Excerpted from A Brief History of Creation by Bill Mesler and H. James Cleaves III. Copyright © 2016 by Bill Mesler and H. James Cleaves III. With permission of the publisher, W. W. Norton & Company, Inc. All rights reserved.

The year is 3,500,000,000 BC. The place is a rocky outcropping that juts out into a shallow, wave-lapped inlet on a landmass that will one day be called Australia. The seas are bright green and have the sulfurous stench of rotten eggs. The moon looms large in the sky, twice as large as the moon does today because it is only half the present distance from the Earth. The sun, however, is only about three-quarters as bright as it will one day be. Still, it bombards the Earth’s surface with deadly ultraviolet radiation unchecked by a protective layer of ozone. The atmosphere is filled with toxic gases, and almost completely devoid of oxygen. That will come much later, the product of photosynthesis by tiny organisms that will one day churn away in the primitive oceans.

But the ancestor of those creatures is already here. It lives in the ocean near the shore, close to a hydrothermal vent that keeps the temperature of nearby water close to boiling. It is a tiny, single-celled organism, no more than a lipid membrane that encases an early but functioning genome composed of DNA, as well as proteins and the RNA with which these parts communicate. Billions of years in the future, scientists will give it a name: LUCA, short for “last universal common ancestor.”

Eventually, LUCA will give birth. This will be a virgin birth, which it will accomplish through binary fission. When it is done, LUCA will have divided into two of what will be essentially clones, distinguishable only by the odd genetic mutation. Soon other such clones will appear. They will share their genetic information with each other in a haphazard fashion. Their genetic code will be pooled, their evolution shared. Collectively, they will exist less like a community of organisms and more like a communal organism.

In time, the shallow lagoon will be filled with such organisms, forming small domed masses that peek out from the surface of the water. They will appear to be made of mud. This will be an illusion. Inside, they will be composed of finely laminated layers of silt and biological material. These are microbial mats, not unlike the masses of scum that float to the surface of modern ponds, filled with complex symbiotic communities of micro-organisms interspersed with fine particles of clay and other minerals that have adhered to their cell walls.

Under a microscope, these communities would appear as tiny oval cells juxtaposed with filamentous bacterial forms. They look a lot like stromatolites, a few versions of which still exist in Australia, as well as on the shores of such far-flung places as the Yucatan, British Columbia, and Turkey. Dynamic and resilient microbial ecosystems, they are capable of thriving in environments hostile to most modern forms of life. In the years to come, these early descendants of LUCA will grow and multiply, carried by winds and currents to distant refuges across the Earth, until one day they will collectively evolve into human beings and every other living thing on the planet today.

Ever since the publication of On the Origin of Species, scientists have speculated about what Charles Darwin tantalizingly called “one primordial form.” Though based on reasonable evidence, the scenario just depicted is purely hypothetical. There are many such guesses about the precise nature of LUCA. The reality is that nobody knows exactly what the environment was like when life arose some four billion years ago. Instead of the hot water of a hydrothermal vent, for instance, LUCA might have lived in a warm little pond not unlike the one once suggested by Charles Darwin. Nor does anyone know what LUCA’s internal chemistry was like. It may have possessed unique features that have been lost to its ancestors over the course of several billion years of evolution and natural selection.

Until the late twentieth century, scientists had precious little to go on. The reason modern scientists can draw a reasonable picture of LUCA at all is due in large part to the evolutionary detective work of one man, the biophysicist Carl Woese. One of the most creative, revolutionary, and underappreciated biological thinkers of the twentieth century, Woese upended nearly everything that biologists had once assumed they knew about the earliest organisms on Earth, and laid the groundwork for a new understanding of how those organisms existed and evolved. After his death in 2012, some of Woese’s most ardent admirers would invoke comparisons to Einstein and Darwin.

Although Carl Woese would go on to become one of the world’s most important biological thinkers, he never much cared about the life sciences when he was young. Growing up in Depression-era Syracuse, New York, Woese was a painfully shy child who fixated on mathematics. Math offered a respite from the chaotic world around him. It was objective, consistent. In his later years, that shyness would keep him away from most academic meetings or conferences, and it probably contributed a great deal to the relative underappreciation of his work by the general public, as well as to the resistance that his most revolutionary ideas would one day meet in the field of biology.

Woese went on to study at Amherst College, where he did his undergraduate work in mathematics and physics. By the time he arrived at Yale to work toward a doctorate, however, he had turned to biophysics, steered, like so many would-be physicists of his generation, into the relatively new science that had been invigorated by Schrodinger’s What is Life? Woese’s work at Yale revolved around radiation and how it might be used to change the molecular structure of viruses, particularly the one that causes Newcastle disease, which afflicts poultry. After graduating, he spent two years in an unsatisfying and ultimately unsuccessful pursuit of a medical degree before securing a position as a biophysicist at General Electric’s primary research laboratory in Schenectady.

