five legs?  Then there'd be extra redundancy.

Is there any consensus about the "why?" behind this part of evolution?
gpt
One note: most animals with legs don't have four of them; six is by far more common, as it is the number of legs on insects.  And insects outnumber tetrapod vertebrates in both number of species and number of individuals by a longshot.  Other even numbers of limbs are common on other arthropods, but four is basically unique to tetrapods.

The reason limbs tend to come in even pairs is because most modern animal groups have bilateral symmetry, which means their left and right sides are more-or-less identical.  You do see odd numbers of limb-analogs radially symmetric organisms like starfish.