First cells

The oldest widely accepted evidence of life is chemical and fossil traces in rocks about 3.5 to 4 billion years old. The exact first cell is not a specimen anyone has. What the evidence supports is that by then, Earth had organisms that could copy themselves and pass on a membrane, a way to get energy, and genetic material.

A cell is a boundary. Outside is the world. Inside is chemistry that keeps itself going. Without that boundary, there is no body to inherit a change. Every later organism, including humans, is a long chain of those copies.

What it did not have was a nucleus, organs, or any behavior. It did not use tools. Its contribution to everything after it is simply that copying. If that step never happened, the rest of this timeline is empty.

Scientists still argue about whether life started at hydrothermal vents, in shallow pools, or somewhere else. The year on the timeline is a rounded window, not a dated event.