Oldest fossils
Stromatolites are composed of algae that grew layer upon layer in shallow, saline lagoons. Water brought sediment which stuck to the algae and the algae responded by growing another layer, and so on, building up a dome-shaped structure. Stromatolites provide the earliest evidence of life on earth and date back more than 3.5 billion years. They can still be found alive today, most famously in Shark Bay, Australia.