The Canon (30 page)

So, too, do we have a sturdy chain of "missing links," of fossils with a variegated mix of traits we might call either "humanlike" or "apelike." There's lovely Lucy, the petite australopithecine female—named after the Beatles song "Lucy in the Sky with Diamonds," which was playing in the tent the day Donald Johanson discovered her—who clearly stood upright but whose skull was a quarter the size of ours; and such early hominids as the semibrainy
Homo habilis,
presumed to be one of the first users of stone tools, and the horse-toothed
Homo ergaster,
and the lantern-jawed
Homo rudolfensis,
and
Homo erectus,
skull swept back as though in a shower cap, and archaic
Homo sapiens,
and early modern
Homo sapiens,
and fully modern
Homo sapiens,
and that quintessential caveman,
Homo neanderthalensis,
which some researchers call Neanderthal while others prefer to drop the
h,
but all agree has been unfairly maligned in serving as shorthand for "extremely primitive, unhygienic, liable to grunt." Neanderthals coexisted with
Homo sapiens
throughout Europe for at least 100,000 years before dying out suddenly, even catastrophically, about 28,000 years ago. The reasons for the Neanderthals' demise remain unclear. Their brains were as large overall as those of their
H. sapiens
peers, although their skulls were of a slightly different shape, flatter and more beetle-browed, suggesting that they had a comparatively smaller frontal lobe, the part of the cerebrum we prize as the seat of our intelligence. Neanderthals, like
Homo sapiens,
fashioned fine stone tools with sharp, flaked edges, but they seemed less interested in art and ornamentation, in painting cave walls or carving pieces of ivory into female figurines with imprudent body mass indexes. Some skeletal evidence suggests that Neanderthals were far more prone to injury, arthritis, and other debilitating conditions than were
H. sapiens.
Or maybe our ancestors couldn't stand the sight of those lowbrows, and exterminated them. Genetic studies strongly suggest that there was never any romantic intermingling of the two hominid species, no evidence that we carry traces of Neanderthal genes. Whatever the cause, when the Neanderthals departed the scene, only one member of the
Homo
genus remained, we the self-designated
drivers and namer of names, with our high, proud foreheads and our three-pound brains.
Homo sapiens sapiens,
so wise we had to say it twise. How can a sentient
sapiens
look at a lineup of hominid and prehominid skulls in a natural history museum and not be impressed at the traits that bind us, and those that set us apart? Descent from a common ancestor, modification by natural selection.

The fossil record, as sketchy as it is in spots, is unerringly consistent in sequence, no matter where in the world you dig. The molecular record, too, reveals the relatedness of all living beings, and also corresponds as one predicts to the evolutionary branching patterns of the major organismic tribes. Our genes are much more similar to those of a mouse than to those of a fly, and they are more similar still to the genes of a chimpanzee, our closest living relation. If you take one strand of a double helix from a human cell, and line it up against a helical strand from a chimpanzee cell, the two strands will stick together—will find the chemical counterpart they expect—along all but 2 to 4 percent of their spans. The 3 billion or so chemical letters that make up our DNA are 96 percent identical to those of a chimpanzee. Looked at another way, 120 million little genetic bases, just about enough to fill one of the 23 sausage-shaped chromosomes you see if you examine your fetus's DNA through amniocentesis, is all that partitions tourist from tenant at the San Diego Zoo. Which is what you'd expect for two species that shared a common ancestor only 5 million years ago. A mere 250,000 generations back, a quarter of a million great-great-greats; a forebear so far, yet so near, I can't help but call it Grandpa Silas.

Another big chunk of evidence for the theory of evolution can be seen in the realm of biogeography, the distinctive distribution of species around the world—at least until we humans started redistributing species willy-nilly as we wandered. Darwin was deeply impressed by the spatial clustering of what he called "closely allied" organisms—species with similar body plans and characteristics. Latin America, for example, is home to a magnificent, highly endangered, and inexplicably obscure family of birds called cracids (rhymes with "acids"), fifty species of large, meaty creatures notable for their vivid variety of headgear: the piping guan's foppish Mohawk crest, the helmeted curassow's bright pink knob bulging up like blown bubblegum between its eyes, the long blue horn of the appropriately named
Pauxi unicornis.
Cracids can be found as far north as the Texas-Mexico border and as far south as Buenos Aires Province—although overhunting and habitat hacking have drastically reduced their numbers—but because they don't fly well, they haven't crossed any oceans. You won't spot a wattled guan in the
Laotian rainforest or a
Mitu mitu mitu
sunbathing on the Solomon Islands. Neither would a wild penguin be caught dead in the jaws of a polar bear. All eighteen species of penguin live in the Southern Hemisphere, many of them around Antarctica, while the polar bear, like its close cousin the grizzly bear, is strictly a resident of the north. In the eyes of biologists from Darwin onward, the concordance between geography and biology, the clustering of "closely allied" species on the same landmasses and the discrepancies between the inhabitants of one continent and those of another, can be traced back to one elegant explanatory engine. "We see in these facts some deep organic bond, prevailing throughout space and time," Darwin wrote. "This bond, on my theory, is simple inheritance." The descendants of a common ancestor, sharing common ground.

