“Giant Sloths” is an homage to Grayson's life's passion—the now-extinct mammals and birds of the Pleistocene in the Great Basin. For the less formally trained Ice Age enthusiasts, he summarizes chronometric dating and the use of scientific and common names. Peppered throughout are accounts of fossil hunters, Dukes, archaeologists, paleontologists, and others, who claimed, at one time or another, to have found evidence of humans hunting, butchering, or otherwise aggravating Pleistocene megafauna. Ultimately, these claims were shown to be false, such as the Nevada State Prison Quarry footprints first publicized as made by extinct megafauna and sandal-wearing humans (proposed as Homo nevadensis) (chapter 1). Fifty years after their discovery, Chester Stock proved that the footprints were made by an extinct species of sloth (Mylodon sp.), not a new species of human.
Grayson takes us on a data-induced trek through the notable discoveries of America's fantastical beasts, including scientific studies, newspaper articles, and pop culture. He presents evidence on 20 genera of mammals and 8 genera of birds that were likely part of the Great Basin's Pleistocene landscape. He treats readers to a detailed description of the peculiarities of this region (chapter 2), especially the distinct floral communities contained within. The Great Basin is not known for its preservational qualities, yet the abundant packrat middens act as vegetational time capsules. The woodrat's crystallized urine (amberrat) preserves macrobotanical remains in middens for 50,000 years—locking in a local vegetation record that can be dissected and dated to understand the changing Pleistocene climate.
Grayson contextualizes the Great Basin megafaunal extinctions with an encyclopedic review of the paleontological and biological data for the 77 mammalian and 20 avian genera that went extinct in North and South America (chapter 3). Included is the growing corpus of ancient DNA (aDNA) data available for some of these animals. Historically, fossil species were differentiated based on morphology and comparative anatomy, often resulting in disagreement on the number of species within a genus. The advent of molecular paleontology and successful extraction of aDNA from fossils elucidates intraspecies connections, enabling resolutions to long-held debates among paleontologists.
Data on Pleistocene plant abundances in the Great Basin indicate that large megafauna were not common during the last Ice Age, explaining the lack of evidence for people hunting or butchering them (chapter 6). In chapter 7, Grayson turns our attention to the ultimate question—why did so many large terrestrial mammal species face an evolutionary end-of-the-line during the waning millennia of the last Ice Age?
Currently, there are three explanations that Grayson describes and dissects. The “Overkill Hypothesis,” proposed by Paul Martin, states that Clovis people hunted megafauna to extinction. We know this hypothesis is null because it hinges on points now shown false: Clovis First (they weren't), Clovis big-game hunters (they weren't), all megafauna went extinct 11,000 years ago (they didn't), and terrestrial animals on islands become extinct following human colonization (they do). While the last is true, it has little bearing on the continent-wide extinctions that occurred up and down the Americas.
Proponents of the Younger Dryas Impact Hypothesis hold that a comet impacted Earth around 10,900 years ago and caused the onset of the Younger Dryas, the end of Clovis and the formation of the black mats that cap Clovis sites, and the extinction of North America's megafauna. Grayson offers a point-by-point rebuttal of the hypothesis. The “Clovis Comet” depends on one grand event that ended the lineages of megafauna; however, an impact of that magnitude would have had a wider array of fallouts. It does not explain why some animals went extinct well before 10,900 or why a population of woolly mammoths made a last stand on a northern island in the Arctic Ocean until 3,700 years ago.
Climate change, specifically the drastic seasonal temperature changes that occurred at the end of the Pleistocene, caused irreversible changes in plant communities, shifts in small mammal ranges, and catastrophic effects on large mammal populations. The altered environmental conditions put Gleasonian individualism into play—the concept that members of a biological community react to environmental shifts at the individual species level, not as a cohesive unit. How mammoths in Florida reacted to those changes can only be understood through detailed population histories and environmental records of Florida mammoths. Repeat this for every species that went extinct in every location where they once thrived, and then we might understand the events that occurred at the end of the last Ice Age. This is Grayson's vision for the way forward to the past.
