The Canon (28 page)

Or compare the following foursome of forelimbs—a bat's wing, a penguin's flipper, a lizard's leg, a human's arm. On the surface, the various appendages look quite dissimilar from one another, and they perform distinct tasks: flying, swimming, darting, purchasing hands-free electronic devices. Yet beneath the miscellany lies skeletal homogeneity, for each forelimb houses the same set of four bones: humerus, radius, ulna, and carpal. The bones are splayed in the bat's wing, converged to a V-tip in the penguin's flipper, but they are readily seen on an X-ray as anatomical homonyms. Embryonic development further confirms the link. If you were to watch a time-lapsed video of how the respective fetuses grow—the baby lizard and penguin in their eggs, the bat and human in their wombs—you would see the four forelimb bones budding out from the same prefatory parts in the embryo. Such structural and developmental cronyism tags us all as descendants of the first vertebrate tetrapods, the valiant four-legged forebears that traded the seas for the soil. Their basic skeletal structure proved so fit for the challenges of terrestriality that all vertebrate forelimbs are modified meditations on the humerus-radius-ulna-carpal theme; we wear the tetrapod coat of arms up our sleeve.

Haldane's droll observation about the proper placement of petrified rabbits is another firmly grounded finding, a fact that must be faced. You won't find rabbit fossils in a bed of trilobite remains. Trilobites, those familiar paleopetroglyphs that look variously like horseshoe crabs, cockroaches, and Game Boy video icons, were for 300 million years the dominant life form in Earth's oceans. There were more than 10,000 species of trilobite, ranging in size from a millimeter—the comma you just passed—to creatures as long as your arm. Trilobites were bottom feeders, seaweed grazers, trilobite biters. They breathed through and swam with their gills, and they had eyes like no other. Whereas the lens of a standard eye is constructed of protein molecules, the trilobite lens was like a marble chip, made of the mineral calcite. Yet by a quarter of a billion years ago, those sharp little eyes had had it. Trilobites went extinct at the end of the Permian period, along with 90 percent of all other marine species then on Earth. The last trilobite likely imprinted its image in perpetuity about 20 million years before any remains of the earliest mammals can be spied in the fossil record, let alone of the first rabbit, which dates from a mere 57 million years or so ago. Paleontologists have seen it and shown it again and again. Fossils are found in the right place and from the right time, newer fossils stacked in layers above older fossils. Trilobites abounded around the world, and whether you're digging in Australia, Austria, or Cincinnati, their fossils are always located in sedimentary beds at just the depth and relative position you'd expect them to be for a creature that thrived a half billion years ago. The same for dinosaur fossils, or the bones of all the archaic, outrageous mammals from the Oligocene, circa 35 million years
B.C.
, like
Indricotherium,
or "giraffe rhinoceros," the most massive land mammal ever and a decent brontosaurus manqué;
Archaeotherium,
a boarlike beast the size of a bull and with scythes where you'd think slicing canines would do; and
Cainotherium,
a distant hoofed relation of the camel but with the face, ears, and forelegs of Haldane's mascot, the rabbit. Wherever you find fossils, they fit. Giraffe-rhino fossils are found in strata that can be dated to the Oligocene, and those fossils are stacked above the Jurassics and the Triassics. The consistency and sequential structure of the fossil record are all facts, big fat faceable facts, and they've been backed up more often than the freeways in L.A.

This is a scientific "theory": not a hunch, not even a bunch of hunches, but a grand synthesis that gathers "facts" or robust findings with petty p values, and infuses them with meaning. A scientific theory also has predictive power. Under its rubric and tutelage, you can generate new ideas about how the world works, and then put those ideas to the test. By using evolutionary reasoning, for example, you might come to certain conclusions about the relationships among different organisms. Long before scientists understood anything about our genetic code, the swizzler stick of DNA that thinks it calls the shots, they had classified organisms into kinship cliques based on their anatomy and behavior. They determined that mice and humans were mammals, and that mice had much more in common with us than did spiders: their organs, brain, cardiovascular system, chemical composition, immune system, reproductive habits, limb and eye count, all were closer to ours than were those of a spider or a fly. Hence, geneticists could easily predict that the murine-human relatedness would extend right down to the threads of our genes, to the individual bases, the chemical subunits, of which our DNA is composed. Sure enough, as scientists began spelling out the genetic codes of a variety of organisms, they found that the closer a creature was to a human macroscopically, the closer it was alphabetically. Mouse DNA is about 70 percent identical to ours, while that of a fruit fly is 47 percent.

