Bitten by the Insect Bug
Scott Richard Shaw’s Planet of the Bugs: Evolution and the Rise of Insects uses the ascendance of insects as a lens through which to view the evolution of life on planet Earth. The book is laid out in a straightforward fashion: each of the ten chapters discusses a single geological period and the key evolution events within that period. However, this is more than the standard recitation of events in sequence: Shaw hopes to replace the typical human-centric understanding of biological evolution with a view that is more nuanced and more accurate.
Shaw shows how the descriptors of each geological period reflect the human-centric view of evolution that has plagued our understanding for centuries. The Ordovician period, for instance, is sometimes called the Age of Fishes. This is a slight to the insects, which were much more numerous during this period (and during every period since). However, we humans are much more interested in the events that directly led to us, and fishes were a critical element in that trajectory, so we have inappropriately overemphasized fishes in our recounting of the history of life. In other words, we have myopically focused on the precursors of humans and overlooked the incredible events that were taking place elsewhere on the tree of life.
Shaw further argues that the lack of focus on nonhumans is not merely an innocent oversight or reasonable preference for understanding our own species. It is, instead, willful ignorance and a dangerous preoccupation since, if we could re-run the tape of evolution, it is highly unlikely that we would end up with Homo sapiens—but in a second run it is likely that we would see worms and arthropods. As Stephen Jay Gould emphasized, contingency is a fact of evolution. The forces of randomness would probably not lead to the same end if we repeated the experiment from day one. The point Shaw is making is that the record does not show an inevitable march to the production of humankind.
Shaw does two things consistently throughout the chapters of the book: he identifies the key forces undergirding the evolutionary events that took place in each period and he discusses in detail the major pivot points during each period. A look at one chapter will illustrate these important aspects of the book.
The Cambrian period, sometimes known as the age of invertebrates, was preceded by three billion years of microbes before the first multicellular animals appeared, according to Shaw (who seems to be overlooking the existence of multicellular animals such as Kimberella before the Cambrian). Once multicellularity was established, specific structures such as external cuticles, exoskeletons, and shells arose. There was a rapid explosion and diversification of these types.
What drove all of this activity? A variety of factors are typically trotted out (e.g., continental drift, ice ages, and meteorite impacts). However, following Nick Lane’s Oxygen, Shaw argues that during the Cambrian, oxygen levels rose to roughly their current levels. Oxygen can be toxic to cells. So cells clustered together in order to escape oxygen’s toxic effects, ultimately giving rise to multicellular organisms. At the same time, aerobic respiration arose, allowing animals to use energy more effectively than their anaerobic counterparts could. The size of an organism that could be sustained thus increased dramatically. Another major step was the emergence of animals that excreted waste products that could subsequently solidify on the outside of the body. This achieved two results: wastes did not have to be sequestered internally, and they were now capable of creating a protective shell.
Impressive as the success of the small shelly fauna was, it pales in contrast to what was happening with arthropods. As Shaw puts it, trilobites were the “rock stars of the Cambrian seas.” By the end of the Cambrian, there were 20,000 species of trilobites living in the oceans. As a result, this period should be known as the Age of Trilobites, and it would be were it not for our human-centric bias. You may be curious about why there are no trilobites today. They were successful in their day, and even today all modern insects share the basic trilobite body plan—a hardened exoskeleton and multi-jointed legs. Most accounts of the decline of the trilobites center on pedestrian forces such as competition with other organisms. But Shaw presents a tantalizing alternative explanation (due to Danita Brandt): trilobites never figured out to molt properly. After shedding their old skin, trilobites secrete a new one but, whereas the new cuticle of a modern insect will rapidly undergo a hardening and darkening process to make it into a rigid protective structure, this process could take days to weeks in trilobites. As a result, trilobites experienced a very lengthy period of vulnerability that ultimately proved to be their undoing before the end-Permian mass extinction event finished them off for good.
Each chapter of the book provides something similar to the synopsis I have just provided for the chapter on the Cambrian Period. In each case, the major evolutionary advances for the period are identified and the underlying causes discussed. One major asset of the book is that Shaw examines a number of alternative hypotheses for explaining the events that take place. He also assesses the available evidence for competing explanations.
This engaging book will appeal to a diversity of audiences. It is sufficiently rigorous that it will prove informative to the trained scientist. It also challenges cherished hypotheses with new evidence. For the lay person with an interest in natural history, the writing is sufficiently clear that such a reader will learn much and, perhaps, be motivated to seek additional information. One quibble: although Shaw wants the reader to be acquainted with geological periods and eras, he does not provide a figure with all of the names and dates collated on it. For a novice, this can be confusing, although the information is provided in the table of contents for the individual chapters.
In short, this is a wonderful book that is highly readable and informative. I strongly recommend it to anyone who is interested in the evolution of life on earth.
Susan W. Fisher is Professor of Entomology Emerita at the Ohio State University.