Last year I was asked to contribute to a book by XS4All (PDF) about the history and future of the Internet. I decided to make some broad brush points on page 102. My colleague Menso also contributed (page 36), or here on his blog.
Long ago there were some monkeys on the African savannah. It was difficult for them as they hunted other animals that were stronger and faster. Other animals could digest the dry grass and live with little water. The monkeys could do none of these things. You would think they would never survive, let alone go on to play an important role in the evolution of the Earth. That they did so is through a unique combination of two things that led to everything else: an opposable thumb and big brains.
Separately, each of these makes little difference. Dolphins have large brains and are certainly intelligent. But without hands to apply that intelligence they cannot build complex civilizations. Chimpanzees have thumbs but lack the brains to make hand axes and build terabit optical routers. So dolphins and chimpanzees are in our zoos instead of vice versa.