Guardian: What is it, were sometimes asked, that makes us human? What is it that marks our species out among all the others on this big ole blue space-marble we call Earth? Tool use? No, any number of other creatures use tools; corvids especially well. Is it that we have sex face-to-face? Bonobos do that and, for all I can tell, so do earthworms. That we laugh? Hyenas and orangutans have that covered, and judging by the look it gave me when I fell down the stairs in my dressing gown, so does my cat. That...