It is hard for us today to imagine the material and intellectual milieus into which Charles Darwin was born, on February 12, 1809 in the southern English town of Shrewsbury. Leeches and arsenic infusions were state-of-the-art treatments in medicine, Thomas Crapper had yet to popularize the water closet, gas lighting in houses was a rare curiosity, and virtually everyone believed that living things existed in a fixed hierarchy ordained by God, from pond scum at the bottom to humans at the top, with the angels hovering just above. Some geologists had already begun to realize that our planet must have had a very long history to produce the phenomena they scoured the landscape to examine; but the received wisdom was that life itself had remained unchanged since the Creation. By one reckoning (arrived at by adding up the “begats” in the Book of Genesis), this event had occurred in 4004 B.C. At the time, it was even considered bold to interpret the fossil remains of extinct animals as the vestiges of the Divine creation destroyed by Noah’s flood. Any modern person miraculously transported back to 1809 would have found this a very alien and uncomfortable world to live in, intellectually as well as physically.Charles’s physician-poet grandfather Erasmus had speculated, some fifteen years before his grandson’s birth, on the possibility that “all warm-blooded animals have arisen from one living filament.” But among scientists only the great naturalist Jean-Baptiste Lamarck, in revolutionary France, had dared to suggest, as early as 1801 but most influentially in the very year of Charles’s birth, that one living species might transform into another. To Lamarck, fossil species were parts of lineages that had changed gradually over time to give rise to those familiar today. There was an alternative possibility in the pattern of change, though, and when Charles Darwin was five years old, the Italian geologist Giambattista Brocchi came up with it. For Brocchi, species did not transform themselves out of existence over time. Instead, they had discrete identities, with births and deaths and finite lifespans over which they might give rise to descendants. Between them, Lamarck and Brocchi defined the possible parameters for the kinds of change that had given rise to the riotous diversity in the living world today. But history can be cruel. Lamarck found himself mocked and discredited, while Brocchi, obscure even in his own day, was forgotten. And biology continued to be a science based on the accumulation of masses of observations, without any unifying intellectual framework.Through brief early experience as a medical student at the free-thinking University of Edinburgh, Darwin was well aware of Lamarck’s work, and almost certainly of Brocchi’s too, by the time he undertook his epic round-the-world voyage on the British navy sloop Beagle between 1831 and 1836. But meanwhile, he would have heard little talk of the transformation of species while attending the University of Cambridge, where all faculty had to be men of the cloth. Still, his observations of geology and natural history in South America convinced him privately by the end of his Beagle experience, or soon thereafter, that species somehow did transform into other species. And with this in mind he soon realized that all living organisms on our planet are thereby related by descent. What is more, it was “descent with modification” that accounted for the striking nested pattern of resemblances among living things, whereby closely related species – descendants of a recent common ancestor – resemble each other in more features than do remotely related forms sharing an ancestor that lived in the more distant past. We humans, for example, are anatomically much more similar to the African apes with which we share an ancestor that lived a mere 7 or 8 million years ago, than to the Old World monkeys, with which we shared a common ancestor almost 30 million years agoAnd thus was born in Darwin’s mind the central theory that unifies biology today. Not without reason did the great geneticist Theodosius Dobzhansky write that “nothing in biology makes sense except in the light of evolution.” In the absence of that notion of common descent, there is no way to understand the distribution of biological phenomena in Nature. To call evolution a “theory,” by the way, is not to demean it. In science a theory is a hypothesis about something in Nature that has resisted all efforts so far to prove it wrong; and since science is a provisional system of knowledge, achieving the status of a theory is about as good as it gets. Science is not about ultimate explanation; it is about understanding what we observe to the best of our abilities.The path to Darwin’s great 1859 book On the Origin of Species was a long one, and it has been retold many times. But the upshot was that Darwin died, in 1882, in a world that was very different than the one he was born into. The nineteenth century was a time of great material progress: by 1882 railroads connected almost every town and larger village in his native land, domestic electric lighting had made its appearance, and Alexander Graham Bell had already received his patent on the telephone. But equally huge was the change in intellectual environment that had come about in Darwin’s lifetime, and largely through his own insights and efforts. Finally, a coherent, testable scientific explanation had become available for the structure of the living world, and for humankind’s place in it. And the revolution that Darwin began in our comprehension of ourselves and of the world we live in is the reason why, each February 12, his birthday is still celebrated in colleges, auditoria, and private homes around the world.