The Aliens Are Coming!

by Ben Miller

  To be blunt, it had the head of a duck. Or, at least, the mandible of a duck; there was something distinctly fishy about its minute eyes, so small that one had to hunt to find them amidst the tufts of fur. Shaw was well acquainted with the so-called “mermaids” of London society; freakish fakes wherein charlatans had procured monkeys’ torsos and grafted them to the tails of fish. Had some Eastern mountebank sewn the bill of a duck on to the body of a mole or beaver?

  He examined the base of the beak, surrounded as it was by a circular flap of somewhat leathery skin. Was the flap there to hide some surreptitious stitching? Try as he might, he could find no traces of thread that might prove the lie. Perplexed, he took a shallow dish and doused the entire pelt in water to see if he might loosen whatever glue was holding it all in place. It refused to fall apart. A horrible truth began to dawn on him. He was going to have to classify this monstrosity by naming its genus and species. But what was he looking at? A bird, a mammal, a fish, or a reptile?

  A WALK ON THE WILD SIDE

  If there is communicable alien life out there in the galaxy, what is it like? The possibilities seem endless. Life on Earth is bewilderingly diverse. We find it everywhere, both microbial and multicellular, eating every conceivable type of food and inhabiting a myriad physical forms. Surely no one could have predicted the existence of the duck-billed platypus from first principles, no matter how much they knew about cellular biology, genetics, paleoclimatology, and the ecology of east Australian rivers? And if we can’t second-guess life on our own planet, what can we ever hope to deduce about aliens?

  Surprisingly, the situation isn’t nearly as hopeless as you might think. As we are about to see, there’s a wealth of information about life on Earthlike planets to be gleaned from life-as-we-know-it, and an abundance of informed speculation to draw on with regard to life-as-we-don’t. Riddles such as the duck-billed platypus have given up their secrets, and the lessons we’ve learned are particularly instructive when it comes to alien-hunting. Not only are we about to meet a menagerie of exotic, exquisitely alien life-forms inhabiting all manner of weird locations, but when it comes to rocky wet planets it turns out the Earth isn’t the only game in town.

  LOOKING IN THE LILY POND

  As we know, the Kepler Space Telescope tells us that roughly one in five Sun-like stars has an Earth-sized planet in its habitable zone; that is to say, at the right distance to have liquid water on its surface. On average, that means the nearest such planet could be as little as twelve light-years away. According to Einstein, that means that even if we find it tomorrow, and invent speed-of-light space travel the day after, it’s still going to take over a decade to get there. And if we fail to invent speed-of-light travel, with present technology the journey is going to take tens of thousands of years.

  I tend to be optimistic about these kinds of things. Knowing a little about humankind, I can’t help thinking that once we have found our nearest Eden we will want to get there as quickly as possible, and necessity, as they say, is the mother of invention.1 But even if we conjure a technology that can drive our ships at, say, 10 percent of the speed of light, the adventure of breathing the air on a planet similar to the Earth has to be several generations away at best.

  So what do we do in the meantime? Are there any clues that we can glean from life on Earth as to what life on other Earths might be like? Happily, there are.

  All life on our planet may share a common ancestor, but it has often inhabited parallel worlds. In some cases, these worlds have been evolving independently for tens of millions of years, with startling consequences. They are, in short, the closest thing to setting foot on an Earthlike planet that we will experience in at least three generations. I’m talking, of course, about some of the greatest wonders of the globe: islands.