Crick and Watson’s discovery of the structure of DNA had by then begun to reshape the scientific world’s understanding of genetics, and Woese spent the next five years at Schenectady trying to decipher the genetic code. The problem, as he saw it, was one of translation. There were just four “letters” of nucleic acid bases, arranged in three-letter “words.” These had to correspond to the twenty amino acids found in proteins, allowing them to be strung together in precise sequences. How this was accomplished was anyone’s guess.

Woese turned his attention to the problem of understanding the cellular translators of the genetic code, ribosomes, the large molecular structures composed mainly of RNA that read out specific genetic instructions from DNA into proteins. The little-understood ribosomes had started to interest Woese as early as his Yale studies on bacteria, but at Schenectady he was able to focus on them exclusively.

Though Francis Crick and others would work out the problem of the genetic code before Woese, he would come to understand the code’s potential in ways they did not. They treated it is a physics problem to be solved mathematically. Ironically, Woese, who had once rejected biology for mathematics, saw the genetic code as a distinctly biological phenomenon rooted in evolution. He saw that the code might be able to serve as a kind of evolutionary time machine, enabling scientists to peek back through successive generations all the way to the dimmest and most distant period in evolution. Instead of trying to gauge the changes between species by measuring the differences between physical manifestations—searching, as Geoffroy Saint-Hilaire had once done, for the similarities between a human hand and a whale fin—Woese believed that evolutionary links could be more conclusively elaborated by tracing the evolution of the cellular machinery that governed the translation of DNA into protein.

Woese would set himself to this task for the next decade, until he understood the history of life well enough that he could completely reshape what had long been thought to be one of the most unshakable foundations of biology, the tree of life.

The first systematic attempt to classify every living thing in the natural world was made by the Swedish physician Carl Linnaeus, one of the eighteenth century’s most influential naturalists. In his groundbreaking 1735 book Systema naturae, or The Natural System, he sorted organisms into three distinct “kingdoms,” consisting of the plants, the animals, and the minerals. By 1758, the tenth edition of Systema naturae had grown to include seventy-seven hundred species of plants and forty-four hundred species of animals, all systematically grouped and categorized. These numbers seemed awfully large at the time, but in the two and a half centuries since, estimates of the varieties of species have grown exponentially. By the beginning of the twenty-first century, scientists would suspect that there may be as many as a billion distinct species of bacteria, about three hundred thousand species of plants, and perhaps ten to thirty million animal species, most of which are yet-to-be-discovered insects.

Linnaeus’s classification scheme was based on fairly easily observable traits: whether things moved or grew, swam or flew, had fur or backbones. He grouped things by physical similarity. Fossils would eventually provide meaning to the scheme, suggesting an evolutionary pattern that linked the different species and providing a body of evidence upon which Darwin’s theory of natural selection would rest.

Darwin was among the first to envision a phylogenetic “tree of life,” a family tree that stretched back through every generation to the beginnings of life. He included a sparse and simple drawing of his tree in On the Origin of the Species. It consisted of a twig-like branching pattern, with extant species represented by the outermost twig points. According to Darwin’s scheme, tracing downward from these points would be like traveling back in time, with junctions in thicker and thicker branches where each twig would have a common ancestor. Humans and chimpanzees, for example, would have diverged from a common ancestor, and if one followed the branch leading to that junction backward, it would eventually join at another juncture with yet another branch leading upward to, say, New World monkeys. One could also travel down the tree through evolutionary time—through the divisions of mammals, the vertebrates, the animals—with the tree gradually narrowing until ending at the single organism that is the root of all existing life. Darwin logically concluded that all living things must have been descended from one common ancestor, what he described as “one primordial form.”

The notion of universal common ancestry became a central tenet of modern evolutionary theory. It was supported by a number of observations, like chirality (first discovered by Pasteur in his work with crystals), the similarities of cellular structures, and the fact that every organism, from microbes to human beings, uses almost exactly the same genetic code. By the modern era, few reputable scientists would argue against the doctrine of common descent.

In time, Darwin’s tree of life was enhanced and reshaped by modern paleontology and radiometric dating. Bones could be dated, and the family lineages of developed species could be more accurately reconstructed. As techniques in microbiology improved, organisms were further divided into single-celled and multicellular organisms, and later into two different categories: organisms with a cell nucleus, called the eukaryotes, and those without, the prokaryotes. Eventually, the living world would be divided into five kingdoms: animals, plants, fungi, single-celled eukaryotes, and prokaryotes. But the pool of evidence for the last two kingdoms was lacking. The fossil record of the most numerous, simplest, and presumably oldest species was glaringly thin, and the place of microbes in the tree of life was shaky. Carl Woese was determined to find a way around that problem.

Bill Mesler is a journalist who has worked the daily Santa Cruz Sentinel, San Francisco Bay Guardian and the Nation magazine. He lives in Washington, D.C.

H. James Cleaves II is a professor at the Earth-Life Science Institute in Tokyo, a visiting scholar at the Institute for Advanced Study in Princeton, New Jersey and vice president of the International Society for the Study of the Origin of Life. His research focuses on the origin of life from a chemical perspective. He lives in Washington, DC.