Jonathan Kennedy - Pathogenesis : A History of the World in Eight Plagues

In the summer of 1837, fresh from a five-year voyage around the world on HMS Beagle, Darwin made a sketch in his notebook under the words I think. The unassuming doodle of what looks like the branch of a tree neatly captures the main features of what would develop into his theory of evolution by natural selection: when populations from a single species find themselves living in different surroundings, random variationcombined with natural selection for traits that provide an advantage in each particular environmenteventually result in them splitting into separate species. This process, repeated again and again over hundreds of millions of years, accounts for the kaleidoscopic variety of life forms on our planet that looks like a vast tree when mapped out on paper.

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 archaeasingle-celled microbes that resemble bacteria. Less than 0.001 percent of all species on the planet are eukaryotes.

The American paleontologist Stephen Jay Gould argues that on any possible, reasonable, or fair criterion, bacteria areand always have beenthe dominant forms of life on earth.

Bacteria are everywhere. They have been found in Antarctic glaciers and on the ocean floor where boiling hot water surges out of the center of the earth. They live miles below ground and miles above it, where they influence the formation of clouds and possibly even lightning. But bacteria are not just ubiquitous: they have also had a profound impact on our planet.

About 2.5 billion years ago our world was almost completely submerged in water, with the exception of the odd volcanic peak piercing through the sea. Methane in the atmosphere created a greenhouse effect that kept the planet far hotter than it is today. There was little or no free oxygen in the water or air, as it was all locked up in other molecules. Life on earth consisted of anaerobic bacteria. Then the world began to change with the emergence of cyanobacteriablue-green algae that use the suns rays to power photosynthesis. This made cyanobacteria much more effective at generating energy, giving them a huge evolutionary advantage. Their numbers boomed. Over a period of several hundred million years, they pumped vast amounts of oxygena by-product of photosynthesisinto the oceans and atmosphere.

This Great Oxygenation Event transformed the planet.

Microbes continue to play a crucial role in maintaining an atmosphere that can support complex life. Cyanobacteria in the oceans still contribute to the oxygen in the atmosphere. In total, phytoplanktonphotosynthesizing microorganisms in the seaaccount for at least half of the oxygen produced by living organisms. And bacteria perform a variety of other vitally important functions. They convert carbon, nitrogen, sulfur and phosphorus into nutrients that can be used by animals, plants and fungi. When these organisms die, they return the compounds to the ecosystem through decomposition. Its no exaggeration to say that bacteria have made the planet habitable for complex life, including humans. Its a bacterial world, and were just squatting here.