Planet of the Bugs: Evolution and the Rise of Insects Read Online Free PDF Page B

on earth, then I highly recommend the scenic drive from Shoshoni to Thermopolis, through the awesome Wind River Canyon in Wyoming. The exposed rocks are a window on the past opened by the formation of the Rocky Mountains. As crust was thrust upward it tilted on its side, depositing the oldest rocks at the top of the canyon. So the drive from Shoshoni initially follows a path of Precambrian rocks devoid of animal fossils. A road sign near the canyon’s entrance marks the start of rocks from the Cambrian period, when life finally showed the inclination to crawl out of the bacterial slime, and the first animals with skeletons evolved. It was the time of the first arthropods, and is especially well known for the rapid diversification of the ones commonly known as trilobites. The scenery is stunning. This is the only highway I’ve ever seen where the ages of animal life are marked by signs.
    In a half-hour or so, you drive through all the ages. After passing through the Cambrian, the “age of invertebrates,” you enter Ordovician time, the “age of fishes.” Soon you are at the Silurian age, the time of the first land plants. Then on to the Devonian time, the so-called age of amphibians. The limestone layers of Silurian and Devonian time suffered extensive erosion in this area of Wyoming, so these layers pass in the blink of an eye. They are followed by the Carboniferous, the time of coal-forming swamps. Next you are on to the Permian, and as you approach the foothills and exit the canyon, you leave the Paleozoic era and enter the Mesozoic, the “age of reptiles,” or as we now say, the “age of the dinosaurs.” Take a brief moment to mourn the passing of the trilobites. But at the same time, you can rejoice in the appearance of the dinosaurs. They are celebrated at the end of the drive at the excellent Dinosaur Center in Thermopolis. Be sure to budget enough time to soak in the sulfurous hot springs teeming with bacteria and to also visit that impressive museum. Finally, as the road levels and the sagebrush prairies open to the valley, you enter Tertiary times, the first years of the Cenozoic era and the onset of the “age of mammals.” Lament, if you must, the passing of the big dinosaurs. But rejoice in their passing as well, because it finally, finally allowed us mammals to make our move on this planet.
    By now, the implicit human-centrist bias in some of that history I just recounted should be obvious. By noting the times of the proliferation of mammals, reptiles, amphibians, and fish, we are merely observing some tenuous history of events leading to the origin of the human species. How ridiculously unlikely it seems that we should be here at all, and how those labels for the geological periods distract from the real pattern of life’s diversity. I’ll be picking apart this human-centrist mythology bit by bit as we proceed. For now, it will suffice to examine the Cambrian.
    The Cambrian period has generally been called the “age of invertebrates.” That’s certainly not because anyone sought to glorify our invertebrate ancestry. It’s simply an observation that we didn’t initially see any of our vertebrate ancestors in fossils from Cambrian layers. Notice that we didn’t call it the “age of arthropods” or the “age of trilobites,” either of which would be apt. Calling it the “age of invertebrates” is a bit like calling it the “age of no humans.” The name subtly derides the success of arthropods by noting the absence of vertebrae rather than touting the evolution of exoskeletons. But subsequently, we did discover our likely vertebrate ancestor in Cambrian times, and what a humbling event that was. A small creature called
Pikaia
was discovered in the 515-million-year-old Burgess Shale fossils of Canada.
Pikaia
was a mere one-and-a-half-inch-long, wormlike creature that burrowed in bottom sediments. She was soft-bodied but did have an internal supporting structure: a primitive notochord, the