Sinh học - Chapter 26: Early earth and the origin of life

For the first three-quarters of evolutionary history, Earth’s only organisms were microscopic and mostly unicellular. The Earth formed about 4.5 billion years ago, but rock bodies left over from the origin of the solar system bombarded the surface for the first few hundred million years, making it unlikely that life could survive. No clear fossils have been found in the oldest surviving Earth rocks, from 3.8 billion years ago.