Other phyla made a more extravagant entrance. A relative of today’s mollusks looked like a pincushion studded with arrowheads. Living lamp shells were foreshadowed by Halkieria, which looked like an armored slug. Opabinia had five eyes sprouting like mushrooms from its head, and it stirred up the seafloor with a clawed nozzle that it could also use to grab prey and stuff into its mouth. Opabinia now appears to be an early relative of living arthropods. Other phyla that today live in humble obscurity—velvet worms or peanut worms, for instance—gloried during the Cambrian explosion in a diversity that they would never again enjoy.
Darwin’s worries over the Cambrian turn out to be unfounded. Now that scientists can read isotopic clocks and recognize molecular fossils, they have shown that the world did indeed swarm with life for billions of years before the Cambrian, as Darwin proposed. The Precambrian, far from some mysterious prologue to evolution, actually takes up 85 percent of the history of life. And paleontologists now have a marvelous collection of Precambrian fossils, including bacteria, protozoa, algae, Ediacarans, burrow makers, and animal embryos. But even with a much smoother fossil record, the Cambrian period is clearly the most remarkable episode in animal evolution. No matter how long animals were already lurking in the oceans, their diversification accelerated 535 million years ago in a tremendous explosion. Thanks to precise uranium-lead dating, scientists have determined that the Cambrian explosion took only 10 million years.
The Cambrian explosion took place completely underwater. As these new animals came into existence, the continents were bare, except for bacterial crusts. But it was not long, geologically speaking, before multicellular life spread onshore. First came plants. Around 500 million years ago green algae gradually evolved waterproof coats that allowed them to survive exposed to air for longer and longer periods of time. The first land plants probably looked like today’s moss and liverworts, forming a low, soggy carpet along the banks of rivers and coastlines. By 450 million years ago centipedes and other invertebrates were beginning to explore this new ecosystem. New species of plants evolved that could hold themselves upright, and by 360 million years ago trees were growing 60 feet high. Out of the coastal swamps would sometimes slither our ancestors—the first vertebrates that could walk on land.
In the history of life, the move to land is a brief coda. Nine-tenths of our evolution took place completely underwater. But from our point of view, the last few hundred million years on land are the most interesting period of all. Fossils of the earliest land vertebrates show that they had branched into two lineages by 320 million years ago. One branch, the amphibians, produced some lumbering giants early on, but today is represented only by frogs, salamanders, and other small creatures. They generally need to stay moist and lay soft eggs that can dry out easily. The other branch, the amniotes, evolved a strong water-tight eggshell. From among these amniotes the dinosaurs emerged around 250 million years ago; they evolved into the dominant land animals and stayed that way until 65 million years ago, when most dinosaur branches became extinct (the only survivors were birds, which are just feathered flying dinosaurs). Although the first mammals appeared alongside the first dinosaurs, they didn’t dominate the land until their reptile counterparts disappeared. Our own primate lineage probably emerged around then, but it wasn’t until 600,000 years ago that the oldest fossils of Homo sapiens were buried in the earth. All humans alive today can trace their heritage back to a common ancestor that lived only around 150,000 years ago.
While these few pages can’t do full justice to the majestic depths of life’s history, one thing is clear: our own time in this universe is almost inconceivably brief. No longer can human history match the scale of natural history. If the 4 billion years that life has been on Earth were a summer day, the past 200,000 years which saw the rise of anatomically modern humans, the origin of complex language, of art, religion, and trade, the dawn of agriculture, of cities, and all of written history—would fit into the flash of a firefly just before sundown.
In the end, Darwin got the luxury of time he craved. But the fossil record, though it documents the pattern of life’s evolution, does not reveal the details of just how it evolved. And on this point, Darwin also never managed to close his case during his lifetime, because he never understood how heredity works.
While geologists and paleontologists were charting the history of life, other scientists in the twentieth century solved heredity’s mystery and linked it to natural selection. The connection lies among molecules and atoms, as did the proof of life’s antiquity. But the atoms of heredity are not embedded in rocks for billions of years. They are lodged in the core of our own cells.
Four
Witnessing Change
The puzzle of heredity—how two people can create a child with qualities of both parents—inspired a lot of wild ideas in the 1800s. One of the wildest, at least from our perspective today, was known as pangenesis. It held that heredity is carried by tiny particles that bud from cells throughout a person’s body. These particles (called gemmules) supposedly stream like trillions of migrating salmon to the sex organs, where they concentrate inside sperm or eggs. And when a sperm fertilizes an egg, the gemmules of both parents blend together. Since each particle comes from a cell from a particular part of a parent’s body, they combine together into a new person with traits of both parents.
Pangenesis turned out to be a failure, but the scientist who proposed it was not relegated to history’s ranks of scientific crackpots. His reputation survived, thanks to a few other ideas that have withstood the test of time. Pangenesis was the work of Charles Darwin.
Along with Earth’s age, heredity was one of Darwin’s great frustrations. Origin of Species persuaded most scientists by the end of the nineteenth century that evolution was a reality, but many of them were skeptical of Darwin’s own mechanism of change, namely, natural selection. Many of them resurrected Lamarck’s old ideas instead. Perhaps there was a built-in direction to evolution, they claimed, or perhaps there were ways that adults could acquire traits in their lifetimes that they could pass on to their children. If Darwin could have shown that heredity forbids these ideas but allows for natural selection, he could have refuted his critics. But that knowledge was beyond Darwin and all the other scientists of his day.
In the years after Darwin’s death, biologists finally began to learn how heredity works. Only then could they recognize that the neo-Lamarckians were wrong. Only then could they recognize how heredity makes natural selection not just possible but inevitable and how it allows new species to form. It took the work not just of geneticists to make this discovery, but of zoologists and paleontologists as well. By the middle of this century, they had combined their research into a collective understanding of evolution that became known as the “modern synthesis.” Younger scientists have used the modern synthesis as the foundation for their research. They’re beginning to understand how evolution happens on a molecular level, and as a result, natural selection is no longer the elusive, imperceptible force Darwin imagined it to be. In fact, scientists can witness natural selection happening in the wild today, as well as the branching of old species into new ones. Scientists don’t even have to watch animals or plants or microbes to see natural selection play out: they can watch it take place within our bodies, or even among artificial life-forms within a computer.