The most significant change since the solar system's formation is the development of life.
Earth is the only planet known to support life .
The first one-celled organisms appeared on Earth at least 3.5 billion years ago, and perhaps as early as 3.9 billion years ago. But larger, more complex life forms did not appear until about 600 million years ago. Life may have evolved elsewhere in the solar system . Scientists reported that microscopic life might have inhabited Mars about 3.6 billion years ago, although additional tests seem to refute the claim. Recent observations by Mars Global Surveyor, however, hint at the presence of large underground reservoirs of water on Mars, which would strengthen the possibility that life evolved on the planet.