In nearly every life, there is a moment when a person realizes, with a shudder, how easily she might never have come to be: how her parents nearly missed meeting, or how some other critical genealogical event almost didn't happen.
In the same way, evolutionary biologists have pondered one of their most intractable questions: how much of the living world is here by chance and might not evolve, if time were turned back and evolutionary history played out again?
A few scientists have begun finding ways to test the repeatability of evolution and have found out that what they thought were the random vagaries of evolution are not so random at all.
"There's a lot of phenomenal data coming out," said Loren Rieseberg, an evolutionary biologist at Indiana University. "There's clearly more to repeatability than we'd suspected a decade ago."
Richard Lenski, an evolutionary biologist at Michigan State, said, "A lot of studies are finding quite a lot of surprising replicability of evolutionary outcomes."
It's a wonderful world
Stephen Jay Gould, the late Harvard paleontologist, crystallized the question in his book Wonderful Life. What would happen, he asked, if the tape of the history of life were rewound and replayed? For many, including Gould, the answer was clear. He wrote that "any replay of the tape would lead evolution down a pathway radically different from the road actually taken."
In fact, to many scientists, it would seem impossible to re-evolve anything like life on earth today, given how life has been shaped by accidents large and small.
But 12 flasks of bacteria in East Lansing, Michigan, are beginning to challenge such notions. In 1988, Lenski and his colleagues set up a dozen genetically identical populations of E coli bacteria in bottles of broth and have followed their evolutionary fates.
Now, more than 30,000 bacterial generations later, Lenski and colleagues have what is becoming one of the most striking examples of repeatability yet. All 12 populations show the same patterns of improvement in their ability to compete in a bottle and increases in cell size. All 12 have also lost their ability to break down and use a sugar, called ribose.
More surprising, many genetic changes underlying these adaptations are very similar. Every population, for example, lost its ability to break down ribose by losing a long stretch of DNA from the same gene.
Other scientists studying cichlid fish have observed how the same varieties of cichlids evolve anew every time they invade a new lake. And Rieseberg and colleagues have found evidence that evolution can repeatedly produce the same species.
These scientists found that one sunflower species on sand dunes has evolved independently three separate times. And each time one of the species newly evolves, genetically it appears to turn out much the same. "With these species, there seems to be only one way to do it," Rieseberg said.
Some scientists, like Simon Conway Morris, a paleobiologist at the University of Cambridge and ardent critic of Gould's view, say the evidence for repeatability is rampant. He argues in his new book, Life's Solution: Inevitable Humans in a Lonely Universe, that some features are so adaptive that they are essentially inevitable -- like the ability to see and, as his title suggests, the intelligence and self-awareness that are the hallmarks of humanity.