A Short History of Nearly Everything: Chapter 26 Summary & Analysis

If a person traces their genealogy back eight generations, they have 250 direct ancestors. That number rises to a million at 25 generations and a billion at 30 generations—that’s more people than existed back then, meaning that we all have common ancestors. Ancestrally, Bryson says, we’re “all family.” In fact, most people share 99.9 percent of their genes. Each of a person’s cells has a nucleus, and each nucleus has 46 chromosomes which contain the instructions necessary to make an individual. Each chromosome is made of a “wonder” chemical called DNA, and each person have a lot of it—up to 20 million kilometers worth of DNA exists inside a human body. DNA has one purpose: to make more DNA. DNA isn’t itself alive, but it is fundamental for life.

A Swiss scientist named Johan Friedrich Miescher first discovers DNA in 1869, and other scientists discover chromosomes in 1889. Many scientists suspect that either chromosomes or DNA (or both) have something to do with heredity because they live inside every cell, but they know little else about them. In 1904, Thomas Hunt Morgan begins investigating chromosomes by breeding fruit flies, which have only four chromosomes. He’s able to deduce that chromosomes have something to do with how traits are passed on, though even by 1933, some scientists still don’t believe in genes. In 1944, Oswald Avery cross-breeds bacteria, decisively showing that DNA is the active agent in heredity. 

A decade later, four competing scientists in England—named Maurice Wilkins, Rosalind Franklin, Francis Crick, and James Watson—finally crack DNA’s structure. Franklin, who’s a woman, makes the most progress despite facing a lot of prejudice in the 1950s. Franklin isn’t allowed to eat with other faculty and is often treated with a lack of respect, or “formalized disdain that dazzles modern sensibilities (actually any sensibilities).” Franklin is the only one with good results from her experiments, but she hides her images of DNA strands from the others—until Crick and Watson take them in 1953 “without her knowledge or consent.” From Franklin’s images, Crick and Watson learn that DNA is a double helix, and they go on to win the Nobel Prize for this discovery. 

DNA’s double helix shape looks like a twisted rope ladder: the rungs (or steps) of the ladder are formed by joining bases—specifically, G with C or A with T. The order in which the letters appear as they progress up the ladder is the human DNA code. When it’s time to make a new DNA molecule, the ladder rips down the middle like a zipper, and each half goes to make a new partnership (or a new rope ladder). This happens in “a matter of seconds, which is quite a feat,” according to Bryson. Most of the time, DNA replicates accurately so that each new ladder is an identical replica of the ladder it ripped away from.

Sometimes, though, a letter gets in the wrong rung on the rope ladder—in technical terms, a “single nucleotide polymorphism” or “Snip” happens—and this can affect the body that the cells eventually turn into. It might mean that the human is worse off (say, they’re more likely to get a disease), or better off (say, they have extra red blood cells). These slight differences play out to affect the likelihood of survival: they’re the mechanism for Darwin’s idea of natural selection. 99.9 percent of one’s DNA is identical to every other human’s, but the 0.1 percent difference is made up by one’s Snips. Everyone’s genome is thus slightly different—but only by a tiny margin.

Bryson says that all animals are, in a way, “slaves to their genes,” which is why salmon and spiders are prepared to die during mating—the impulse to disperse genes trumps their impulse to survive. Scientists experimenting with cross-breeding species realize that humans share 60 percent of their genes with fruit flies and 90 percent with mice. It’s hard to isolate genes for a specific trait, though, as most inherited traits arise from combinations of genes that are harder to pin down than single genes. The more we learn, the more complicated the picture seems, which is why cracking the human genome seems more like the beginning of something much bigger. So far, we know what genes are, but we don’t really know how they work. 

It turns out that scientists now also need to crack the human “proteome,” a new concept capturing the information that creates proteins, which is far more complicated than the human genome. Despite the complexity of our genes, Bryson thinks it’s a “profound” truth that we are closely related to everything on Earth—even half the chemical functions that happen in bananas also happen in humans, prompting Bryson to conclude that “all life is one.”