Excerpted from Creation: How Science Is Reinventing Life Itself, by Adam Rutherford. Published by Current/Penguin. Copyright © Adam Rutherford, 2013. Used with permission.

In the 150 years since the Origin of Species was published, millions of scientists have poked and pulled at the theory of evolution; unpicked, tweaked, and yanked at it in every conceivable way. They have observed countless species from aardvarks (or anteaters) to zebras to study their behavior. They have created simulations of myriad populations first with mathematical models, later in computers, and have pushed and pressurized their artificial environments to see how they adapt over successive generations. They have bred and crossbred inestimable species to observe how inheritance works, to see where advantage lies in the next generation. They have let tanks of bacteria reproduce for decades and witnessed descent with modification in action. In the modern era, we have deciphered their genetic codes and seen exactly the differences in DNA that reflect one species becoming two, each finding a niche into which they are better suited. We’ve seen populations of bacteria adapt to the hostile action of antibiotics and, distressingly, emerge resistant. While the initial model that Darwin laid out has been modified and fleshed out, the “one big argument,” as he described it, has survived intact throughout the necessary battering that an idea of this magnitude demands. This is why it is called the theory of evolution by natural selection. The colloquial meaning of the term theory as a hunch, or guess, or plain old stab in the dark, is woefully puny next to its scientific meaning. When scientists talk about a theory they are aiming at the top of the pile of ideas: a set of testable concepts that all point to and predict a description of reality that is so robust it is indistinguishable from fact.

Darwin drew up his masterpiece as the study of cells was beginning to emerge from the stagnant pond of spontaneous generation. But his description of evolution is not about the beginning of new life; it is, as is implicit in the title, about the origin of new species. These arise when an organism has acquired so many mutated traits that they are no longer capable of reproducing with what were once their kin. When we are taught natural selection, typically we refer to prominent visible traits—antlers, hair color, and, once in a while, anteaters’ tongues. But we now know how biology works at a cellular level, and can transpose evolution by natural selection to the microscopic world of which Darwin was largely unaware. The fictional anteater’s tongue is longer because, by the chance of variation within its population, that individual has more (or possibly larger) cells in that tongue, and the genes that generate this disparity of tissue will be passed on through sperm or egg to the next generation. Similarly, when you get a paper cut the reason your platelets form a plug and a wedge to help stop blood loss is effectively because creatures carrying cells in their blood that didn’t perform that clotting function as effectively have been deselected by nature thousands of generations (and species) ago (probably by allowing the creature concerned to heal less efficiently, or possibly leak to death). Crucially, we now know that what is being selected is not the individual, nor the cell, but the carrier of the information that bears advantage. As with all biology, the information that confers clotting is held in DNA inside cells, the molecule that will play the central role throughout the whole of this story.

Cell theory and natural selection are reflections of the same truth: life is derived. It’s incrementally and ultimately spectacularly modified, but, in essence, life is the adapted continuation of what came before.

Public acceptance of evolution by natural selection has waxed and waned over time, and was disputed by scientists at least in its first fifty years or so. But now, at least among scientists and those who generally understand it, natural selection is the only valid explanation for the variety of life on Earth. While science by definition expects to be corrected over time, it now seems extremely unlikely that Darwin’s idea will be replaced wholesale. When you tie it together with cell theory, both ideas are compellingly fortified.

Although the concept of evolution—simply that organisms change over time—predated Darwin, in 1859 this idea, as well as that of natural selection, was new and truly revolutionary. They both rebutted the prevailing view across the majority of human history, that creatures were each created distinctly. Without the hard-fought work of Darwin and the closely observed work of the eighteenth- and nineteenth-century microscopists, the idea of multiple routes for life, separate origins not just for plants, animals, fungus, but for every single type of organism, might have seemed plausible. Even taking into account the innumerable variety of distinct cell types, each with highly specialized functions, separate or plural origins might seem reasonable.

Instead, thanks to Darwin and cell theory, we can link every organism on the grandest pedigree. As Darwin writes in the final paragraph of his masterpiece, “There is a grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one.” These are some of the finest words set to paper, oft quoted, but some things are just worth repeating. Yet he did pose a question in those last five words: “into a few forms or into one.” Which is it? What lies at the base of the tree of life? A single form, a cell, or several? The answer to this deep-rooted historical question lies not in the past, but in the molecular guts of every single living cell. By examining the mechanism by which cells pass on their characteristics and by which those characteristics mutate, we will arrive at an answer to the question of whether life has a single origin—and we will begin to see what it might have looked like when it first emerged.