As a fundamental keystone of modern biological theory, the theory of evolution radically transformed our understanding of living species. Ultimately, it created a solid foundation to answer one of the greatest questions of all time: Where do we come from?
Since the beginning, humans have developed different explanations for their (and the world’s) origins. Most creation myths and religious theories have been attributed to the design of a God — like in Judaism, Christianity, and Islam — or groups of deities, like in many indigenous peoples’ cultures. In Hindu cosmogony, Brahma is described as a demiurge who disposed of and organized the foundational elements of the universe and very few ditched the transcendent in a quest for their roots.
Ancient Greek philosophers such as Anaximander proposed that animals could be transformed from one kind into another, and according to Empedocles they were made up of various combinations of preexisting parts. Before them, Chinese philosopher Zhuang Zhou described what some historians consider a precursor to natural evolution. In the Zhuangzi, one of the two foundational books of Taoism, master Zhuang describes the connections among species, their environment, and how different surroundings and conditions influence the way species evolved. Although the processes described in the Zhuangzi aren’t the evolutionary mechanisms we theorize about today, they’re remarkably similar to Darwinian evolution regarding the relationship among species — especially if we consider that master Zhuang lived in the fourth century BC.
At the same time, in Europe, Early Church Fathers, Gregory of Nazianzus and Augustine of Hippo weren’t sold on the idea that all plants and animals were created by God. Some, they argued, must have developed over time from God’s creations.The reason was more religious than biological, but also practical: It would have been impossible for Noah to hold representatives of all species on his Ark, hence some must have come into existence after The Flood.
Fast-forward 15 centuries, and the British theologian William Paley uses natural history, physiology, and other contemporary knowledge to elaborate on the argument of the Earth’s origin from design. In his Natural Theology, published in 1802, Paley uses a watchmaker analogy to state that a specific design implies a designer. Watches and timepieces have been used as examples of complicated technology in philosophical discussions by thinkers like Cicero, Voltaire, and René Descartes. And because these artifacts are so complex, if someone was to find one abandoned on the street, they would certainly reconnect it to the work of a skilled watchmaker, rather than to the idea that watches have always been lying abandoned like stones and rocks. By extension, Paley went on to argue that the complex structures of living beings require an intelligent designer, God, who had carefully designed “even the most humble and insignificant organisms” and all of their minute features.
But although it gained traction, Paley’s argument wasn’t supposed to last unchallenged for long. Just seven years after the publication of Natural Theology, Charles Darwin was born and his father, Robert Darwin, was a wealthy and established doctor. After a summer spent as an apprentice treating the poor of Shropshire, England, Darwin began his studies at the University of Edinburgh Medical School. But it didn’t take long before his interest in lectures and surgery faded and his fascination in taxidermy and biology grew. During his second year at the university, he joined a student natural history group, which used science to challenge orthodox religious concepts. How Darwin became the father of the modern theory of evolution lies right in these two episodes of his history.
Annoyed by his son’s poor commitment to medicine, Robert Darwin decided that a bachelor of arts and a career as an Anglican Parson suited him better, and so he sent Charles to a religious college in Cambridge. There, Charles studied Paley’s work, became passionate about entomology and biology, and eventually decided that it was time for him to travel to the tropics to study natural history. But not before taking a preparatory geology course, graduating, and convincing his father to fund his trip around the world aboard the HMS Beagle.
Darwin spent five years traveling in the Southern hemisphere and by the time he returned to England, he was already a celebrity in scientific circles. That is when he started speculating about the possibility that “one species does change into another” to explain the geographical distribution of living species like rheas — large, flightless birds, which are similar to ostriches. But it took Darwin another 20 years before On the Origin of Species by Means of Natural Selection went to press, and the cycle of scientific contributions that would change our perception of the world started by Nicolaus Copernicus, Galileo Galilei, and Isaac Newton, was closed.
Darwin’s argument accepted the facts of adaptation (remember Zhuang), but he demonstrated that the variety and multiplicity of plants and animals was the byproduct of natural selection, rather than the creativity of a design agent. “Many more individuals are produced than can possibly survive,” he wrote. So, “there must in every case be a struggle for existence,” where the variables that better overcome challenges are passed on, generation after generation.
As a process that promotes (or maintains) adaptation, Darwin proposed natural selection primarily to account for the adaptive organization of plants and animals. In this framework, evolutionary changes through time and diversification were by-products of natural selection, fostering adaptation to different environments. And in 1871, Darwin extended the theory of natural selection to human evolution, arguing that human beings shared a common ancestor with African Apes.
A contemporary of Darwin, Augustinian monk Gregor Mendel provided the missing link in Darwin’s theory. In 1866, Mendel published a paper which laid out fundamental principles of the theory of heredity, which are still relevant. Mendel and Darwin never met, nor did the English scientist have the chance to read the monk’s studies — Mendel’s work was rediscovered only at the beginning of the 1900’s.
From Darwin’s theory stemmed a variety of alternative hypotheses from scientists and philosophers. In the scientific community, two major movements took shape. On one side stood biometricians led by Karl Pearson — an English statistician who defended Darwinian natural selection as the major cause of evolution through the cumulative effects of small, continuous, individual variations. On the opposite end were the mutationists, who, following Mendel’s theory of heredity, stressed the contribution of spontaneous alterations of genes to the rise to new species.
The rise of genetics in the 1920s and 1930s added some clarity to the issue of evolution once and for all to a restricted circle of scientists. Theoretical geneticists on both sides used mathematical arguments to show that continuous variations (like body size, or number of eggs laid) could be explained by Mendel’s laws, and that acting cumulatively on small variations, natural selection could yield evolutionary changes in form and function. Mutationism was taken off the table, but most importantly, theoretical geneticists provided a framework for the integration of genetics into Darwin’s theory of natural selection.
In 1937, Theodosius Dobzhansky made it unmistakably clear that Darwin and Mendel’s theories go hand in hand — and he did so by providing experimental evidence which supported their theoretical arguments, and which ultimately funded the synthetic theory of evolution.
By 1950, acceptance of Darwin’s theory of evolution by natural selection was universal among biologists. Three years later, the application of molecular biology to evolutionary studies by American geneticist James Watson and the British biophysicist Francis Crick, led them to deduce the molecular structure of DNA. The race to understand where we come from and what makes us unique didn’t stop there. In 1981, Martin Evans of the University of Cambridge and Gail Martin of the University of California, San Francisco, conducted separate studies and derived pluripotent stem cells from the embryos of mice, which opened up the field of regenerative medicine to treat damaged or diseased tissues through cell-replacement therapies.