Why do all mammals have two eyes, but specific insects and arachnids have more?

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Why do all mammals have two eyes, but specific insects and arachnids have more?

In: Biology

Actual ELI5: All vertebrates evolved from fish which already had two eyes. Spiders evolved from some of the earliest land Arthropods. Hunting spiders tend to have evolved extra pairs of eyes, which help them detect things to their sides and give them depth perception.

Insects have different sight needs than humans and other mammals do. They tend to ‘see faster’ (real thing, look it up) so much so that a congruent vision for them only starts to form at about the moment when humans are unable to discern any more frames per second. For insects, seeing quickly is far more important than seeing with a lot of depth, like humans or owls. They can see all around them very fast (which is why it is hard to swat them) whereas mammals and birds have excellent depth perception. Better for hunting and navigating the tops of trees.