Scientists could go further in their predictions about molecular genealogy. Sure, we're genetically closer to mice than to flies, but how can it be that nearly half of our DNA still resembles the recipe for a creature with compound eyes, backward-bending legs, and a persistent desire to take up residence in a Porta-John? For that matter, we share one-fifth of our genetic code with yeast, an organism that has neither
cabeza, abdomen,
nor anything else requiring multicellularity. What sorts of genes could possibly be tying us to fungus?

As it happens, there are many basic chores that every cell must know how to do. Whether of wildebeest, baker's yeast, human humerus, or fly glomerulus, a cell must be able to take in nutrients, throw out the trash, stay in shape, and divide when told. One would predict, then, that the genes encrypting such fundamental tasks would be the genes least likely to change over evolutionary time, no matter who inherits them—and that is precisely what geneticists have found. The cell's maintenance and division genes are among the most well preserved specimens nature has to offer. We should all look, after half a billion years of evolutionary heaving and hawing, as dewily unchanged as do the genetic instructions that tell a cell to split in two. In fact, science has put the timelessness of DNA's blue-collar codes to spectacular use. Through studying the genes in yeast that oversee cell division, researchers have learned far more about human cancer than the malignancies themselves would deign to divulge. Tumor cells are ugly, messy, hard to handle. Yeast cells are pliable and generous (remember, they gave us wine). Should we ever declare victory in the ragged "war on cancer," the theory of evolution can claim credit for having sharpened the spears.

Another reason evolutionary theory may sometimes seem less bedrock-solid than it is stems from some of the internecine haggling among evolutionary biologists over details—the sort of squabbles that for almost any other scientific discipline would be of interest only to the contestants and their listservs; but with Darwinism as the national blood sport, everybody wants to be cc'd. Evolutionary biologists do argue over the mechanics of evolutionary change, how fast it happens, how to measure the rate of evolutionary change, whether transformations occur gradually and cumulatively, putter and futz, generation after generation, always working to stay ahead by a nose, until, whaddya know, you're wearing a Chiquita on your beak; or whether long banks of time will pass with nothing much happening, most species maintaining themselves in a comfortable stasis until a crisis strikes—an asteroid hits the Earth, or volcanoes dress the skies in flannel pajamas of
sulfur and ash—at which point massive evolutionary changes may arise very quickly.
^*
They debate what constitutes a species, and where you draw the line between two truly distinct species, each worthy of formal codification through Linnaean nominalization, and two different subpopulations of the same species. They scuffle over the nexus between evolution and romance, and whether a female chooses to mate with a male based on her careful assessment of his underlying genetic quality; or because she noticed that every other female was chasing after the guy and figured maybe they knew something she didn't; or because his nose reminded her of a favorite food item, and she was hungry.

Yet no matter how they swat the details, evolutionary scientists do not dispute the fundamentals. They do not argue over the reality of evolution, or that existing species evolved from previous species. And they do not dispute the engine that drives evolutionary change, as elucidated so brilliantly by Charles Darwin and Alfred Wallace 150 years ago: natural selection. Natural selection is the force that transforms drift and randomness into the gift of extravagance. It takes the doctrinaire sloth of the second law of thermodynamics, the tendency of every system to get frowzier over time, and hammers it into a magic, all-purpose, purpose-making machine that turns around and breaks entropy at the knees.

The basic premise of natural selection is simple. Parents give birth to multiple offspring—more offspring, as a rule, than can be expected to survive. Those offspring are like their parents, but not exactly like them. Each child's DNA is a uniquely shuffled, braided, clipped, and restated version of its elders' DNA. Genes that had lain dormant in the mother find their spine in the young, while a dominant trait in Dad is silenced come the son. And then, there may be a few total novelties in the mix. A new mutation, a slight change in the chemical spelling of a gene as it is bequeathed from parent to progeny: What do you expect, when you're copying out DNA, a sentence 3 billion letters long? Life, like everybody else, makes mistakes, and mutations are part of the fun. As Yogi Berra pricelessly put it, "If the world were perfect, it wouldn't be." And if there weren't thermodynamically inevitable bugs in the DNA copying program, we'd all still be bugs of a different sort:
unicellular, genetically identical archaeobacteria happily burbling by a hot spring, as the Old Ones did in the beginning.

