The Canon (29 page)

In the vast clan of insects and their arthropodal relations, all conceivable strategies have been sampled, all weapons amassed. Imitation, obfuscation, threat of death or indigestion—name your poison, there's an arthropod bartender ready to serve. The whip scorpion sprays a scorching one-two cocktail punch: oily caprylic acid to penetrate even the toughest outer sheath of an attacker, and water-based acetic acid to burn the tender tissues beneath. The devil's rider walkingstick backs up its defensive camouflage with chemical artillery, shooting streams of
terpenes, potent chemicals similar to the active ingredient that makes catnip detestable to everything except, inexplicably, cats; and to see a blue jay get hit in the face with walkingstick spray is to see a blue jay that will never again question a twig. Some millipedes contain high doses of a progesterone-like compound that may serve as a long-term defense strategy by crimping the fertility of millipede foes. That outcome would not be of much use to the consumed specimen serving as the sacrificial Pill, but, in reducing the number of future predators, it would give a leg up to the millipede's surviving relations.

Insects have the means and motive to synthesize more defensive chemicals than we humans have had time to tally or test. They also have a flair for outwitting us when we turn our chemical arms against them. When we think of the dismal history of DDT, we think of springs silenced of birdsong and skies brushed free of bald eagles. Yet the real failure of our insecticidal campaign against mosquitoes and the diseases they carry was in how quickly the buzzing suckers came to shrug off our sprays. By the time DDT was banned in the United States, in 1972, nineteen species of mosquitoes—about a third of the known malarial vectors—were immune to the pesticide. Have your dependent loved ones perchance encountered any head lice lately? If not, then either you are a homeschooler, or your kids are very unpopular. In recent years,
Pediculus capitis,
a bloodsucking parasite with a particular fondness for the comparatively soft scalps of children, has joined the schoolyard metal detector and the thirty-pound backpack as a staple of modern childhood. The reason is simple: head lice have become murderously hard to kill. They're virtually immune to soft-core toxins like pyrethrins—the active ingredients in lice shampoos sold over the counter—and pediatricians are understandably reluctant to recommend that stronger poisons be applied within seepage range of impressionable young brains. That leaves tedious and inefficient parental nitpicking as the primary defense against lice, which pretty much guarantees that there will always be a parasite reservoir somewhere, ready to infest fresh heads in Topeka today or reclaim seasoned ones in Des Moines mañana.

The pace at which insects become resistant to our poisons, and bacteria to antibiotics, is often fast enough to observe. One year, baited traps took care of our ant problem; the next year, I watched in horror as the ants marched right through the little hockey puck disks without missing a ta-rah, en route to their main course at the cat food dish. Or the crickets that populate and freely defecate in our basement: Every spring, my husband sprinkles poison in all their favorite crannies and hatcheries. Up until last spring, the treatment worked, and the crickets crumbled. This year, either the treatment didn't work, or it worked the way radiation did for ants in the 1950s sci-fi classic
Them!
If you have doubts that evolution happens, I invite you to stop by our basement, where the crickets now look like kangaroos.

Whatever the pest in question, the evolution of pesticide resistance conforms to the Darwinian algorithm. Random genetic variations arise all the time in a population, especially among fast breeders. Most of those variations are either of little consequence or are decidedly inadvisable, and they are accordingly ignored by selective pressures, or are quickly swept from the gene pool. Every so often, however, a mutation of enormous utility like toxin resistance springs up, and the novel trait soon becomes the species norm.

Genetic quirkery can also sow biodiversity. If a mutation happens to affect a key gene that controls an animal's basic development, the resulting aesthetic or behavioral changes may be so profound that the beneficiary of the mutation looks or acts like a whole new species. And if that remastered organism and its progeny somehow become separated from their unmutated peers—say, by rising sea levels that transform a peninsula into an island—the odd stock may indeed evolve into a distinct species that will no longer couple with its former fellows. Scientists at Stanford University, for example, recently traced the evolution of the stickleback fish family to a handful of comparatively simple genetic changes. There are about fifty species of stickleback fish found throughout the Northern Hemisphere. Some live in the ocean and are protected against predators by a full-body armor of thirty-five bony plates. Others swim through freshwater lakes and rivers, freed of their cousins' cumbersome chain mail and thus able to dart about speedily and feed competitively. The researchers have determined that a few mutations in a single gene underlie this dramatic discrepancy in stickleback anatomy, specifying maximum plate growth in the marine fish, more or less suppressing it among lake dwellers. The ease with which major overhauls in stickleback format can be accomplished helps explain why the family managed to diversify its ranks so rapidly: freshwater sticklebacks diverged from their oceanic counterparts a mere 10,000 or so years ago, at the end of the last Ice Age, and they have further speciated and specialized themselves in whatever body of water they have colonized.

