A quick short answer: Things that evolve can't de-evolve to gain new abilities, be them any better. We can't get worse than we already are. If they evolved from humans, it wouldn't be feisable outside of atrificial genetic modification. They would need to de-evolve their good legs for worse spider legs then best spider legs, which is impossible. If they evolved from spiders, it wouldn't make sense, because it would have to change pretty much every thing about thier respitory systems, size, prey, and things like that. Smaller things breath VERY differently compared to us. So much so that smaller creatures would just die if they get too big without evolveing different respitory systems. They square-cube law would also make it harded for the air to get to them, as they breath through tubes as of now. They just wouldn't be able to evolve in that way without being worse for a period of time, which would likely make them unabe to reporduce, killing them off.

A quick short answer: Things that evolve can't de-evolve to gain new abilities, be them any better. We can't get worse than we already are. If they evolved from humans, it wouldn't be feasible outside of artificial genetic modification. They would need to de-evolve their good legs for worse spider legs then best spider legs, which is impossible. If they evolved from spiders, it wouldn't make sense, because it would have to change pretty much every thing about their respiratory systems, size, prey, and things like that. Smaller things breath VERY differently compared to us. So much so that smaller creatures would just die if they get too big without evolving different respiratory systems. They square-cube law would also make it harder for the air to get to them, as they breath through tubes as of now. They just wouldn't be able to evolve in that way without being worse for a period of time, which would likely make them unable to reproduce, killing them off.