time. An asteroid colliding with Earth gives no warning, and there is no time for any sort of anti-impact adaptations.

Because it was so catastrophic, the Permian extinction certainly affected the biological makeup of the world. The differentiation of a Paleozoic fauna from a Mesozoic fauna allowed John Phillips to both define these terms and subdivide the stratigraphic record into large-scale units—because the change was so striking. The Paleozoic world of trilobites, rugose and tabulate corals, goniatitic ammonoids, and most straight nautiloids, among many, many more kinds of life characteristic of the Paleozoic, were replaced by new, higher taxa.

The changes on land were equally dramatic, with the therapsids being largely replaced by new kinds of reptiles that ultimately gave rise to the dinosaurs, crocodiles, and a host of other Mesozoic icons. All of this change may have been the result of the catastrophic part of the extinction, what could be called the Permian-Triassic boundary event of the longer Permian extinction, the biggest hydrogen sulfide burp of 251 million years ago. But here it is proposed that a second group of evolutionary changes occurred because of the longer-term effects in the late Permian: the changeover from a glacially-gripped world to a hot one—where ultralow carbon dioxide was replaced by high levels of this gas—and perhaps most importantly, it was a world that experienced the most catastrophic drop in oxygen levels known for the past 600 million years, the time of animals. The most consequential change occurring over this interval was the evolution of endothermy, but there may have been equally consequential changes occurring as well, including the evolution of live births. While we have no evidence either way about the latter, there is solid evidence that endothermy appeared in the interval of time when oxygen was dropping fastest. Was this just coincidence, or was it cause and effect? Let’s propose a new hypothesis concerning why endothermy was evolved by multiple lines of vertebrate animals.

METABOLISM AND THE EVOLUTION OF ENDOTHERMY

Here let’s return to a subject raised in the last chapter, but one whose history is even more important in this chapter’s interval of time than last’s. Metabolism is the term used to characterize the acquisition and



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