Did you know that the theory of evolution did not begin with Charles Darwin? The idea of evolution was part of Western thought for more than 2,000 years before Darwin changed the world with his legendary book On the Origin of Species.
The experiences and observations of Charles Darwin significantly contributed to his theory of evolution through natural selection. This module explores those influences and describes evolution as a force for biological change and diversification. The first in a series, it details how the theory challenged the cultural mindset of the time, including the effect of his major works: On the Origin of Species by Means of Natural Selection and Sexual Selection and the Descent of Man.
Few people have changed the world with the power of an idea. Charles Darwin, the British naturalist who lived during the 1800s, was one of them. While we might equate the idea of evolution with other revolutionary scientific breakthroughs, such as Einstein's general theory of relativity, people seem to care less about what it means to live in a universe where the speed of light is fixed than in a world in which humans descended from hairy apes.
That is a tricky question because of its implications about the very nature of life, humanity, and religion. It is the reason why some greet Darwin's name with a gut-level sense of distrust even though his contributions to our understanding of life are as solidly confirmed as are Einstein's contributions to our understanding of the universe. So, it is no surprise that more people have an inkling – too often wrong – of what is meant by Darwin's concept of natural selection than by the terms of Einstein's famous equation E = mc2.
On the Origin of Species
Darwin's legendary book, On the Origin of Species by Means of Natural Selection; or, the Preservation of Favoured Races in the Struggle for Life, is frequently listed as one of the greatest books ever written. The three critical ideas he developed in it are:
Darwin began developing these ideas as a result of his experiences during a five-year voyage on the British survey vessel H.M.S. Beagle, which sailed around the world on a mapping expedition during the early 1830s (Figure 1). Darwin was on board to work as the ship's naturalist, to record information about the geology, sea life, land animals and plants, and people that the Beagle would discover. When he set sail in 1831, Darwin was twenty-two years old, fresh out of college, fascinated with science, and deeply interested in geology and natural history. He was planning to become a clergyman, partly because he thought it would allow him enough free time to pursue his other interests.
One of the main ideas of Darwins' book On the Origins of Species was that all forms of life share family roots.
Ideas about evolution through history
Darwin was keenly aware that the idea of evolution was in the air and was being hotly debated in some circles. Actually, it had been part of Western thought for more than 2,000 years, at least since the Greek philosopher Aristotle proposed there were natural laws that explained how the world came to be. These laws were meant to be alternatives to the usual myths and stories about the origins of the universe and of people that all native cultures seem to generate. Some of Aristotle's proposals were quite specific. He believed, for example, that there were "higher" species and also "lower" species, and the lower ones gave rise to the higher.
As Europe emerged from the Middle Ages, scientists interested in biology considered evolution an idea of historical importance. One of Darwin's own grandfathers, Dr. Erasmus Darwin, had even written extensively about evolution. But what changed the climate of Darwin's times was that the natural sciences were becoming modernized and professionalized, with their own societies, meetings, and publications. This allowed the fuzzy notion of evolution to rise to the level of a scientific hypothesis, which might be proven or disproven by research, evidence, and a method of reasoning.
Evolution vs. fixity
As the mid-1800s approached, the idea of evolution posed a serious challenge to the then-popular view that species were unchanging fixtures of nature. This concept, called the Fixity of Species, was a perspective that European zoologists and botanists adopted as part of their culture, to reflect Western religion and the story of creation as laid out in the Bible. A key feature of the scientific argument for "fixity" was the notion that the structure of each species was based on a model, ideal form. In other words, botanists would make the case that all wild briar roses were supposed to look like replicas of one another because a wild briar rose was meant to be built in a precise, definite way or it would not be a wild briar rose. Why? Because each wild briar rose was a product of God's "perfect" acts of creation. And if each was meant to be perfect, there was no reason for any to change, and no possibility that they ever did.
The fixity idea, however, was not satisfactory to all. Some geologists and zoologists thought that species might actually change over time. In fact, the possibility of evolution being a fundamental feature of nature eventually became the crucial question of nineteenth-century science. One of the reasons why this happened was that fossils were slowly being discovered, some in highly "imperfect" environments that seemed not to follow the logic of creation – such as the occurrence of ocean seashells found buried on the tops of mountains such as the Alps and the Himalayas.
Darwin allowed himself to wonder if species were fixed or prone to evolution. With the intense experience of five years of living and working on the Beagle, collecting and describing a vast number and variety of natural history specimens, he developed into a first-rate naturalist – actually, the best in the world. He came to see species differently than those who saw perfection in them. Darwin did not focus on the sameness of individuals; rather, he thought it was important that individuals, like you and me, vary in spite of the fact that we belong to the same species. He realized that the variations could become the raw material for evolutionary change.
In Darwin's day, most people believed that
Clues to evolution: Birds and fossils
One of the clues that moved Darwin to totally accept the principle of evolution involved a group of small birds called mockingbirds. Mockingbirds are unspectacular animals with a wingspan of about 10 inches. They live in many habitats in North, Central, and South America, from southern Canada to Chile and Argentina. Darwin observed and collected them on the Galapagos, a cluster of small islands off the coast of Ecuador (Figure 2), and sent his specimens back to London for study.
