Pathogenesis: A History of the World in Eight Plagues

by Jonathan Kennedy

If we trace the Tree of Life to the base of its trunk, we find the Last Universal Common Ancestor (LUCA): a single-celled, bacterium-like organism that is the distant progenitor of all living things, humans included. This one shared antecedent is the reason why everything from blue whales to giant redwoods to bacteria have inherited common features such as DNA to store genetic information and a molecule called ATP that is the universal source of energy. Moving up the tree, the trunk splits into three boughs that represent the great domains of life. Two of these consist of organisms invisible to the naked eye: bacteria and archaea—single-celled microbes that resemble bacteria.[*1] The third branch depicts eukaryotes, which are distinguished by storing their DNA in a nucleus and using specialized structures called mitochondria to produce energy. This category encompasses all complex life including animals, plants and fungi, but it makes up only a few small twigs in the vast Tree of Life. There are about 8.7 million species of animals, plants and fungi on earth,[6] compared to an estimated 1 trillion—1 million million—types of bacteria and archaea.[7] Less than 0.001 percent of all species on the planet are eukaryotes.