a striking feature of metabolism is the similarity of the basic metabolic pathways and components between even vastly different species. for example, the set of carboxylic acids that are best known as the intermediates in the citric acid cycle are present in all known organisms, being found in species as diverse as the unicellular bacterium "escherichia coli" and huge multicellular organisms like elephants. these striking similarities in metabolic pathways are likely due to their early appearance in the evolution of life, and kept because of their efficiency.