Among the deeper questions looming in science is the degree to which the outcome of evolution is governed by chance. Stephen Jay Gould famously addressed the question in his book Wonderful Life, proposing that if time were wound back 500 million years or so and allowed to run again, evolution would produce a completely different mix of living things - one without us.
Penn State evolutionary biologist Blair Hedges says his observations have led him to disagree. He once wrote to Gould, raising the objection that he has seen too many examples of different lines of animals taking parallel evolutionary paths, converging on the same shapes and survival strategies.
The most striking examples come from his work in the Caribbean - especially Haiti, the site of a recent expedition that brought back nearly three dozen lizards and frogs, some of them remarkably similar to animals that evolved independently elsewhere.
In fact, Hedges argues that if life has arisen on other planets, evolution would produce some of the same patterns and themes seen here on Earth. And there's a reasonable chance you'd see some kind of creature that, like us, has a yen to explore the cosmos.
The Caribbean offers a natural experiment in parallel evolution, he said. DNA testing he has done suggests that all the species of frogs and lizards there came from small "founder" populations that arrived from South America around 30 million years ago and then evolved separately on the different islands.
The results, he said, can help explain how evolution works and to what degree its outcome is deterministic or random. Just as Darwin used finches on the Galapagos to understand the mechanism of evolution, Hedges is seeking insights from frogs and lizards on the larger islands - Jamaica, Cuba, and Hispaniola, which includes the Dominican Republic and Haiti.
Haiti has an enormous diversity of animals but they're confined to remote patches of forest, hard to access by foot or car. Ninety-nine percent of Haiti's original forest has been cut down or burned. For his expedition two weeks ago, Hedges used Google Maps to pinpoint four mountaintops that he believes had never been explored by biologists before.
To get there, he and his team were deposited by helicopter, spending a day at each place. To limit weight, they doubled and tripled up in tents and subsisted on trail mix, granola bars, and caffeine pills.
It paid off: His team collected members of 33 species, some of them never before recorded by scientists. (I tried my hand at animal collecting with the team, and a more complete account of the trip will appear soon.)
The deprivations don't faze Hedges, 54, who on these trips spends much of his days and nights out in the forest collecting living specimens. He's been considered one of the most prolific discoverers of new animals, having described 79 new species of lizards, frogs, and butterflies over the years.
He keeps coming back to the Caribbean, he said, and the more animals he finds, the more common patterns he sees from island to island. Yet DNA analysis shows they are the product of independent lines of evolution, adapting in parallel through a process called convergence.
If, as Gould imagined, the tape of life were rerun, Hedges said, "I don't think it would be exactly the same . . . but there are some very repeatable patterns."
A perfect example, he said, was a rare and critically endangered animal found on his latest trip - a 6-inch-long striped lizard named after Philip Darlington, the Philadelphia-born naturalist who discovered it in Haiti in the 1930s. This particular species hadn't been seen in 27 years despite several expeditions that set out in search of it.
The darlingtoni is one of a number of similar lizards, called twig anoles, that are known for sleeping while clinging to twigs, and looking very much like twigs.
Despite their very specialized behavior and body type, twig anoles have evolved independently on Haiti, Jamaica, and Cuba, Hedges said. They have become slender bush anoles and stockier ground anoles - with representatives of each type appearing, apparently independently, on all the islands.
Frogs, too, have diversified on the islands and often followed surprisingly parallel paths, he said. Another surprise find was a Haitian aquatic frog whose young hatch directly from eggs, without going through a tadpole stage. Similar frogs have emerged independently in Jamaica.
Hedges says his views are similar to those of Simon Conway Morris. The Cambridge University paleontologist was featured as a hero of Gould's Wonderful Life but later attacked Gould's conclusion, arguing instead that evolution is predictable and that human existence was inevitable.
Conway Morris is a controversial figure among scientists for his public embrace of Christianity. Though he's not a literal biblical creationist, his view of humanity as inevitable is thought to accommodate the notion of being made in God's image better than Gould's view of the human species as an evolutionary fluke.
Hedges said he didn't think the predictability of evolution had any religious implications. But he does think it might have a bearing on the nature of extraterrestrial life - a topic on which he bases the last lecture in his senior level biology class.
Life, he said, simply finds optimal solutions to common survival problems. "We should not expect to see organisms that look identical to those on Earth, but we will probably see things that are, more or less, familiar to us," he said. "I believe that this expectation applies to even basic things such as cells and photosynthesis."
Hedges said he did get a response to his letter to Gould, who replied that evolution might run the same way twice if the time frame were limited to a few million years, but not over longer time scales.
And, clearly, life on Earth is shaped by chance events, such as the giant asteroid that hit 65 million years ago, Hedges said. That accidental impact pushed aside the dinosaurs and opened a niche for large mammals.
Harvard biologist Jonathan Losos, who has done his own studies of lizards in the Caribbean, said he was inclined to side with Gould - that chance rules over predictability.
The twig anoles do show that there is a pattern to evolution, Losos said, but one that is limited. "The anoles of each of the islands have diversified independently and produced the same set of specialists," he said.
But that's in the short term. "I still think contingency and historical factors and chance play a major role in determining evolutionary direction," Losos said.
Others, such as University of Chicago biologist Neil Shubin, say it's not surprising to see patterns in evolution since different creatures use common genetic recipes and developmental patterns that may set them on predictable evolutionary paths.
There are random events along the way, Shubin said, but that doesn't mean that anything is equally possible for living things. Certain genetic mutations are more likely than others, he noted, but they don't arise with any purpose. If an animal could benefit from a longer neck, or perhaps a bigger brain, genetic mutations won't appear in anticipation of these needs.
Brandeis University biologist Greg Petsko said that while the question of chance in evolution could be informed by science, it is, essentially, a philosophical issue.
That may explain how Gould and Conway Morris can draw opposite conclusions from the same fossils.
"If you believe that things would come out essentially the same if you rewound the clock, you are arguing that the whole of evolution represents an inevitable march towards us - that we are the fittest, as it were," Petsko said.
"If you believe, as I do, that it would probably turn out quite differently, even with a constant geology, then you are saying that there isn't anything all that special about Homo sapiens, that man is an experiment, just one among billions of experiments, and one that will probably fail, as most have in the past."