sentinel

These wanderers must have looked on Earth, circling safely in the narrow zone between fire and ice, and must have guessed that it was the favorite of the sun's children. Here, in the distant future, would be intelligence; but there were countless stars before them still, and they might never come this way again.

So they left a sentinel, one of millions they have scattered throughout the universe, watching over all the worlds with the promise of life. It was a beacon that down the ages has been patiently signaling the fact that no one had discovered it.

-- Arthur C. Clarke "The Sentinel" (the short story that was the genesis for 2001: A Space Odyssey)