What process might have caused the first cells to spontaneously generate over a very long period of time?

The process that happens in the given statement above is the chemical evolution. The chemical evolution allows cells to generate where as when time prolongs or goes on, complex molecules are being created in which they came from simple molecules. They are being created because of the reactions that occur chemically, generating or evolving them as time goes by.

What have scientists learned about plant behavior by studying the dodder plant