From a purely materialist point of view, humans are intelligent because intelligence is an evolutionarily adaptive trait.

Something that just occurred to me re: the above: if intelligence is evolutionarily adaptive for humans, why would it not be similarly adaptive for other species? The level of manual dexterity humans possess may allow them to make better use of their intelligence, but it seems like other species should still be able to derive some benefit. So, from the purely materialist view point, it seems counterintuitive that we're the only "highly intelligent" species on the planet.