How easily the small eludes the big. We say that bugs will inherit the Earth, as if it wasn’t theirs already. Bugs made the Earth. Long ago, tiny spineless creatures with legs arrived on the wet shoreline, probably to escape predators at sea, and made land habitable for plants. The simultaneous accommodation and war between plants and insects, the six-legged descendants of those first crawlers, would shape the terrestrial ecosystem.
Early in Planet of the Bugs, Scott Richard Shaw, who is a professor of entomology and curator of the Insect Museum at the University of Wyoming, takes us on a drive through the Wind River Canyon. The exposed rocks are a kind of display case of geological history, with the ages of animal life identified by road signs. Near the canyon’s entrance, one of them marks the first rocks from the Cambrian period, the “age of invertebrates.” In the next half-hour, drivers pass through the Ordovician, the “age of fishes,” the Silurian (land plants), the Devonian (amphibians), the Carboniferous (swamps), the Permian, and onward to the Mesozoic era (reptiles and dinosaurs), which preceded the “age of mammals.” It’s all tidy—and wrong, Shaw says. The “age” labels attached to geological periods are a mere “human-centric mythology” that he’ll pick apart as he takes us through the eons. Shaw, our erudite and passionate guide, makes bugs the star. Give him a few hours and you may briefly escape our species bias.
For example, the signature fossils of the Cambrian are the trilobites, a part of the larger category of arthropods, which today includes insects, spiders, lobsters, shrimp, millipedes, centipedes, and scorpions. All have a segmented external skeleton and several jointed legs. As Shaw quips, calling the Cambrian “ ‘the age of invertebrates’ is a bit like calling it the ‘age of no humans.’ ” The very word “invertebrate” means no spine, or “not us.” Why not, instead, tout the evolution of exoskeletons, the big biological advancement of the age, and call the Cambrian the “age of trilobites” or the “age of arthropods”?
As you observe the segments on a lobster’s tail, think of the segments of your spine. They’re not so different, except that our bones are inside. It’s easy to see the advantages of outer bone, which provides support and protection against the elements and predators; we need shoes and coats because our exteriors are soft. The advantages of an internal skeleton might be flexibility and a sensitive surface (shells don’t feel much). But arthropods gain flexibility from segmented bodies and sensitivity from exterior sensory spines. Yes, it’s tough to keep growing if you live inside a suit of armor, so arthropods periodically molt, shedding their shells temporarily. According to Shaw, the only real vertebrate advantage is that we can grow continuously, which has allowed us to grow to bigger sizes and evolve big brains. No ant is writing about me, after all. Yet when it comes to survival, small size is generally a plus.
In the Cambrian, we find the first vertebrate: a one-and-a-half-inch-long wormlike creature with a primitive spine called Pikaia. Pikaia may not have been abundant and could easily have disappeared without leading to much at all, Shaw argues. These fossils vanish at the beginning of the Ordovician, perhaps because Pikaia turned into fish or the last Pikaia was gobbled by one. However, calling the Ordovician the “age of fishes” is another case of bias, he says, pointing out that, by the mid-Ordovician, there were only five families of fish but 50 families of cephalopods (large predatory squids with coiled shells). A better label, therefore, would be the “age of cephalopods.”
Because trilobites gradually became less diverse, it’s sometimes thought that they lost an evolutionary battle with fish. This would have been a significant triumph for vertebrates. But trilobites didn’t disappear for another 250 million years, at the end of the Permian. Over that time, arthropods such as crustaceans and multi-legged myriapods developed more efficient molting methods, and new predators—squids (an invertebrate) and sea scorpions (an extinct arthropod)—appeared in the waters. Fish were there, too, but they weren’t the main act.