Irreducible complexity...
Sad how Evolution deniers just keep recycling long discredited arguments. Even Darwin had trouble explaining the human eye, and in the mid nineteenth century this was an argument against natural selection, by the end of the nineteenth century it was well understood. When you don't have any good arguments, all you have left is recycling old ones. And if these guys really understood Evolution, which they clearly don't, the would bring up the evolution of speech, which is one of the big unexplained problems with evolution.