Some accuse evolution of being pretty directionless. After all, the whole process involves random genetic mutations. But if evolution is just a toss of a million-sided die, why do so many animals turn out eerily similar? Nope, it's not intelligent design. We take a look at the creepy look-alikes brought on by what biologists call "convergent evolution."
Could a koala frame you for a crime? Probably not you, but it could certainly frame your species. Koala fingerprints and human fingerprints are so alike that experts can mistake one for the other. Which makes no sense, since koalas and humans split off from each other between 125 and 150 million years ago. Our genetics haven't crossed over since (although that would be one cute baby), we're not the same size, we don't do the same things, and yet the pads of our fingers look exactly alike. How did that happen?
With the emergence of epigenetics, we are getting hints that passing on certain characteristics to one's offspring may not be entirely random. Lifestyle, stress, and nutrition in the previous generation can play a part in the next generation, and may even shape the species. Still, the heavy lifting of evolution has always been in random genetic mutation. Nature will throw whatever mistakes and quirks happen in the double helix at the wall and see what sticks. Convergent evolution happens because only a certain number of things stick to a certain kind of wall. There are only so many ways to climb a tree, live in desert sands, or go between the sea shore and the ocean. As niches get more specific, more specific methods are needed to fill them, and distinct animals will inevitable evolve specific similarities.
Convergent evolution can be prompted by any set of conditions. White snow brings out white plumage, fur, or scales, in all kinds of unrelated species. Similar predators will chase totally different species into the same trees, or under the same rocks, or force them to fight with the same poison. Any specific food source that isn't already being depleted will bring out similar characteristics in different species. And, of course, much depends on how similar the species are in the first place. This is why placental mammals and marsupials are the poster species for both divergent and then convergent evolution.
When Marsupials Went Away and How They Came Back
Marsupials and placental mammals were identified as different species 125 million years ago, splitting off from a common ancestor via divergent evolution. They seem to have been working their way back towards each other ever since. Marsupials dispense with the last stage of pregnancy and simply give birth to a severely underdeveloped offspring. The baby animal works its way around to a pouch or protected spot on the marsupial and grows from there. Mammals came up with a special thing called the placenta, which nourishes the fetus in the uterus for much longer, and so they give birth to more developed babies. Although we think of marsupials as Australian, since that continent supports the most dominant and diverse marsupials, it's likely that they got there from South America via an iceless Antarctica millions of years ago.
Placental mammals and marsupials found their way with similar genes to similar environments, and converged so spectacularly that they've been featured on intelligent design blogs ever since. Some would say that their similarities are more the results of parallel evolution, but considering the distance and the time that separate the animals, and the uncannily similar animals they developed into on separate continents, they do display a gift for convergence.
The flying squirrel has its marsupial equivalent in the flying phalanger. The anteater meets its match in the long-tongued ant-eating numbat. (These not only developed, distinct from other species, lost teeth, developed massive salivary glands, and pumped up their stomachs enough to eat ants. They became the same animal multiple different ways.) Placental mammals and marsupials even fill the same evolutionary nitches. A small forest-living kangaroo in Australia stores fruit by burying it, the way squirrels do in the rest of the world. Since trees with the most kangaroo-or-squirrel-accessible fruit benefit most from this, entire convergent ecosystems spring up. Each pair of animals aren't within over a hundred million years and several oceans of each other, and yet each could pass - on sight - for close relations. But while marsupials and mammals are the most widespread examples of convergent evolution, they aren't the weirdest.
Convergent Evolution's Weirdest Hits
Nature developed antifreeze. Twice. At each end of the earth fish have special substances in their blood. These substances, called glycoproteins, slightly lower the temperature that fish blood can be at before it freezes. The proteins surround ice crystals and keep them from spreading. They'd be remarkable if they popped up once on a planet. However, fish that survives in polar waters don't do so well in equitorial waters. And it would be a long swim between them. So two different sets of fish came up with the exact same adaptation to help them keep alive in the cold. Convergent evolution goes down to a molecular level.
Dolphins and bats couldn't be less like each other while still being mammals. And yet they have the same gene, called prestin, which encodes for special proteins. Where do these proteins go? The inner ear. Both animals find their food, and their way around, by echolocation. They seem to have independently stumbled on proteins, and a gene sequence, that helps them in this. They converge in ways that we can't see, but they still converge.
In Madagascar, an island cut off from major land masses before there were even monkeys, there is an aye-aye, a lemur with a long thin finger that it uses to prise bugs out of tree bark. Across the world there is the long-tailed possum, which has a similar finger. These two animals have little in common, except an environment without woodpeckers. They had a food source, they weren't beaten to it by the birds, and so through generations, the best fed and most fertile aye-ayes and possums were the ones with long fingers.
Marsupials and placentals don't just imitate each other in the modern day. We've all seen pictures of the long-extinct saber-toothed tiger, but it had its own marsupial equivalent in its own time. The thylacosmilus was a marsupial with not only saber canines that jutted from its upper jaw, but what looked like long downward-sweeping wings from its lower jaw. There were a lot of members of the extended canine club among extinct mammals of a vaguely tiger or wolf like appearance. It's possible that these were in response to selective mating. Even what animals find sexy can cause convergent traits.
Other marsupials, which seem to be carrying the day, don't limit themselves to converging with placental mammals. Honey possums are tiny mouse-like creatures that fill the roles of butterflies. They weight at most eleven grams and have developed long tongues that they use to lick the nectar out of flowers (while pollinating them).
And then, of course, there are the koalas and their ability to leave wrongful evidence at crime scenes. Chimps have fingerprints. Gorillas do, too. Most monkeys and tree-faring animals do not, although they have other things that give their grip texture. Koalas almost never get out of trees, though, which leaves biologists puzzled. As far as they can tell, the koala's rather picky eating habitts - they only eat leaves of a certain age - might cause them to need to grasp things more carefully and creatively than other creatures do. The fine whorls and loops on a fingerprint can give people, and animals, extreme control when grasping and manipulating certain objects. This is possibly way we share our prints with only the animals that need to be especially dexterous.
Or especially devious. Koala prints a crime scene would make for a slam-bang episode of Law & Order, though, if someone could only think up a plot.
Top Image: David Iliff
Numbat Image: Martin Pot
Skull Picture: Claire Houck