How smart are humans in
absolute terms?
For the purposes of this OP, "intelligence" is defined as "the effectiveness with which an organism is able to purposely interact with its environment in order to realize its goals".
Defined as such, intelligence can in principle be measured in non-relative terms. For instance, you could create a series of problems or challenges to see how well an organism was able to meet them. That sort of thing is routinely done by investigators with both humans and animals. Best of all, such challenges measure human intelligence in more absolute terms than does comparing ourselves to other species.
By the way, there is a sense in which humans are no longer the smartest things on the planet. About four years ago -- in a grossly under-reported story -- researchers brought online the world's first artificial super-intelligence. An artificial super-intelligence is vastly smarter than both humans and other forms of artificial intelligence.
What distinguishes ASI from both human intelligence and conventional artificial intelligence is that ASI can recognize and solve problems that neither humans nor conventional AI are even capable of recognizing are problems.
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And now for something much better than the OP...