The complexity of the stromatolites found at Isua, if they are indeed stromatolites, suggest that life on Earth was already sophisticated and robust by the time of their formation, and that the earliest life on Earth likely evolved over 4 billion years ago. This conclusion is supported in part by the instability of Earth's surface conditions 3.7 billion years ago, which included intense asteroid bombardment. The possible formation and preservation of fossils from this period indicate that life may have evolved early and prolifically in Earth's history.