Pity the poor platypus. A creature so confused that when the first specimens arrived in England at the end of the eighteenth century, scientists searched for hours in vain to locate the stitches crafty Chinese merchants must have used to assemble their hoax. With its reptilian reproductive mode (a mammal that lays eggs!), duck-bill, 10 sex chromosomes, and variety of anachronistic traits, it seems like a quirky relic of a bygone age, the antithesis of a thoroughly modern model of evolutionary advancement like ourselves.
Certainly, we humans are a marvel of natural selection. Our big brains, opposable thumbs, and great running endurance superbly adapted us to our ancestral savannah homes and from thence to take over the world. But don’t sell platypuses short—reputation to the contrary, they are extremely well-adapted to the Australian streams in which they live: lush, dense fur lets them be active at temperatures near freezing, webbed feet and a powerful tail make them agile swimmers, and their fabled bill is not only extraordinarily sensitive to touch, but houses electroreceptors capable of detecting the slightest twitch of a crayfish’s muscles, allowing them to find food underwater while their eyes and ears are tightly closed.
Despite our many differences, we humans and platypuses (not platypi, that would be mixing Greek and Latin) share an important distinction: not only are we both well adapted to our ecological niches, but we’re both evolutionary singletons, species that have no parallel, no evolutionary doppelgänger, either today or in the past. Why that is so is a mystery. If big brains, bipedality, and opposable thumbs are so useful, why didn’t natural selection lead to the evolution of human-like creatures multiple times? As for the platypus, streams like the ones they live in occur throughout the world, yet the duckbill is singular.
Convergent, repeated adaptation is actually widespread in the animal world. Consider sharks, dolphins and ichthyosaurs (extinct marine reptiles from the Age of Dinosaurs), all stream-lined, fast-swimming marine predators with a dorsal fin and a broad, powerful tail for propulsion. This similarity is the result of natural selection sculpting the same adaptive features from different evolutionary starting points (a fish, a land mammal and a land reptile, respectively). In the same way, the prickliness of New and Old World porcupines is independently derived, as are cacti and just as spiny African euphorbs, and many other cases of convergent evolution. Even the saber-toothed tiger of the La Brea tar pits is not unique: similar colossal canines with steak-knife serrated edges evolved several times in prehistoric felines, as well as in an extinct, lion-like relative of the opossum from South America.
But for every example of adaptive convergent evolution, there is another case in which natural selection has failed to repeatedly take the same course. The dominant plant-eater in Australia today is the Kangaroo, a bipedal bounding herbivore unmatched anywhere else. Everyone knows Woody Woodpecker, but in Madagascar, the job of extracting grubs from trees falls to a primate, the aye-aye, which uses chiseling incisors and a long snaggly finger instead of Woody’s pounding beak and long, bristle-covered tongue.
Why convergent evolution occurs sometimes and not others is now the focus of intense scientific research. Some scientists are sequencing genes and even entire genomes to determine whether similar-looking species are actually closely related evolutionarily—in many cases, it turns out that they aren’t, their similarity the result not of inheritance, but of convergent evolution. Other scientists are conducting evolution experiments, taking advantage of our newfound knowledge that evolution can occur very rapidly to test whether populations exposed to identical environments adapt in the same way. Initially, these experiments were conducted in the lab on fruit flies and microbes, but increasingly evolutionary biologists are taking the same approach in the field, conducting evolution experiments on organisms as diverse as lizards, guppies, and evening primroses in completely natural settings.
Understanding what determines whether convergence occurs has important implications. For example, knowing when pests or microbes are likely to adapt in the same way can guide efforts to counter them. But the biggest implication may be more ethereal. Recent years have revealed the existence of scores of potentially habitable, earth-like planets, the nearest only four light years away. Some scientists estimate there may be billions in our Milky Way galaxy alone. If life has evolved on some of these planets—and given the sheer numbers, many consider it inevitable—what would it be like?
If we’re to believe Hollywood, quite a lot like life here on Earth. From Star Trek to Star Wars to Valerian and beyond, most interplanetary science fiction movies populate their worlds with lifeforms quite similar in general appearance and biology to what has evolved here on Planet Earth. Guardians of the Galaxy takes this approach to a new extreme with Groot, a humanoid (albeit with a tri-syllabic vocabulary) evolved from a plant-like ancestor. Even some scientists agree. “They will look an awful lot like us,” said one biologist; “the emergence of something like ourselves [is] a near-inevitability” agreed another.
Experimental studies of evolution and the history of life on Earth disagree. Populations and species evolving independently in different environments sometimes adapt in the same way, but oftentimes they don’t. Critically, the more different they are at the outset, the more likely they are to take different evolutionary routes—the contingencies of history, endowing the different species with different genes and traits, make it more likely that natural selection will find different ways to adapt. And life forms on different planets would certainly start out different—who knows what their genetic system would be, to cite just one difference? Not to mention that planets, even if earth-like, would differ in many ways from Earth 1.0 and consequently the selective pressures would likely be at least somewhat different as well. Just like the hexapods in last year’s movie Arrival, alien life forms are likely to be, well, alien, bearing little resemblance to us or the terran species with which we’re so familiar.
But that conclusion shouldn’t be surprising. All we have to do is look at independent worlds of evolution here on Earth. New Zealand has no native land mammals and in the 80 million years since it detached from Australia, birds have evolved to fill the ecological roles occupied elsewhere by mammals. But a kiwi bears little resemblance to a badger and a moa isn’t much like an antelope. That’s not to mention the carnivorous parrots, hamburger-sized snails, bats that run around on the ground like mice and shrubs that grow their leaves on the inside with the branches on the exterior. Evolution in a bird-dominated world turns out very different. And the same is true on every large land mass that has evolved in isolation—Madagascar, South America, not to mention the Cretaceous world of the dinosaurs—the same types of ecological actors evolve, but they fill their roles in diverse and unique ways.
Will other worlds be populated by humanoid species? Probably not. But why focus on us? I’d rather speculate about extraterrestrial platypusoids, as well as analogs to other evolutionary singletons, such as elephantoids, chameleonoids and kiwioids. And in all cases, the answer is the same: if they only evolved once here on Earth, there’s really not much reason to expect similar life forms to evolve on other planets. With all of those Earth-like planets out there, it seems likely that life has evolved on some of them. But my guess is that when we finally meet our galactic neighbors, they won’t be erect, bipedal, big-brained animals, nor furry, web-footed creatures with a duck-like bill.
Jonathan Losos is Monique and Philip Lehner Professor for the Study of Latin America, Professor of Organismic and Evolutionary Biology and Curator in Herpetology, Museum of Comparative Zoology at Harvard University. He also serves on the National Geographic Society’s Committee for Research and Exploration.