This question si very similar to how can an embryo (fertilised egg) form into a baby and how can a baby grow into a fully sized adult. The reason all these things happen is because of cell division. One cell divides to form two cells, then four cells, then 8 cells and so on. All of these cells are capable of making proteins by following the instructions on the DNA they contain and so they can grown leading to the formation of big things from small things.
Comments