RNA and lipid membranes
Sometime around four billion or 3.5 billion years ago, lipid membranes and RNA or DNA got together to form the earliest living cells on Earth. Nobody understands exactly how this happened. But somehow some RNA or DNA molecules, or both, got into the water that was inside the bubbles of lipid membranes. It probably happened accidentally at first, when RNA and DNA molecules just happened to be in the water where a lipid membrane was forming. But being inside lipid membranes turned out to be helpful to the RNA and DNA molecules, because the lipid membranes protected the molecules from water currents and other things and helped keep them from breaking apart. These were the first cells.
Natural selection favors the first cells
So natural selection caused more of the RNA and DNA that was inside bubbles to survive, while more of the RNA and DNA that was outside bubbles died. After a while most of the RNA and DNA was inside lipid membrane bubbles – inside cells.
How these cells reproduced
Like the RNA and DNA that were inside them, these simple cells were able to reproduce themselves. One cell could become two new cells. The cell began by using RNA molecules to make a copy of the cell’s DNA. Then, with one copy of the DNA in each half of the cell, the cell’s membrane would pinch itself in the middle and split apart, making two new cells, each pretty much the same as the old cell (except if copying mistakes had happened during the copying of the DNA).
Did you find out what you wanted to know about the first cells? Let us know in the comments!