The evidence for evolution abounds, within us, beneath us, crowning and surrounding us. Antievolutionists complain about the "gaps" in the
fossil record, and lacunae there are, by the chasm. Several hundred thousand fossil species have been identified and named, but researchers suspect that the known bones represent a mere one-thousandth of one percent of all species that have lived. "Of course there are plenty of gaps in the fossil record," said Dawkins. "There's nothing wrong with that. Why shouldn't there be? We're lucky to have fossils at all." Think of all the obstacles that a corpse must overleap en route to immortality. First, it must avoid the fate that awaits most dead organisms: getting picked apart and scattered by scavengers; decomposed by worms, mold, and microbes; husked and battered by the elements; or any or all of the above. The best defense against nature's multilateral recycling program is a quick burial, which generally requires dying someplace where a thick layer of sediment is likely to sweep over the carcass soon after it is deposited: in or around lakes, rivers, swamps, and lagoons, for example, or on the ocean floor close to shore and its sandy, silty runoff. The sedimentary blanket helps to prevent decomposition, at least of the organism's toughest tissues—bones, teeth, shells, tusks, woody stems. Over time, the sand and silt sedimentation turns to stone, and so, too, may the bones and other bioremains within, as, bit by bit, mineral particles come to replace the original organic molecules while maintaining their positional integrity.

Yet even then, a fossil is not safe. Its sedimentary cemetery may end up getting buried under so many subsequent layers of rock that bed and fossils are melted beyond recognition, or are ripped apart by the constant shuffling of the crust's plates, or blasted to ash in a volcanic snit. Finally, there is the considerable problem of discovery. After spending tens of thousands to millions of years patiently petrifying underground, a fossil must fight its way to the surface again if its record is to be read. It must hitch a ride on the edge of an uplifting plate, to emerge on the exposed side of a hill or mountain. Or it must be on a sedimentary plateau that has been painstakingly carved into by wind or water, revealing a Dagwood-sandwich stack of rockmeats, a fossil feast. Paleontologists do most of their hunting on hillsides and canyon gullies, where fortuitous collusions of geology and meteorology have served up samples of archaic sediments, glimpses of long ago that, on most of the earth, are stashed far below.

The ease with which paleontologists can affix an age to an outcrop of rock and the fossils embedded therein varies from site to site, but it can nearly always be done. Fossils younger than 55,000 years or so can be dated with high accuracy by measuring the ratio of two forms of carbon, carbon 14 and carbon 12, that still linger in the organism's remains; the less carbon 14 there is relative to carbon 12, the older the fossil. When it comes to appraising older fossils, scientists must look to the stone that houses the bone. Rocks as they form often become infused with a host of so-called radioactive isotopes, unstable versions of atomic elements like uranium 238, potassium 40, and rubidium 87 that attempt to tranquilize themselves by periodically and methodically spitting out excess particles from their unwieldy cores. Because scientists have determined the rates at which these radioactive atoms decay into stability—a pace known as the element's half-life—they can use the changeling tracers as geological clocks. Their Geiger counters clacking, researchers compare the proportion of still volatile to safely spayed isotopes in their disinterred treasure, and so they can get a reasonable grip on how long the rock and its entrenched fossils had been festering underground, to time frames dating back hundreds of millions of years.

Yes, it's hard to be a fossil, harder still to be a found fossil with a dependable isotopic birth certificate. Of course there are gaping gaps in a record so reliant on a defiance of nature's tireless composting piety, and on the blind luck of revealing uplifts a million years hence. Wouldn't you be wary if the gaps were too
few?

