The first time that what went around came around "a quid pro quo with bacteria"

We humans love old saws, old adages, and the like. The best ones persist indefinitely because, at their core of meaning, they embody great truth. One of these adages, in an older form, is "quid pro quo". A more contemporary version is "you scratch my back, I'll scratch yours", and more recently still: "what goes around, comes around". I think you will immediately agree that the concept is ubiquitous in both nature (termed symbiosis) and human behavior (affectionately known as mutually beneficial relationships) which is really nothing more than a subset of nature. I contend that the first meaningful quid pro quo "happened" some few billion years ago, involved bacteria and an early form of Eukaryote cell, and that we humans owe our very existence to this reciprocal back scratching.

This is a story of the evolutionary origin of the eukaryotic cell, described by the Theory of Endosymbiosis. The Eukaryota include the organisms that we are most familiar with - all animals, plants, fungi, and protists. They also include the vast majority fossilized animals. Despite their mind-boggling diversity in form, Eukaryota share fundamental characteristics of cellular organization, biochemistry, and molecular biology. A critical characteristic they share is the presence of mitochondria, a small organelle is the cell that produces energy. Plants cells contain chloroplasts that function similar to the mitochondria.

Mitochondria are the cell's "powerhouses," the sites where aerobic respiration breaks down food in the presence of oxygen into water and carbon dioxide, generating energy. Plants and many protists also contain plastids, where photosynthesis takes place, allowing these organisms to manufacture their own food from carbon dioxide and water. Chloroplasts and mitochondria are curious organelles indeed; they contain their own DNA and replicate independently. In fact, these organelles are descendants of symbiotic bacteria living inside the eukaryote host. They have become so dependent on their host that they can no longer live outside. Eukaryotes - including humans - are, in a very real sense, not individual organisms but colonies, and living proof that evolution frequently does not proceed according to the violent stereotypes of continuous struggle, of "nature red in tooth and claw." Sometimes the winning strategy is cooperation.