Kind of hard to say without knowing what you mean by ‘developed’ – if you mean most intelligent or cognitive I’d have to say any of the higher primates like gorillas or chimpanzees.
If you mean most evolved/adapted I don’t think we could say truthfully that humans are the most developed! I;d say that observing any of the animals that live in extremely extreme environments (acidic bodies of water, deep sea creatures, waterfalls, artic waste etc) would lead us to say that are more ‘developed’!
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