Pigs have very similar biological characteristics to humans, which makes them useful as a source for various hormones (for instance insulin, before industrial production was invented) and tissue (heart valves being the most common). Chimps would work well for these also but are slightly harder to come by.
In any case, as others have stated, chimpanzees have much higher genetic similarity to humans than pigs.
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Spending as long as I do here, it's easy to forget that Oddworld has actual fans.
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