A Short History of Nearly Everything | Study Guide

Bill Bryson

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A Short History of Nearly Everything | Part 5, Chapter 22 : Life Itself (Good-bye to All That) | Summary

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Summary

Bryson begins the chapter with a discussion of lichen, a hardy organism that thrives in areas, such as the Antarctica, where few other living things can survive. Bryson uses the lichen to illustrate the point life "just is." It is in no rush to evolve, and in fact, if Earth's history were compressed into a single day, life would begin at 4:00 a.m. with the first single-celled organisms, and humans would arrive "one minute and 17 seconds before midnight."

The author then examines how life arose on land, saying, "Few occasions were more eventful than when life moved on to the next stage in the narrative and came out of the sea." Bryson makes the argument life forms left the water because it was favorable for them to do so. To live on Earth they had to develop ways to take oxygen from air, a process first achieved by plants about 450 million years ago. Animals—which required anatomical changes to survive on land and bear their own weight—left the water about 400 million years ago.

Biologists still do not know which species was the "first terrestrial vertebrate." The reason for this is, in part, because of the nature of the fossil record but also because of the secrecy of a Swedish scholar named Erik Jarvik. For almost 50 years, Jarvik refused to let anyone else study what he claimed was the first tetrapod, or animal with four limbs that end in a maximum of five fingers or toes. Tetrapods include dinosaurs, whales, birds, humans, and fish. Jarvik began his analysis of a "tetrapod" fish found in the 1940s and refused to let anyone else study it. After his death scientists realized the fossil was not of a tetrapod, and no one has been able to produce a convincing tetrapod fossil since.

What we do know is mammals were once synapsids, or animals with one hole in their skull. Over time, protomammals evolved, and by the end of the age of the dinosaurs 65 million years ago, these early mammals provided the evolutionary foundation for the rise of the age of mammals.

Bryson then delves into a discussion of extinction episodes, naming the five major extinctions: the Ordovician, Devonian, Permian, Triassic, and Cretaceous. The Permian was the most dramatic of the extinctions; as many as 96% of all species disappeared. However, scientists are unsure of what caused it in part because, as Bryson says, "it is so very hard to exterminate life on a grand scale." There were also smaller extinction "episodes" in which certain populations were devastated. Possible causes for large and small extinction events include global warming, global cooling, changing sea levels, solar flares, and meteor impacts like the one that defined the KT boundary discussed in Chapter 13. Bryson sums up the chapter with four points: "Life wants to be; life doesn't always want to be much; life from time to time goes extinct; ... [and] life goes on."

Analysis

As Bryson has described in previous chapters, the history of the planet Earth is framed around destructive events, and the history of life on Earth is no different. However, one of the main points of the chapter is the strong relationship between extinctions and evolutionary change. The extinction events that have characterized the past also led the way for the rise of mammals. In his discussion of extinctions Bryson argues, "One of the reasons it is so hard to produce convincing explanations for extinctions is that it is so very hard to exterminate life on a grand scale." While scientists have multiple theories regarding the extinction of life on Earth, there are very few, if any, that are not subject to some form of controversy.

For instance, Bryson describes the controversy surrounding the KT impact, which caused the mass extinction of dinosaurs, in terms of the questions it provokes. While scientists generally agree the extinction event was the result of a meteorite, the results are unclear. Bryson asks why the event resulted in the extinction of the dinosaurs, but not in the extinction of other large animals, such as crocodiles.

Similar controversies surround the discussion of the Pleistocene era extinction of approximately 10,000 years ago. During this period, a massive number of large animals, such as mammoths and saber-toothed tigers, went extinct; however, not all large animals disappeared, and they didn't die off at the same time. How modern scientists perceive extinction events is important because, as the Pleistocene extinction shows, they have implications for the future of existing species.

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