After the voyage, Darwin consulted one of the most experienced ornithologists (bird specialists) in England, John Gould, about their taxonomy (see our Taxonomy module). Darwin was surprised to learn that he had misclassified some of the birds because it was difficult for him to tell the species apart from the subspecies. The physical traits of mockingbird species and subspecies blended into one another. For Darwin, this meant that the guidelines he had been trained to use to identify and classify animal and plant species, based on the idea that each one ought to have an idealized "perfect" form – Fixity of Species – was an arbitrary rule created by taxonomists, nothing more than an untested assumption. It logically followed that if species were not designed to be a series of perfect individual replicates, then evolutionary change – or "transmutation" of one species into another – was a possibility. Darwin saw immediately that some of Gould's species could have come into existence if one subspecies changed a little bit more than usual, perhaps as it got isolated on a separate island.
A second clue that led Darwin to embrace evolution had to do with fossils. Fossils are formed when an organism dies and its remains become hardened by absorbing minerals from the earth in which they were buried. Thus, fossils are direct evidence of life in the past and have great importance when considering a time-dependent concept such as evolution. In Argentina, Darwin collected fossils of gigantic armor-plated beasts, megatheres (Figure 3), which were unlike anything else anywhere in the world – nearly. Only the tank-like armadillos, which Darwin had also seen in South America, bore any resemblance to them. Considering these extinct and living forms together, Darwin theorized that megatheres and armadillos might be related. He thought they might be part of a large group of South American mammals that had evolved body armor as a protective adaptation. He speculated that an ancient "cousin" of the megatheres might have been the ancestor of the armadillo.
The Galapagos mockingbirds and the Argentine megatheres provided Darwin with two complementary views of evolution. One helped him picture biological change by comparing living animals. The other helped him see it by comparing an extinct species with one that was living. Darwin collected pieces of the evolutionary puzzle during his five years of sailing on the Beagle, but to solve the puzzle by putting the pieces together into a basic model for the public to see would take him several more decades of effort. His work was capped by publication of Origins in 1859, more than twenty years after he began his voyage on the Beagle.
Response to Origins
Origins was immediately recognized as a major scientific success. In one of the quirkiest episodes in the history of science, this happened to be the second time that Darwin published his explanation of evolution. A year earlier, Darwin learned that another naturalist, Alfred Russell Wallace, had also thought of evolution by natural selection, and they eventually wrote a joint paper on the subject in order to share the credit. But the Darwin-Wallace essay did not compare with Origins, which included examples and reasoning that Darwin developed over a twenty-year period. Origins was much more than a statement on the controversial idea of evolution; it laid out a new system of thought, another way of asking scientific questions, assembling scientific evidence, and scientifically testing hypotheses.
Some people were less than happy with the book's publication. Since its central idea was that evolution is an ever-present, unstoppable, fundamental law of nature, Origins became an angry flashpoint for those who cared less about the biological history of animals and plants than they cared about the deeper implications of the really big idea it represented – that in the middle 1800s there were new, logically sound, evidence-based ways of looking at life that challenged the religious ways of thinking that had been broadly accepted for centuries. (See Figure 4 for a parody of his theory of evolution.)
This makes it all the more interesting that the "Question of Questions" was not at all touched on in Origins. Darwin knew all along that this new science of evolutionary biology could be applied to human beings precisely the way he had applied it to mockingbirds and armadillos. Like the mockingbirds, people vary in appearance across countries and continents, and from one island to another. Like the armadillos and megatheres, the skeletons of modern humans closely resemble extinct fossils then being discovered in the Neander Valley of Germany, fossils that would come to be known as Neanderthal man. Darwin said nothing about this in Origins for, in his extraordinary thoroughness, he wasn't ready yet. He was also unprepared for the difficult personal battle that would have resulted if he had.
When Darwin's book On the Origin of Species was published,
Human evolution: Descent of Man
About twelve years later, in 1871, Darwin did publish a book specifically about human evolution, Sexual Selection and the Descent of Man. By then, the fury against his ideas had died down in England, and evolution was not a hotly contested issue any longer. By then, other highly accomplished scientists had written about people evolving, most notably Thomas Henry Huxley, in Evidence as to Man's Place in Nature, which appeared in 1863. The idea was slowly being absorbed by society. But nothing could match Darwin's brilliant thinking about the evolutionary process, so no one could match what Darwin would have to say about the subject of man.
Descent of Man was as much about bringing out the few facts then known about human evolution as it was about the meaning of evolution as a way of thinking about our ethics and personal values. Darwin knew that evolution was one of the most important ideas for the human species to comprehend. He knew that seeing us from an evolutionary perspective was more than peering through a telescope to look back at our own primitive origins. Evolution was also a mirror and a microscope for looking at ourselves as we are today.