Yet another line of proof is best captured with a line of mnemonic ditty: "Kings pour coffee on fairy god-sisters." This is my favorite way of remembering the taxonomic system that we use to classify species. You have your kingdom, then your phylum, your class, order, family, genus, and species. It's a nested sequence of categories, from big-picture suzerain to a specific little sister—the word "specific," conveniently enough, being the etymological progeny of "species." The narrower the niche, the more traits the pigeonholed will share; the broader the category, the larger the number and the wilder the heterogeneity of its members. No matter how hurly-burly any corraled crowd becomes, though, the beings bunched together in one batch will have more in common with one another than they do with those in any other like-tiered grouping. Let's take a quick look at ourselves. We
sapiens
are the only living species in our genus,
Homo,
although the fossil record shows there have been other
Homos,
like Neanderthal and Erectus, before us. Our family is Hominidae, and we share it with four living species of great apes—chimpanzees, bonobos, gorillas, and orangutans—as well as dozens of extinct predecessors of varying apely or humanesque traits. We hominid apes join with some 200 species of monkeys, lemurs, tarsiers, lorises, and the like in the order Primate; and with another 4,600 or so species in the class Mammalia, a cadre united by our hair, four-part hearts, two-part ears, and motherly udders; even those egg-laying outliers of mammaldom, the duck-billed platypus and the anteating echidna, dribble milk from their mammary glands that their hatchlings lap up. Our phylum, Chordata, subphylum Vertebrata, celebrates our backbone, and brackets us together with more than 50,000 other vertebrates, like reptiles, birds, fish, and amphibians. Our kingdom is Animalia, and here we run into the great throngs of arthropods and other
spineless animals: insects, spiders, scorpions, millipedes, lobsters, crayfish, and crabs; and oysters, octopuses, gastropodal makers of dye; and the worms and the sponges, corals, sea pens, sea cucumbers; many millions of animae with wide-open mouths or mouth pores, defined by our need to feed on somebody, somehow. Not so for the 260,000 species in the kingdom Plantae, those hidebound Rumpelstiltskins that spin sun into gold; yes, even the Venus flytrap, should no insect come calling, can rally its chlorophyll and get by eating light.

If we continue clambering up the tree of life, however, into a fairly recent addition to the classification scheme that has yet to be incorporated mnemonically, we'll join with trees and other plants, as well as with algae and yeast. Above kingdoms are two "empires," the eukaryotes and prokaryotes—we eukaryotes being those whose cells are equipped with a nucleus in which the double helix is cradled, and prokaryotes, like bacteria, with their DNA floating unbounded in the viscous cell belly, the better to divide if you just give it twenty minutes. And if you rise higher still to view the code by which life carries on, eukaryote and prokaryote become one. Inside every cell, and every viral parasite of a cell, you will find the same chemical alphabet, the same nucleic acid letters that tell a single epic story in a billion different ways. Above empire, kings, and caffeine, we have the Gaia of genes.

As the writer and naturalist David Quammen has observed, this phylogenetic sorting system, this nesting of category within category, and the tiered pattern of resemblances that brings ever more species into the fold and finally culminates in a single ancestral supertrait—the shared chemistry of our genes—is not the ordinary way we organize collections of items. I, for instance, have a large collection of bookmarks from around the world, dating back to the early nineteenth century. I've organized the antique ones by theme—the bookmarks that advertise pianos, or Pear's soap, or chocolates, or tires, or Smokey the Bear, or the Scottish Widows Fund—but there's no systematic way to link the tire bookmarks to the perfume bookmarks to the Mr. Peanut 1939 World's Fair commemorative bookmarks. The same goes for my daughter's collection of boxes. She likes to arrange them in aesthetically pleasing configurations, but there's no obvious morphological hierarchy, no reason for saying the jeweled box is more like the painted wood box than it is like the carved wood box. Why can't bookmarks and boxes, or rocks, or earrings, be systematized like matryoshka dolls? Because, Quammen writes, "Rock types and styles of jewelry don't reflect unbroken descent from common ancestors. Biological diversity does." And the number of traits two species share, or the degree to which their
DNA strands might happily, stickily, intertwine, is often a measure of how recently the two species diverged from a common ancestor.