  CONTINENTAL DRIFT

  As we discussed in Chapter Six, at the time of the Permian extinction some 250 million years ago, all the present-day continents were joined together into one giant landmass, or supercontinent, known as Pangaea. Thanks both to its size and a runaway greenhouse effect, the center of Pangaea became a dune-filled desert. We can still see its legacy today in the form of colossal deposits of sandstone; in fact, my own county of Cheshire in the UK is home to a sandstone ridge that is a relic of the Permian.2

  The Permian, as you may know, marks the end of the Paleozoic Era, when plants and animals colonized the land. The Mesozoic Era, which followed immediately after, saw Pangaea begin to break up. The first big rift came toward the beginning of the Jurassic, when Pangaea split into two: the northern lump, called Laurasia, was made up of North America, Greenland and Eurasia; while that in the south, called Gondwana, was made up of South America, Africa, Antarctica, India, and Australia.3

  By the beginning of the Cretaceous Period, roughly 140 million years ago, Gondwana was starting to break up. One by one, continents broke off the southern land mass and headed north to join Laurasia. Africa was the first to leave, splitting from South America to create the famous jigsaw puzzle by which the right hip of South America fits neatly into Africa’s left. India followed soon after, making a beeline for Asia, while Africa headed north toward Europe. In the middle of the Cretaceous, about eighty million years ago, it was New Zealand’s turn to take the plunge, while Madagascar split off from a still-migrating India.

  Australia and South America were the last to leave, remaining joined to Antarctica until well into our present era, the Cenozoic. Australia departed first, in the Paleogene Period, roughly forty-five million years ago. South America and Antarctica then hung on until the beginning of the Neogene Period, roughly twenty-three million years ago, at which point the Drake Passage opened up and the forests of Antarctica were replaced by permanent snow.

  For the next twenty million years, South America migrated north, until eventually, at the beginning of our present Quaternary Period, an upwelling of volcanic rock created the Isthmus of Panama, and it became joined with North America. Finally, the globe had taken its present form, and a permanent ice cap began to form in the Arctic. The transformation from the tropical hothouse of the Jurassic, Cretaceous, and Paleogene into the icehouse of the Neogene and Quaternary was complete.4

  What all this means is that from the early Cretaceous onward, Gondwanan life effectively became marooned on a series of islands and continents, each of them like a tiny Earth. Fascinatingly, these alternate worlds paint a very different picture of how evolution might have turned out. On North America, for example, placental mammals came out on top, with marsupial mammals forced to extinction; on South America, however, the reverse was the case, and marsupial mammals dominated. In Australia, it was marsupial mammals that made the cut while the placentals perished; in New Zealand, no mammals survived at all.5

  IF IT WALKS LIKE A DUCK . . . IT’S NOT NECESSARILY A DUCK

  Madagascar is a case in point. Located in the Indian Ocean, just off the coast of East Africa, it is home to a fascinating array of mammals, none of which were present on the island when it rifted from India. As far as primates go it has no monkeys or apes, and no dogs or cats among its carnivores. All of the mammal species it does have—and it has a great many—are the descendants of creatures that either swam, floated, or flew across from Africa and India.6

  At some point round about sixty million years ago, a family of lemurs appears to have made the trip from East Africa. Not that they booked a package tour, of course; our best guess is that they were clinging to a tree during a hurricane when they got washed out to sea. A bunch of tenrecs—small, shrew-like mammals—followed in their wake roughly thirty million years ago, and other rafts bobbed up on the shore around twenty million years ago, carrying rodents and carnivorans, the mongoose-like ancestors of carnivorous mammals.

  It’s the lemurs that most people have heard of; present-day Madagascar is home to nearly one hundred different species, all descended from that first vagabond crew of early primates that crossed the Mozambique Channel on a fast-moving ocean current. The remarkabl
e thing about lemurs is not only are they box-office catnip, as demonstrated by the fabulous Madagascar movies, but they are a striking visual echo of what the first primates must have looked like. Gaze into the eyes of a lemur, and you can almost imagine you are eyeballing one of our shrew-like ancestors in the tropical rainforest of the Paleocene.

  But while it’s lemurs that get the glory, as far as our story goes it’s the carnivorans that are the real stars. Do yourself a favor and Google a creature called a fossa. What pops up is something that looks like a cougar; so like a cougar, in fact, that many nineteenth-century taxonomists placed it in the cat family. The fossa has a head like a cat, a body like a cat, and a tail like a cat. It climbs trees and can semi-retract its claws, just like a cat. Yet, intriguingly, a fossa isn’t a cat. Instead, DNA studies have shown it to be a direct descendant of the African carnivorans that washed up in the Miocene some twenty million years ago.