Besides, gaps are perpetually being at least partially plugged. In 2001, researchers digging in the low hills of northern Pakistan, at a site once submerged beneath the warm, shallow waters of the Tethys Sea, discovered two superb caches of whale fossils that help trace the mammal's brazen backward flip from terra firma to aqua primordia. Biologists had long assumed that whales—which in their streamlined design and fealty to water seem so piscine that Herman Melville deemed them fish, but which breathe air, breastfeed, and bear hair follicles like any other mammal—were descendants of land mammals that returned to the oceans some 50 million years ago. The whale's fossil trail, though, was so spotty that biologists could only guess at what the prenautical whale might have looked like. Now they have compelling evidence that the progenitor to Moby Dick looked and ran like a wolf but ate like a pig, for it was closely related to ancient artiodactyls, the group of hoofed ungulates that today includes pigs, camels, cows, and hippos. Even before venturing into the water, the new fossil trove shows, the protocetacean had specialized ear bones that now are found only in whales and dolphins, suggesting that the whale's impressive audio skills, its capacity to hear freed Willy keening half an ocean away, may have evolved to track sound prints on land. Instead, it heeded the songs of the sirens and followed them into the sea.

Moreover, there are some beautiful fossil series that show persuasive procession of one species into the next. Among the most famously fleshed-out fossil record is that for the horse. According to the sequence disinterred to date, the first horselike genus was
Eohippus,
or
Hyracotherium,
an agile, four-toed creature the size of a Labrador retriever, which daintily dined on shoots, berries, and leaves in the woodlands of Eocene North America, 53 million years ago.
Eohippus
gave rise to several lineages, horses of different sizes, toe numbers, teeth ridges, and, undoubtedly, colors, although fur does not fossilize well so we'll probably never know. Some species were suited for life in the deep woods, others for open grasslands. One successful savanna specialist,
Hipparion,
migrated across the Bering land bridge to the Old World about 10 million years ago, and soon spread across southern Eurasia and Africa. Back in North America, all of
Eohippus
's descendants gradually died off, and by the start of the Pliocene, 5 million years ago, only the horse called
Dinohippus
remained. Large and rugged,
Dinohippus
had long legs and single-toed, padless hooves ideal for galloping across the open plains that proliferated as the climate grew cooler and drier. It also had big, thickly enameled teeth designed for a lifetime of grinding on tough scrub grass and the tougher silica that inevitably comes up with it.
Dinohippus
begat a slightly more graceful version of itself,
Equus,
the modern genus of horse. At some point,
Equus
crossed paths with the comparably toothsome and leggy
Hipparion,
and for whatever reason—greater fecundity, lucky horseshoes—managed to supplant it. All of today's breeds of horse, from a grizzled Central Park carriage horse to a thoroughbred, sparrow-boned stallion, are members of the equine club. So, too, are existing species of zebra and wild ass. Their evolutionary heritage is a canter cast in stone.

Another group for which the fossil record is surprisingly rich is ... our own. If you want to implicate a supreme being in the story of human evolution, you might consider inserting it here, as the mastermind behind the fortuitous events that yielded a wealth of prehuman remains in settings that normally are hostile to fossils. A supremely levelheaded being who wants only to guarantee that those in a position to ponder their roots have a look at the family tree. We have fossils of primogenitor primates from 80 million years ago, shrewlike mammals that began spending less time on the ground and more in bushes and trees, where they evolved large, forward-facing eyes well suited for ferreting out insects and dexterous fingers for plucking found insects off leaves. We have fossils from 50 million years ago, when the archaic arborealists started to diversify and give rise to early monkeys and apes, and these
fossils formed in the semitropical forests of Africa, where humidity and armies of mulchers generally reclaim all biodetritus before it has a chance to be archived in sediment. We have souvenirs from ancestral apes like
Dendropithecus, Proconsul, Kenyapithecus.
"There actually seem to have been more potential ancestors than we would have needed prior to 12 million years ago," the Stanford biologist Paul Ehrlich writes in
Human Natures.
We have, he added, "an embarrassment of fossil riches."