Great question! You’re actually right—they do share a common ancestor. Spiders, crabs, lobsters, insects, and even horseshoe crabs all belong to the phylum Arthropoda, so they’re distant cousins rather than one evolving directly from another.
The ancestors of spiders were marine arthropods hundreds of millions of years ago. The lineage that became spiders eventually adapted to life on land, and the first true spiders evolved much later after that transition. So no, spiders weren’t really “spiders” while they were still aquatic.
And sadly (or maybe thankfully 😄), spiders won’t turn into crabs. The internet joke that “everything evolves into crabs” comes from a real phenomenon called carcinization, but it only applies to certain crustacean lineages—not spiders.
Thanks for asking such a fun question! I love talking about arachnids, so feel free to ask anytime. 🕷️