Yet not every case of similitude in nature is proof of a close bloodline. Sometimes organisms on one continent will bear a startling resemblance to species located halfway across the globe, to which they are only very distantly related. For example, the cacti of the Americas are quite difficult to distinguish from a group of African succulent plants called euphorbias. In both families, you have some species that are shaped like slightly squashed dough balls and others that grow tall and upright, like aspiring totem poles. Euphorbias and cacti display a similar preference for spines or thorns over leaves; are sheathed with a thick, waxy skin; and store water in their hollow cores. If you bought a euphorbia and nicknamed it Saguaro, your aunt from Tucson might not see any cause to correct you. Yet the cactus and euphorbia families are as scantily related as two plant groups can be, and each has much nearer floral cousins without a spine to their frame.

The same with the anteating echidna of Australia, the anteating pangolin of Africa, and the giant anteater of Latin America. The three mammals share more than a fondness for ants and termites. Each has an extenuated, depilated snout, wormlike tongue, bulging salivary glands, a stomach as rugged as a cement mixer, vestigial teeth, and little scythes on its feet. Yet the trio's last common ancestor probably darted among dinosaurs. The echidna, remember, is still laying eggs, and its nearest kin, the platypus, looks like a Muppet.

Importantly, the anteating trio, the bicontinental succulents, and a plethora of other cases in which the anatomy matches but the taxonomy clashes only serve to underscore Darwin's sweeping authority. All exemplify the phenomenon of convergent evolution, of widely dispersed lineages confronting similar problems, and, through the guiding hand and cracking cat-o'-nine whip of natural selection, independently devising the same basic solution, the same set of tools to get the taxing job done. Both the euphorbias of sub-Saharan Africa and the cacti of the Sonoran plateau of North America have evolved in some of the harshest, parchest, and most sun-beaten habitats on earth, and there are only so many ways for a plant to weather a life
in extremis.
You can adopt a round conformation, which lends you the least amount of surface area relative to your volume: that way, you have a minimum of covering exposed to the harsh sun and drying winds, but a relatively big central holding tank to store whatever water may fall during a brief desert shower. Alternatively, you can grow tall and upright, so that little of your surface sits directly beneath the glare of the midday sun, while
again giving you internal space for a personal reservoir. Leaves increase your total surface area and wick away moisture from within, so best to dispense with them entirely and let your stem do the photosynthesizing. Thick waxy skin inhibits evaporation and deters the sharp incisors of thirsty desert rodents, while thorns not only add to the defense against hydrotheft, but also help channel dewdrops and rainwater down to the plant's shallow roots. Yes, if you plan to succeed under fire, you'd better have a tough hide and plenty of big pricklers on your side.

Another risky profession is myrmecophagy, the consumption of ants, and it doesn't help if you plan on ordering a side of termites. Ants and termites are among the most successful of all arthropods, such a dominant presence in whatever habitat they choose to colonize that other insects like beetles or cockroaches are consigned to puddling around their outskirts. Edward O. Wilson has estimated that ants alone make up at least half of the world's insect biomass. Much of the success of ants and termites lies in their social skills, their ability to work together seamlessly as highly specialized but de-individualized members of their collective—to behave as a ruthless "superorganism" and model for the Red Menace of McCarthyism and the spandexed Borg of
Star Trek.
Nowhere is the insects' militant nationalism more evident than in their commitment to homeland security. When attacked, ants and termites reply en masse, stinging, biting, shooting out streams of formic acid, swarming into eyes, ears, nostrils, pants. Hence, while an ant colony or termite mound of millions of individuals would seem to present an irresistible target to nearly every passing food pipe, in fact for many creatures resistance is prudent. If you have designs on this refractory form of sustenance, you can't be an amateur or do it part-time. A hammer won't work; you need specialized gear.