Why do spiders have eight legs? Why do we have 2 legs? Here's one answer: Our ancestors - and the spiders' ancestors - with different numbers of legs didn't live and reproduce. 8-legged spiders and 2-legged people survived and reproduced.

We have 2 legs because our ancestors had 2 legs - our distant relatives are all the other 2-legged primates walking and climbing around. Spiders have 8 legs, because their ancestors had 8 legs. Spiders and horseshoe crabs evolved from the same ancestors!

Here's another answer: things like spiders evolved with lots of legs, so they can still get around if a leg or 2 are pulled off: Spiders Evolved Spare Legs

That's an idea that makes sense and has some evidence to support it.

Here's a spider joke:
Q: Why do spiders have eight legs?
A: Because if they had six they would be insects!

As scientists we can come up with some reasons for why an organism evolved a certain way, but we can never know for sure. Certain designs have utility and certain ones may just be neutral, since nature doesn’t design with a future plan. In the case of spider’s 8 legs, one potential explanation is that this provides extra legs. Studies on spiders with 6 or more legs has found that they are just as good at making webs and catching food. Spiders can also release a leg if its caught by a predator to escape. This explanation makes sense, but as I said earlier, it may not be correct or may not be the only explanation.

What a cool question, Samantha.

The short answer is that we don’t exactly know. “Why” questions are my favorite type of question, but they can be especially tricky when we ask why an animal has a certain trait, such as why a spider has eight legs. It’s hard to know the correct answer because the environment that the spider lived in when it first had eight legs happened many, many, many years ago.

Some scientists believe the first spider was alive about 400 million years ago! Imagine that! If spiders did first develop to have eight legs that many years ago, it would be difficult for scientists to understand what forces in the environment made it so that spiders would have eight legs.

We recently learned that spiders actually may not need all eight legs to function normally. Believe it or not, between 5% to 40% (depending on the species) of adult spiders will be missing at least one of their legs. In 2011, a group of researchers in France decided to take a closer look to find out what happens when spiders do lose one or two legs. They found that the spiders missing one or two legs were able to survive just as long, catch just as many prey, and produce just as many egg sacs as the spiders who did not lose any of legs. However, it’s very important to notice that the researchers only studied one species of spider, and that these spiders were studied in a laboratory environment rather than their natural habitat.

Spider legs are modified appendages from an ancestor that had many more pairs of legs, but most of the legs of spiders got modified into other things. There probably isn't any particular reason why spiders evolved to have eight, insects six, horseshoe crabs ten, or any other number for the jointed-legged animals. It's just one of those happenstances that evolution does some times.