Through mutations and DNA shuffling, discrepancies arise in the gene pool that give nature something to select from. Now nature has choices among the plethora of offspring; let the winnowing begin. A microbe is born with a metabolic mutation that allows it to digest more and grow bigger than can its compatriots on the stromatolite program. The microbe greedily consumes whatever resources it can wrap its fatty acid membrane around, including—hey!—the poor mother bacterium that happened to spawn it. Soon the hyperphagic youth begins spawning spores of its own, many outfitted with the advantageous metabolic mutation, and the tribe drives the more sedate unicells into oblivion. A few hundred or hundred thousand generations go by, and another happy gaffe arises, this time in a gene that dictates the performance of a component of the microbe's membrane. As a result, the microbe proves unusually sensitive to cues from its neighbors, able to tell who's where, what they're doing, and how to profit from their labor. Before you or anybody else knows it, this ancestral bugging device has bred an army of eavesdroppers, and the world of unicellular, non-cooperative, asocial narcissism gives way to multicellular, interactive, community-based narcissism. In the wake of this sublime innovation, feudalism, monarchism, democracy, plutocracy, the postmodern corporation, Monopoly, and Clue were sure to follow.

Natural selection, then, is a two-step exercise of almost unlimited potential. First, minor inherited variations arise in a population by chance. A frog is born with a mutation that lends her head an odd, rhomboid cast. Other frogs stare and make rude belching noises as she hops past. The croak, of course, is wholly on them: little Braque girl turns out to look just enough like a fallen leaf to blend into the forest floor when an amphibivorous bird comes pecking, and so she outlives her taunters. Additional mutations among her descendants fortuitously enhance the camouflage effect, and every time a better cloaking device arises, natural selection favors the bearer just enough that the mutation soon becomes the species norm. Today, the renowned Solomon Island leaf frog looks so much like a leaf that, again, you can't help but shake your head in near disbelief. Ridiculous! How can a random mutation just "happen" to carve a few corners into a frog's figure? How can a deviation of an ordinary amphibian gene whip up something so goofy that also happens to be so useful? Let alone a string of random genetic changes that just happen to improve on the masking effects of the mutations preceding them. What are the odds that a series of snafus would shake out as the perfect disguise?

Quite high, in fact. Frogs are under relentless pressure from a broad range of predators. Birds, snakes, turtles, mammals, other frogs, scorpions, tarantulas, Jacques Pépin—all seek the dense, crunchy energy packets that frogs and their legs embody. A few industrious vipers can wipe out more than a hundred frogs in a single twilight hunt.

But frogs compensate for their extreme vulnerability by breeding like a certain other prey species that hops. Beyond ensuring that at least some frogs will survive to reproductive age, fecundity breeds evolutionary opportunity. Given the great number of froglets produced in a single generation, the episodic appearance of wonderful blunders is to be expected; and every defensive leap forward will be quickly selected. Soon the accidental has become foundational, the species standard from which other revisions may or may not arise.

Insects, too, have both the incentive and the mechanism to evolve a dazzling pageant of "Where's Waldo?" routines. Everybody in the world eats insects, either willingly or between visits from the city health inspector. Insects blunt the sting of brevity with stunning fertility. Among my favorite examples of prolificacy is
Blatella germanica,
the common German cockroach. If left unchecked, a single female can, in her twelve or so months of life, give rise to 40 million offspring. With insect odds, all bets may as well be FDIC-guaranteed. The dun coloration of the German roach suits its urban habitat, but whatever the occasion, an insect can dress for it. The Javanese leaf insect not only sports the central rib and radiating veins of a leaf, but also the appearance of little holes and torn edges that you'd expect of a leaf partly eaten by ... insects. A stick insect looks like a stick and acts like a stick, which means not too stuck-up or suspiciously static. Just as a real tree part twists in the wind, so a seated stick insect will intermittently, woodenly, sway back and forth—animal imitating vegetal imitating dryad at night. But my vote for the dandiest drop-dead disguise goes to the swallowtail caterpillar, which resembles freshly deposited guano.