Z. D. Blount, American Scientist, 105 :1, 2017, 156-165. Replaying Evolution Is the living world more a result of happenstance or repeatable processes Zachary D. Blount Amazon’s television series The Man in the High Castle, based on the classic novel by Philip K. Dick, presents a nightmarish alternative 1962 in which the triumphant Nazi and Japanese empires occupy a fractured, defeated United States. This alternate history is spun from the imagined consequences of a minor change in a real event. On February 15, 1933, Giuseppe Zangara opened fire on president-elect Franklin D. Roosevelt in Miami, Florida. Zangara was only 25 feet away, but his attempt failed because he shared the wobbly bench on which he stood with a woman who, as she strained to see, jostled the bench at just the right time to spoil his aim. The show’s version of history did not include the fortuitous jostle. Although the result of such a change might not have been the dystopia the show envisions, history would have been quite different had Roosevelt died that day. The Long-Term Evolution Experiment (LTEE) has followed 12 lineages of Escherichia coli bacteria for more than 66,500 generations since 1988. A sort of experimental time machine, it allows scientists to rerun evolution and compare its outcomes. Human history has been wrought from the particulars of unique events and personalities. Indeed, the historical record is rife with instances like the attempt on Roosevelt’s life, where even slight changes could have dramatically altered the course of events. These instances illustrate how the existence of the current world depended on the process of history, linking past to present in a complex web of causality. In other words, human history is contingent. Contingency, philosopher John Beatty has written, essentially means that history matters “when a particular future depends on a particular past that was not bound to happen, but did.” It arises because the future flows causally from the past, but many futures are possible at any given time, and which one comes to pass is determined by the precise, chance-laden way in which a complex tangle of improbable events interacting in improbable ways plays out. Contingency is why we can more or less explain the past, but the future is unpredictable. Even a cursory survey makes it hard to deny that contingency has played an important role in human history. But human history is embedded within the 4-billion-year evolutionary history of the living world. Has this grander history of life been similarly subject to contingency, making the modern living world as a whole as much a unique product of coincidence and accident as the current state of humanity? It’s a startling question with mind- boggling implications. We tend to think of natural phenomena as regular and deterministic, proceeding from a beginning along an inevitable path, sure as the planets in their orbits or a ball rolling down a hill. Whether or not the process by which life has developed and evolved on Earth is really so deterministic, however, is an open question. Indeed, whether evolution itself is contingent in the same way as human history is one of the most vibrant and important debates in biology. Evolution is different from many other natural phenomena in that it is fundamentally historical. Like human history, evolution plays out over time and involves a fundamental tension between chance and necessity. Natural selection deterministically adapts organisms to their environment by incessantly winnowing the wheat of beneficial variation from the chaff of the neutral or the detrimental. The variation on which natural selection works arises from random genetic changes, however, and even the most beneficial mutations can be lost, especially in small populations, due to the lottery-like effects of genetic drift—random fluctuations in gene frequencies. Moreover, just as in human history, evolution must take place in the context of what has evolved before, which can alter the prospects for future change. Finally, evolution is a process that is strongly affected by