Why do most (all?) mammals have four limbs instead of another random number?
In: Biology
The reason is really that we all descend from a common ancestor which had four. The reason the four limbed ancestor was successful and so became the progenitor of all mammals rather than a random two, six, eight, etc limbed animal is probably because of a balance between function and energy requirement. More limbs provide utility such as balance, maneuverability, dexterity, etc but they are also very costly to grow, move, and use. Adding more limbs offers a diminishing return in utility (4 is a lot more useful than 2, 6 is only a bit more useful than 4, 8 is barely any more useful than 6, etc) but a linear increase in complexity and energy expense. Organisms with more limbs than they “need” would be more likely to starve since they require more energy. Organisms with fewer may find it more difficult to access food or defend themselves. So the organisms with the optimal number thrived, and that happened to be four.
The reason we have 4 and not 3 or 5 is thanks to symmetry. Again, a distant ancestor was bilaterally symmetrical and the ancestor with that degree and type of symmetry ended up being the one best suited to large, motile, land based creatures.
This has been a passing curiosity of mine too. There are so many benefits of more and/or even less appendages in many circumstances, besides the help of tails. How has a majority of creatures rounded out to four?
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