When did life begin on earth? We know it did not begin when the earth was formed 4.6 billion years ago. The earth was constantly being struck by meteorites at that time. Also, there was no oxygen in the air. The first fossils of life were formed about 3.5 billion years ago. This makes scientists believe the first life began between 3.9 and 3.5 billion years ago.
Scientists believe that a kind of atmospheric "soup" of hydrogen, ammonia, and methane reacted when lightning struck them. They formed the building blocks of life. These building blocks are called amino acids. They form proteins. Proteins are the basic substances of living things.
Proteins were the first step. Other substances needed are DNA and RNA. These substances allow living things to reproduce. This is a basic characteristic of living things.
DNA carries the instructions for cell function. RNA can act as an enzyme that also helps cells function.
Some scientists believe that life began deep under the sea near hydrothermal vents. Here, ocean water seeps through the cracks in the ocean floor. The water is heated by the magma. It rises through the ocean. All of the energy and nutrients needed for life are found near these hydrothermal vents.