Humans are late arrivals on Earth. For nearly 75% of Earth’s history, life consisted of single-celled microbes without a nucleus (prokaryotes). Volcanoes and erosion sculpted Earth 3.5 billion years ago. Life consisted only of cyanobacteria, which trapped sediments in intertidal zones to form mound-like structures (stromatolites). Cells with a nucleus (eukaryotes) evolved about two billion years ago, after oxygen had accumulated in the ocean. Over time, they developed greater complexity, with different parts taking on specialized functions—allowing evolution of sexual reproduction. Explore the ancient ocean in an image gallery or in our Ocean Over Time interactive.