What are the biological requirements for intelligence?

For some time I have wondered about the differences between plants and animals, two distinct "kingdoms." Maybe someday I'll have enough spare time to look into the matter (so to speak.) A variation of that question popped into my mind today: What are the biological requirements for intelligence? Man evolved intelligence in the animal kingdom. What specifically enabled that evolution of intelligence in man? Not the "pop", superficial explanations, but what exactly is it that permits man to exhibit intelligence? Put another way, why were plants unable to evolve in a parallel manner into "intelligent" individuals? Are there in fact biological requirements for intelligence that only the animal kingdom has to offer? Or, could intelligence, in theory, occur in plants through some path of evolution within the plant kingdom? In any case, in short, what exactly are the biological requirements for intelligence? And I do mean intelligence in the sense of human-level intelligence. That does beg the question of other forms of "intelligence" that may be wholly incomparable to human intelligence.

Now, this also broaches on the question of machine intelligence, computational intelligence, or artificial intelligence. If in fact there are biological requirements for intelligence, can those requirements in fact be met by non-biological entities such as computers as we know them. Of course that does beg the question of whether we could simply develop a computer program which is a simulator for biological life. That then raises the question of whether plants could evolve a machine-like structure which in fact was such a simulator for animal life.

In any case, we are left with the question of what the requirements would be for human-level intelligence in machines, and whether there may be biological functions that cannot easily (or maybe even possibly) be simulated in machines.

By "machines", I mean computers as we know them today, a device which can execute what we call computer programs or computer software.

That begs two questions. First, are there radically difference computer software architectures that might enable programming of human-level intelligence? Second, are there radically different device architectures which would permit software architectures that cannot easily (or maybe even possibly at all) be developed with computer devices as we know them.

To phrase the initial question another way, could we in theory genetically engineer plants to develop forms of intelligence?

More abstractly, could another "kingdom" develop which was neither plant nor animal, but capable of exhibiting human-level intelligence? Maybe in another solar system, another galaxy, or a parallel universe? Or, is there in fact some fundamentally basic requirement for intelligence which even in theory can only be satisfied within the animal kingdom?

One final question... What biological requirements would need to be met for artificial devices, presumably capable of reproduction by themselves, to in fact be considered "biological" and a new "kingdom" paralleling the animal and plant kingdoms?

-- Jack Krupansky