"Humans are obviously unique.
But it's suprisingly hard to say why" says the front page of New Scientist 24. May 2008.

Yes, it must be very hard if you're a fundie materialist, because the thing that makes humans unique is our ability to send our mind away from the material plane to visit lower and higher worlds. This superpower is what we call "imagination", but unfortunately it has a bit of an undeserved bad rep. But without it we would not just be deprived of porn and soap operas and The Sims, we would also lack religion, matemathics and other forms of science that require us to first guess the unknown before we can use it to understand the known.

By "lower worlds" I mean the worlds we create, such as daydreams or computer games. By "higher worlds" I mean the worlds that create us, such as matemathics and perennial religion. The lower worlds are easy to change but woefully temporary. The higher worlds are longer lasting than the daily world, but painfully hard and unyielding.

Because higher and lower worlds are both accessed by imagination, it is not always obvious which is which. Take great art, for instance. Some of the Cro-Magnon cave paintings still radiate a strange beauty/power after more than 10 000 years, especially to those who see them firsthand or who have an artistic disposition themselves. On a shorter time scale, Bach and Beethoven are long gone from this world, but the beauty they brought here remains. Verticalists will say that these works are a form of "revelation", which is (broadly speaking) content from a higher world being channeled into ours (or even further, into worlds we create, thus lifting them up to or beyond our own). The concept is borrowed from religion, so your vocabulary may vary.

In any case, this is what makes humans unique. There are birds able to make tools, different from but on a similar scale of complexity as the handaxes our ancestors used 100 000 years ago. But to the best of our knowledge, no animal can follow us into the higher and lower worlds, through the doorways of imagination.