We and all other living creatures require a continuous source of chemical free energy to remain alive. This is the reason for eating: We take in highly ordered molecules that have low entropy, high energy, and high free energy, and eject disordered molecules with high entropy, low energy, and low free energy. The ultimate free-energy source for all activity on Earth is the sun (right), and the mechanism for trapping free energy by synthesizing glucose is plant photosynthesis.

All of plant nutrition, and half that of animals, is based on one molecule, glucose (C6H1206). Even more remarkable, all life on Earth uses the same metabolic machinery to extract free energy from glucose - not just the same overall reactions, but the same steps, the same intermediates, and the same controlling enzymes. Not every organism uses the entire scheme. Some have lost parts of the machinery, and others never evolved them. Nevertheless, there is a common irreducible metabolic core to all life. We, slime molds, redwoods, and bacteria all share a common chemistry. This is the strongest evidence that life evolved once on this planet, and that all of its inhabitants are related.