Here, Liz Else (New Scientist, (3 June 2011) tells us, that “recursivity” or “thoughts within thoughts” make us human:
Chimps, bonobos and orangutans just don’t tell stories, paint pictures, write music or make films – there are no great ape equivalents of Hamlet or Inception. Similarly, theory of mind is uniquely highly developed in humans: I may know not only what you are thinking, says Corballis, but also that you know what I am thinking. Most – but not all – language depends on this capability.
Actually, no, other sources say, these qualities don’t make us human. They identify us as human. They are a key characteristic, and not the only one, that we expect of humans, and would expect of intelligent space aliens.
The emerging point is that recursion developed in the mind and need not be expressed in a language. But, as Corballis is at pains to point out, although recursion was critical to the evolution of the human mind, it is not one of those “modules” much beloved of evolutionary psychologists, many of which are said to have evolved in the Pleistocene. Nor did it depend on some genetic mutation or the emergence of some new neuron or brain structure. Instead, he suggests it came of progressive increases in short-term memory and capacity for hierarchical organisation – all dependent in turn on incremental increases in brain size.
This is a critical insight in non-materialist neuroscience: The ability to think, not some supposed module in the ocean of the brain, explains things like language, religion, etc.
That said, a Mark Hauser retro-man will surely come out shortly with a study claiming to have found the quality in chimpanzees. But to see it, you’d have to be him.