The asteroids never formed a planet because the gravity of nearby Jupiter kept pulling them apart. The solar system has probably changed little since the planets and moons formed. The Sun churns steadily along, converting hydrogen to helium in its core. The planets evolved somewhat as they radiated away the heat left over from their birth, or as ring systems came and went. 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 have found evidence that microscopic life might have inhabited Mars about 3.6 billion years ago, although the finding has not been confirmed. Water once flowed across the Martian surface, and might still exist in underground reservoirs, raising a slight hope that Mars might yet be inhabited by simple organisms.