Deep underground, in Mexico’s Cueva de Villa Luz, the cave walls drip with slime. The rocky surfaces are teeming with colonies of mucus-producing bacteria. No sunlight reaches these organisms far beneath Earth’s surface. Instead, the bacteria survive by capturing energy released as they oxidize hydrogen sulfide gas that exists within the cave. As a by-product of that reaction, the microbes produce sulfuric acid, making the slime that oozes from the cave walls—dubbed “snottites” by researchers—as corrosive as battery acid.

Snottites might be stomach turning, but they’re intriguing, too. The organisms that produce snottites are called extremophiles because they live in places where humans and most other animals cannot survive. Such microorganisms may tell us something about life when Earth was young.

All cells require an archive of information, a membrane to separate the inside of the cell from its surroundings, and the ability to gather materials and harness energy from the environment.

From cave-dwelling bacteria to 100-ton blue whales, the diversity of life on Earth is astounding. Yet all of our planet’s organisms, living and extinct, exist on branches of the same family tree. Bacteria that produce snottites, swordfish, humans, hydrangeas—all evolved from a single common ancestor.

When and how life originated are some of the biggest questions in biology. Earth is nearly 4.6 billion years old. Chemical evidence from 3.5-billion-year-old rocks in Australia suggests that biologically driven carbon and sulfur cycles existed at the time those rocks were formed. In the eons since, the first primitive life-forms have evolved into the millions of different species that populate the planet today.

How did the first living cell arise? Scientists generally accept that life arose from nonliving materials—a process called abiogenesis—and thousands of laboratory experiments performed over the past sixty years provide glimpses of how this might have occurred. In our modern world, the features that separate life from nonlife are relatively easy to discern. But Earth’s first organisms were almost certainly much less complicated than even the simplest bacteria alive today. And before those first truly living things appeared, molecular systems presumably existed that hovered somewhere between the living and the nonliving.

All cells require an archive of information, a membrane to separate the inside of the cell from its surroundings, and the ability to gather materials and harness energy from the environment. In modern organisms, the cell’s information archive is DNA, the double-stranded molecule that contains the instructions needed for cells to grow, differentiate, and reproduce. Without that molecular machinery, life as we know it would not exist.

DNA is critical, and it’s complex. Among the organisms alive today, the smallest known genome belongs to the bacterium Carsonella rudii. Even that genome contains nearly 160,000 DNA base pairs. How could such sophisticated molecular systems have arisen?

The likely answer to that question is: step by step. Laboratory experiments have shown how precursors to nucleic acids might have come together under chemical conditions present on the young Earth. It’s exceedingly unlikely that a molecule as complex as DNA was employed by the very first living cells. As you’ll see in the chapters that follow, scientists have gathered evidence suggesting that RNA, rather than DNA, stored information in early cells and, indeed, did much more than that, catalyzing chemical reactions much as proteins do today.

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While some kind of information archive was necessary for life to unfold, there is more to the story. Living things must have a barrier that separates them from their environment. All cells, whether found as single-celled bacteria or by the trillions in trees or humans, are individually encased in a cell membrane.

Once again, scientists can only guess at how the first cell membranes came about, but research shows that the molecules that make up modern membranes possess properties that may have led them to form spontaneously on the early Earth. At first, the membranes were probably quite simple—straightforward (but leaky) barriers that kept the contents of early cells separated from the world at large. Over time, as chance variations arose, those membranes that provided a better barrier were favored by natural selection. Moreover, proteins became embedded in membranes, providing gates or channels that regulated the transport of ions and small molecules into and out of the cell.

A third essential characteristic of living things is the ability to harness energy from the environment. Here, too, it’s feasible that a series of natural chemical processes led to entities that could achieve this feat. Simple reactions that produced molecular by-products would have enabled more complex reactions down the road. Ultimately, that collection of reactions—combined with an archive of information and enclosed in some kind of primitive membrane—evolved into individual units that could grow, reproduce, and evolve.

Such a series of events may sound unlikely. However, some scientists argue that given the chemicals present on the early Earth, it was likely—and even inevitable—that they would come together in such a way that life would emerge. Indeed, relatively simple, naturally occurring materials such as metal ions have been shown to play a role in key cellular reactions. Billions of years after the first cells arose, some of those metal ions—such as complexes of iron and sulfur—still play a critical role in cells.

That’s one reason researchers are so interested in studying organisms found today in extreme environments. The sulfur-hungry snottites in the Cueva de Villa Luz help us to understand life 1–2 billion years ago, when oxygen was less plentiful and hydrogen sulfide more abundant. Still earlier, when life arose about 4 billion years ago, the planet’s atmosphere contained no oxygen—humans couldn’t survive in such a world and neither could snottite bacteria. However, by studying modern extremophiles living in oxygen-free environments, scientists may uncover clues about how Earth’s first cells came together and functioned.

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Did life arise just once? Or could it have started up and died out several times before it finally got a foothold? If, given Earth’s early chemistry, life here was inevitable, could it have arisen elsewhere in the universe? The study of life’s origins produces many more questions than answers—and not just for biologists. The mystery of life spans the fields of biology, chemistry, physics, and planetary science. Though the questions are vast, our understanding of life’s origins is likely to come about the same way life itself arose: step by step.

CASE 1 QUESTIONS

Special sections in Chapters 2–8 discuss the following questions related to Case 1.