  Let’s think about that for a moment. There have never been any jungle cats on Madagascar, yet the fossa looks just like one. How can that be? We’ll never know for sure, but the smart money says that the fossa is a rather striking example of what we call convergent evolution. The same way of making a living—chasing small mammals through tropical trees—has produced two animals that are remarkably similar in appearance, despite the fact that they are only distantly related.

  The fossa is one example, but Madagascar is home to many others. One of my favorites is the tenrec, several of which have evolved to look exactly like hedgehogs, despite being only distant relatives. Not that examples of convergence are limited to Madagascar; Australia, New Zealand, and South America are all full of cases where unrelated creatures have ended up looking remarkably alike.

  Where we find placental flying squirrels and moles in North America, for example, we find marsupial flying squirrels and moles in Australia, even though their last common ancestor was probably doing its best trying to avoid being trodden underfoot in the Cretaceous jungle. The saber-toothed placental tiger of Ice Age North America had a much earlier doppelgänger in the saber-toothed marsupial tiger of South America.7 Even more striking, in my humble opinion, is the outward resemblance of the Australian thorny devil lizard, Moloch horridus, to the North American desert horned lizard, Phrynosoma platyrhinos. Both are desert-dwelling ant-eating lizards with blotchy markings and protected by exaggerated spines, and yet they are about as unrelated as it is possible for two lizards to be.

  Such whole-animal convergences are striking, but they are only part of the story. Partial convergences are even more common, where creatures have very different body plans but similar body parts or behaviors. Flight, for example, has evolved at least four times, in insects, pterosaurs, birds, and bats, while, as we have already learned, camera eyes have evolved independently both in vertebrates and octopuses. The bearing of live young is estimated to have evolved over one hundred times among lizards and snakes, usually in response to cold climates, while hosts of animals we previously thought were related—flightless birds, for example—turn out to be only distant cousins.

  RED IN TOOTH AND CLAW

  It was just these kinds of partial convergences that so confused the European investigators of the duck-billed platypus. Following Shaw’s examination in 1799, the classification of what is properly known as Orinthorhynchus anatinus8 only became more problematic when, in 1802, the surgeon and anatomist Sir Everard Home reported that it possessed a cloaca, a single opening for the alimentary, reproductive, and urinary tracts. That would seem to class the platypus among either amphibians, reptiles, or birds, and Home even theorized that it laid eggs. At this point the French anatomist Etienne Geoffroy Saint-Hilaire waded in, declaring that both the platypus and its countryman the echidna, or spiny anteater, represented an entirely new class of vertebrates. He named this new class the monotremes, from the Latin for “single opening.”

  Not that his fellow naturalists took much notice, preferring to join the mammal vs reptile fracas. The pendulum swung even further in the direction of reptiles with the 1823 discovery by the German anatomist Johann Friedrich Meckel that the platypus’ spur was venomous. Venom, of course, is associated with reptiles like snakes and lizards. Three years later, however, Meckel published a paper that proved beyond doubt that the platypus had mammary glands, propelling the pendulum hard in the opposite direction. The entire platypus debate now hinged on one crucial question: Did it lay eggs?

  The Aborigines were adamant that it did, but so rigid was the belief that mammals only gave birth to live young that, to begin with, few academics took the idea seriously. Even as late as 1884, the Sydney Morning Herald declared that any evidence to the affirmative must be “examined and reported on by scientists in whom the world has faith, then all the scientific world will stand convinced and will believe where they have not seen.”

  That same year, a tyro Scottish zoologist named William Hay Caldwell decided to blow his entire academic grant on traveling to Australia to settle the matter once and for all. An ecologically sensitive program of shooting and dissecting platypuses had been in progress since 1834, overseen by the Australian naturalist George Bennett, the curator of the Australian Museum, Sydney, New South Wales, but the obnoxious Caldwell adopted what can only be described as a slash-and-burn approach.

