All life is related by descent from a common ancestor, shares a genetic code, and consists of similar biochemical building blocks. Therefore knowledge gained from investigations of one type of organism can, with thought and care, be generalized to other organisms. Biologists use model systems for research, knowing that they can extend their findings from such systems to other organisms. For example, our basic understanding of the chemical reactions in cells came from research on bacteria but is applicable to all cells, including those of humans. Similarly, the biochemistry of photosynthesis—the process by which all green plants use sunlight to produce biological molecules—was worked out largely from experiments on Chlorella, a unicellular green alga. Much of what we know about the genes that control plant development is the result of work on Arabidopsis thaliana, a relative of the mustard plant. Knowledge about how animals, including humans, develop has come from work on sea urchins, frogs, chickens, roundworms, mice, and fruit flies. Being able to generalize from model systems is a powerful tool in biology.