"They might be there, they might not," says Hawking. "But recent experiments like the Kepler mission have changed the game. We now know there are so many worlds, and organic molecules are so common, that it seems quite likely that life is out there, but intelligence is a great unknown. It only took 500 million years for life to evolve on earth, but it took 2.5 billion years to get from the earliest cells to multi-cell animals, and technological intelligence has appeared only once, so it may be very rare. And when it does evolve, we only need to look in the mirror to know that it can be fragile and prone to self-destruction."