Reasons for an upright hexapedal humanoid to evolve [duplicate]

Question

I'm currently developing a fantasy setting, where at least two of my sapient species will be hexapods(six limbed animals). One is a species of quadrupedal "giants", using four legs(among other stuff) to support their larger size, and the other is a race of tree dwellers that use the extra limbs to easily move through the trees while still having multiple limbs free. They're probably going to be similar to mammals, but that could change.

I could just go with the simple reason of "feesh has six fins", but I don't want practically all of the animals in my setting to have six limbs. I need some reason for these animals to develop an extra set of limbs somewhere down the line. Does anyone have any relatively plausible ideas for how this might happen?

Also, I believe that my planet will be pretty similar to earth for the most part, so it wouldn't come about due to something like abnormal gravity. Anyways, thanks for responding, if you do, and have a nice day!

Answer 1

Sometimes vertebrates have mutations that lead to extra limbs. This is more common than we'd like to admit in chicken (google it up!). Usually the extra limbs are not functional, but if they are, then natural selection will do its part.

If the extra limbs help the animal thrive and reproduce more, then they stay. Since mutations are a thing, the only reason we don't have more than two arms or two legs in our own world is because the ones resulting from mutation have never been useful. It only takes one individual in your world having functional, helpful extra limbs, and also having children to start a new trend towards hexapodism.

Some reasons why extra legs could be beneficial:

• More stability, both when standing and running;
• Two more limbs to push against water when swimming.

Reasons why extra arms could be beneficial:

• Better climbing;
• Better tool usage;
• Two more clawed hands to hold onto prey;
• Two more limbs to paddle when swimming/rowing.

Answer 2

Your six limbed races are originally from somewhere else.

And in that somewhere else maybe all the creatures do have 6 limbs. Some ancient event allowed your sapients from elsewhere to colonize your world. Maybe their ancestors had spacefaring or plane-traversing tech, and showed up on your world. Maybe a third advanced race saved these species from a cataclysm on their homeworld and relocated them, sort of like relocating pandas to Molokai to save them. Maybe some ancient summoning brought representatives of these species from their original place and they did well in their new home.

Their differences from the other denizens of your world could be grist for your writing mill. Maybe there are a couple of other 6 limbed creatures. Maybe these things have other phenotypic commonalities which set them apart from the natives of your world.

Answer 3

There were two main ancestors or a single six limbed one which split up early

You could make it so that there were two main ancestors to which all animals can descend from, one with 4 limbs and other with 6. Alternatively, the fish with 6 fins could've diverged early into 2 species, one who lost 2 limbs and the other who kept them. In both cases the ones with 4 limbs would've been more successful overall than the 6 limmbed ones, justifying their larger numbers without eliminating their hexapodal counterparts, which had a degree of success themselves. Either solution will solve your problem without the need of drastic mutations that added 2 perfectly functional limbs, as that outcome would be a lot harder to happen naturally.