Complex thought - Complex language; both an aspect of only one creature on the planet.
Recent research on putty-nosed monkeys (Cercopithecus nictitans) in Nigeria has given science reason to think otherwise. In this article the researchers found the monkeys had varying shouts of warning depending on the nature of the threat - causing other monkeys to react in a situation specific manner.
Why is it that findings like this seldom surprise me? Still, I delight in them and, in this case, it's a bit more remarkable in that the research wasn't even directed at our closest cousins in the primate family. This would indicate that the evolution of this characteristic probably took place long before our split with chimps on the evolutionary tree.
What's the significance of this? Well, what was the significance of our ancient ancestors developing these skills? They are the abilities of an evolving species that would indicate the possibility of future, even more complex, thought and speech. And, it shows yet another feature of Homo sapiens that isn't all that unique.