A Short History of Nearly Everything: Chapter 22 Summary & Analysis

 Humans want to think we have a purpose on Earth, but science shows us that sometimes life “just is.” Lichens, for example, live out their existence by simply clinging to rocks, absorbing minerals for food. If Earth’s history were compressed into a single day, life would get going at four a.m. with microbes. It takes until 8:30 p.m. for sea plants and jellyfish to show up. Around 10 p.m., land plants emerge, and around 11 p.m., the dinosaurs turn up, lingering until 11:40 p.m. Humans show up at 11:59 p.m., and they’ve so far been on the scene for just one minute. Bryson suggests that perhaps most life isn’t ambitious because complex organisms tend to go extinct, while simpler ones—like jellyfish—are still around. 

 In order for marine animals to evolve into land life, creatures have to develop a different internal architecture (including a load-bearing spine) and learn how to consume oxygen from the air instead of water, which is no easy feat. Though we usually imagine our ancestors as fish with legs, or as early amphibians, they’re most likely mites (living on plants) that crawl onto land when plants do. Dead vegetation falls into swampy sediment instead of fully decomposing and releasing carbon dioxide into the air, resulting in much higher oxygen levels that enable land life to rapidly grow larger. Early millipede fossils reveal specimens as large as six feet long.

Eventually, insects evolve to fly. Early dragonflies are as large as ravens, and plant life is much larger too—ferns alone rise 50 feet high. In 1948, an eccentric Swedish scientist named Erik Jarvik acquires an “Ichthyostega” fossil, which scientists think might be the common ancestor fish for all tetrapods (a category that includes dinosaurs, whales, birds, humans, and fish). Unfortunately, Jarvik locks it away in his office and it doesn’t surface again until after his death in 1998, when scientists realize Jarvik that mis-described it—it’s not, in fact, a tetrapod. Since then, no other viable early tetrapod fossils have been found, so this part of our history remains a mystery.

Scientists divide land-creature evolution into four “megadynasties.” The first consisted of primitive reptiles, amphibians, and turtles. One branch of these creatures evolves into the earliest “protomammals” (called therapsids) who comprise the second megadynasty. Meanwhile, another branch evolves into dinosaurs, who destroy nearly all the therapsids except for small burrowing mammals about the size of mice. The dinosaurs take over and comprise the third megadynasty, followed by our own “Age of Mammals.” Bryson says one of the strangest features of life is the prevalence of extinction. Over 99.99 percent of all species that have ever existed have gone extinct. The typical life span for complex species before extinction is about four million years, which is about where humans are right now.

Though it seems counterintuitive, extinctions are actually good for life’s evolution—mass extinctions are typically associated with massive leaps forward. For example, darting fish only began to thrive after immobile filter feeders went extinct. Earth has had five mass extinctions in its history (all before the era of dinosaurs), each of which wiped out approximately 70-95 percent of Earth’s species. There were also 12 or so smaller-scale extinction periods, though one of them almost wiped out horses. (Bryson can’t imagine the world without horses.) Figuring out why mass extinctions happen, however, is much more difficult. Scientists speculate that anything from climate change to “catastrophic” solar flares might be responsible for Earth’s mass extinctions. 

Bryson explains that the KT asteroid impact that wipes out the dinosaurs is enormous. The impact’s force exceeds the power of eight billion atom bombs. The KT impact also happens when Earth is more oxygenated—and therefore more combustible. The asteroid lands in a shallow sea bed made of sulfur, triggering several months of burning acid rain, while debris from the impact blocks out the sun for months, maybe years. Yet curiously, 30 percent of species survive this event, including crocodiles, snakes, turtles, amphibians, and corals. It turns out that the impact kills 90 percent of land animals (including all the large ones) but only 10 percent of sea animals (sparing large sea creatures like sharks).

Luckily for humans, the land survivors include our ancestors: tiny, nocturnal, burrowing mammals. With the dinosaurs gone, mammals thrive. Bryson says this historical age sees guinea pigs as large as rhinos, rhinos as large as two-story buildings, and fierce 10-foot-tall carnivorous birds. Unfortunately, too few fossils have been discovered thus far for scientists to paint an accurate picture of the early age of mammals. Similarly, the 300 or so dinosaur fossils discovered in the 19th and 20th centuries capture just a mere glimpse of life in that age.  Bryson says that humans feel sure about our status as life’s “dominant species,” but actually, “we are here only because of timely extraterrestrial bangs, and other random flukes.”