ABSTRACT
Human biochemistry and physiology have ancient origins-the central pathways of energy metabolism, such as the citric acid cycle and glycolysis, were present in the last common ancestor over 3.5 billion years ago.1 Because they are shared by all contemporary organisms, it seems almost certain that this common ancestor had already evolved mechanisms for amino acid, nucleotide, carbohydrate, and lipid metabolism. These primordial pathways have been retained throughout subsequent evolutionary divergence, into all current life domains, because they represent successful solutions for their particular biochemical functions. They produce end products and/or energy as ef- ciently and simply as possible, given existing biophysical laws.