  In the Australian winter of 1884, assisted by an army of local Aborigines, he set up camp on the banks of the Burnett River in northern Queensland, and began slaughtering all the platypuses he could find. On August 24, following a three-month period in which he had destroyed more than seventy platypus, he shot a female which had not only just laid an egg, but also had one in her uterus, ready to be laid. His triumphant telegram, “monotremes oviparous, ovum meroblastic,” was seen as the last word on the matter. Not only did the platypus lay eggs, it said, but those eggs were reptilian. The platypus was officially a conundrum.

  THE EVOLUTION OF MAMMALS

  If the study of islands and continents that have been isolated for tens of millions of years is the next best thing to finding an Earthlike planet, then to my mind at least the discovery of the duck-billed platypus is the next best thing to capturing an alien. And thanks to the Platypus Genome Project of 2008, we now have a much clearer idea of how this extraordinary creature fits into the grand sweep of evolution.

  Part of the answer, as you might have guessed, is that the platypus is a descendant of a third group of mammals, the monotremes, which sits alongside the marsupials and placentals. The fossil record of monotremes is patchy, but suggests a radiation in the late Triassic or early Jurassic, eventually becoming extinct everywhere except in Australia, where they continue to thrive.

  So far as we can tell from DNA studies, the last common ancestor of monotremes, marsupials, and placentals probably lived some time during the Triassic, and appears to have been a furry egg-laying creature that suckled its young. Throughout hundreds of millions of years of evolution, the platypus has continued to lay eggs, while the other two surviving branches of the mammal family tree—marsupials and placental mammals—have instead evolved the ability to give birth to live young.9 So much for eggs. What about the platypus’ other extraordinary traits, like its venomous spur and its duck bill? Fascinatingly, this is where convergent evolution comes in. Platypus venom is remarkably similar to reptile venom, but turns out to have evolved completely independently. The platypus’ bill provides even more of a surprise. Everyone knew the platypus had to be doing something clever to be able to catch half its body weight in insect larvae at the bottom of muddy streams in the dead of night with its ears, eyes, and nostrils closed; that something turned out to be what’s known as electroreception.

  Electroreception is very much de rigueur in fish, but almost unheard of in mammals. It turns out that the platypus’ extraordinary bill, as well as mimicking shovelers such as the duck, is home to a vast array of receptors. As it swims, it swings its head from side to side, detecting the minute electric fields given off by its prey. In other words, despite having t
heir last common ancestor way back in the Devonian, fish and platypus are both capable of sensing electric fields. Their evolutionary journeys couldn’t have been more different, but given the same selection pressures—trying to make a living in murky water—the final destination was the same.

  So what does all this tell us about aliens? Well, for starters it tells us that just because a planet is Earthlike, four billion years of evolution isn’t necessarily going to produce anything remotely human. After all, on New Zealand and Australia placental mammals didn’t even make the cut, let alone the subdivision that we belong to, the primates.10 And even if primates do evolve, and subsequently give rise to ground-dwelling, bipedal apes, there’s no guarantee that they will fare any better. The Madagascan lemurs, for example, convergently evolved several “ape” species, but they all went extinct.11

  On the other hand, thanks to convergence, what comes around goes around. The islands and remote continents of the Earth show us that certain adaptations arise again and again: when it comes to life on planets like our own, we should expect to find the same notes, just not necessarily in the same order. Things like wings, eyes and teeth will be common on Earthlike planets throughout the galaxy, even though the creatures that possess them may be as unfamiliar to us as the duck-billed platypus was to nineteenth-century naturalists. These are, after all, the solutions that work time and time again, and toward which evolution will always stumble. And here’s the rub. Intelligence, the vital commodity we need to find if we are to communicate with aliens, turns out to be a convergent trait.

  FEATHERED APES