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 from hydrogen sulfide gas within the cave. As a by-product of that reaction, the microbes produce sulfuric acid. The stalactite-like slime formations oozing from the cave walls—dubbed “snottites” by researchers—are as corrosive as battery acid.
Snottites might be stomach turning, but they’re intriguing, too. 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.
From cave-dwelling bacteria to 100-ton blue whales, the diversity of life on Earth is astounding. Yet all of the planet’s organisms, living and extinct, exist on branches of the same family tree. Snottites, swordfish, humans, hydrangeas—all evolved from one single common ancestor.
When, where, and how life originated are some of the biggest questions in biology. 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 100 million or so organisms thought to populate the planet today.
How did the first living cell arise? Before scientists can hope to tackle such a question, they must agree on a definition of life. That’s not necessarily as straightforward as it sounds. In our modern world, the features that separate life from non-life 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 domains of the living and the nonliving.
All cells require an archive of information, a membrane to maintain the inside of the cell different from the outside, and the ability to harness energy from the environment. In modern organisms, that archive is in the form of 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 also complex. Among the organisms alive today, the smallest known genome belongs to the bacterium Carsonella rudii. Even that relatively small genome contains nearly 160,000 DNA base pairs. How could such sophisticated molecular systems have arisen by chance?
All cells require an archive of information, a membrane to maintain the inside of the cell different from the outside, and the ability to harness energy from the environment.
The likely answer to that question is step by step. Laboratory experiments have shown how precursors to nucleic acids might have come together under the chemical conditions present on the young Earth. It’s exceedingly unlikely that a molecule as complex as DNA was the first archive of information employed by the very first living cells. As you’ll see in the chapters that follow, scientists have gathered evidence that hints at what the earliest nucleic acid molecules might have looked like.
While some kind of information archive was necessary for life to unfold, life requires more than a collection of nucleic acids replicating in a warm primordial pond. Living things must have some barrier that separates them from their environment. The cells of all living things, single-celled organisms or multicellular creatures, are each encased in a cell membrane.
Once again, scientists can only guess at how the first cell membranes came about. But research shows the molecules that make up modern membranes possess some interesting properties that may have led them to arise spontaneously. 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 and that provided molecular gates were subject to an early example of the same natural selection process that is still happening today.
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 may have produced molecular by-products that 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 breathe, grow, reproduce, and evolve.
Such a series of events may sound unlikely. However, some scientists argue that given the chemicals present on early Earth, it was likely—if not 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 iron–sulfur minerals—still play a critical role in cells.
That’s one reason researchers are so keen to study creatures like the sulfur-hungry snottites in the Cueva de Villa Luz. When life arose, the planet’s atmosphere contained no oxygen—humans couldn’t survive in such a world. By studying modern extremophiles—including bacteria that thrive in caves and near superheated hydrothermal vents at the bottom of the sea—scientists may uncover clues about how Earth’s first cells came together and functioned.
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 involves 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.
Answers to Case 1 questions can be found in Chapters 2-8.