A Short History of Nearly Everything: Chapter 16 Summary & Analysis

Being alive isn’t easy. As far as humans know, it’s only possible to be alive on Earth—buried in a remote corner of the Milky Way—and the zone that sustains life on Earth is only 12 miles high from the ocean floor to the atmosphere, which seems tiny compared to the “cosmos at large.” It’s even worse for humans since we need to live on land, meaning that 99.5 percent of Earth’s volume is off-limits to us. We can climb 500 feet high easily, but 500 feet underwater the pressure would compress our lungs to the size of a soda can.

Umberto Pelizzari holds the world record for free diving, at 236 feet deep. Other organisms do survive at much deeper depths. The pressure at the deepest part of the ocean, the Mariana Trench is like standing under 14 cement trucks. Curiously, the human body itself—which is made mostly of water—wouldn’t be crushed, but the gases in our bodies would make our lungs implode. Until recently, scientists thought that this would happen to anyone diving below 100 feet, but free divers have proven otherwise.

The real danger comes from nitrogen, which comprises 80 percent of the air humans breathe. Under deep pressure, nitrogen turns into bubbles that migrate into our blood and tissues. If a diver ascends too quickly, the bubbles start to fizz—just like soda or champagne does—and clog the blood vessels, depriving them of oxygen. Divers call this excruciatingly painful sensation “the bends.” The only way to avoid the bends is to dive in and out so quickly that nitrogen bubbles don’t have time to form—this is what free divers do. The other solution is to ascend slowly, letting the bubbles dissipate gradually.

In the early 20th century, the eccentric father-and-son scientists and divers John Scott and J. B. S. Haldane work out the rest intervals needed for a safe. The Haldanes acquire a decompression chamber so they can test what happens to the body at different pressures. When John tries simulating a dangerously fast ascent, the dental fillings in his teeth explode. His wife, whom he coaxes in next, has a 15-minute seizure. Curiously, another effect is nitrogen intoxication. At certain pressures, nitrogen is intoxicating, like alcohol, though nobody knows why. Bryson says all this means that it’s pretty hard for a human to leave the surface world.

Human bodies aren’t particularly robust: we’re sensitive to heat and cold such that deserts and ice caps are off limits. In fact, humans can only really live on 12 percent of the total land area and four percent of the whole surface (including the seas). Nonetheless, there are worse places to call home. If we want another home, we’d need four things.

The first thing humans would need for another home is an excellent location: “the right distance from the right sort of star” that burns slowly and keeps us alive but doesn’t boil us. In fact, if Earth’s orbit were five percent nearer to the sun, everything would be boiled away. If it were 15 percent farther away, it would be frozen over. Second, we’d need the right kind of planet. Earth’s molten core created its atmosphere and its crinkly crust—without those, Earth’s surface would be smooth and covered evenly in water, meaning only sea life would evolve.

The third thing we’d need for another home is a “twin planet,” which for us is currently Earth’s moon. The moon happened to be created by an asteroid impact that sent part of Earth into orbit around itself, so it’s much larger than it would be otherwise. According to Bryson, the gravitational pull of the moon stops Earth from wobbling on its axis with “goodness knows what consequences for the climate and weather.” Finally, we’d need good timing. If history hadn’t played out just the way it did—say, if the dinosaurs hadn’t been wiped out—we wouldn’t be here.

Oxygen is the most abundant element on Earth, making up 50 percent of Earth’s crust. The quantities of the rest are “often surprising.” Silicon is the second most common element, although the abundance of an element doesn’t necessarily relate to how important it is. Carbon only comprises 0.05 percent of Earth’s crust, but without carbon, life wouldn’t be possible. Other elements are critical for sustaining (rather than creating) life, such as potassium and sodium. Humans evolved to be compatible with naturally-occurring elements, but there are still very “narrow ranges of acceptance.” Selenium is essential, for example, but too much of it would kill us. Normally, our tolerance for an element is proportionate to its abundance in Earth’s crust.

Element properties become more “curious” when they’re combined. Sodium explodes in water “with enough force to kill,” and chlorine is lethal—but when combined, these two elements make ordinary table salt. On the whole, humans tolerate elements that are water-soluble. Lead is poisonous because we weren’t exposed to it at all before it was used commercially, and plutonium—which comes from space—is 100 percent toxic to us. Other life forms might depend just as vitally on ammonia or mercury but find oxygen deadly if it doesn’t naturally occur on their planets. Of course, humans evolved to tolerate the elements around us, so it’s not all a matter of luck—but we’d likely be lost elsewhere in the universe.