system
Given the following post, respond as if you're commenting on Reddit:
human
Is it just a coincidence that both spiders and octopi have 8 legs? Or do they share a common ancestor?

They seem fairly divergent in all other aspects, but 8 is an interesting commonality.
gpt
They don't have a common ancestor that also has 8 legs. Spiders are arthropods and octopi are a late brance of molluscs. Basically that means that the common ancestor of arthropods (which also includes crabs for example) and molluscs (also including slugs and bivalves) split way before spiders and octopi (order level) even existed. There were many species in between who do not had 8 legs.