How Darwin Really, Truly Solved the Mystery of Life
Many are confused about the origin of "Origin"
"When on board H.M.S. 'Beagle,' as naturalist, I was much struck with certain facts in the distribution of the inhabitants of South America, and in the geological relations of the present to the past inhabitants of that continent. These facts seemed to me to throw some light on the origin of species…"
—The first sentences of Charles Darwin's On the Origin of Species (1859)1
When Charles Darwin first boarded the H.M.S Beagle, eager to embark on the most colossal adventure of his life, he was still a conventionally religious young man. Like most naturalists of his time, Darwin had no doubt that God was the Creator of the Universe, the Grand Designer of both the celestial and the earthly. He knew that God created the stars and that God conjured the organic, breathing life into species in all their forms and placing them in the various regions of the Earth. Darwin believed that God wisely chose to set camels in the deserts, mountain goats on mountains, whales in the ocean, and sheep in grassy fields—and that God bestowed upon each plant or animal, fish or insect, all the peculiar and helpful accoutrements that each required to flourish in its particular habitat. This apparent evidence of the mind of God in every aspect of the organic world, a view detailed in William Paley's influential book, Natural Theology, enabled one to study God's awesome plan by carefully examining the creatures that were His handiwork. This became the predominant goal of most natural historians of the time. And this was Darwin's goal.2
But as Darwin journeyed with the Beagle around the world, faithfully collecting and recording the flora and fauna from each exotic stopover, he began to notice a problem. Yes, natural theology adequately explained why so many different organisms had traits that were peculiarly adapted to their specific location and lifestyle. The long, thin beaks of hummingbirds, the webbed feet of beavers, the sticky silk webs of spiders—all were evidence of God's careful and inventive arranging. What natural theology could not explain, however, and what Darwin did not expect to discover, was a persistent global pattern in the distribution of life, a continuous organic thread running from region to region, stretching uninterrupted through space and time. What natural theology could not explain, Darwin had started to realize, was biogeography.
Darwin knew that the basic premises of independent creation, permits, or even demands, the occurrence of identical or closely related species in similar but widely separated environments. We should, in the creation-based view of the pre-Victorian naturalist, find matching species on opposite sides of the world as long as the environments are sufficiently alike—similar plants and animals, for example, in the Arabian Desert and the Mojave Desert of North America or in the icy worlds of the Arctic and Antarctica. But this contrasted with the organic world that Darwin had begun to explore. Everywhere around the globe, taxa that were the most alike were almost always near each other geographically, regardless of changes in environment. As Darwin studied the diversity of life on South America, he discovered species being replaced by very similar ones as he traveled along the continent, even into different climates.3 Organic differences accrue with distance.
Darwin observed a similar pattern when he visited the Galápagos Archipelago and mused about this problem in his "Journal of Researches," now known today as The Voyage of the Beagle:
It is probable that the islands of the Cape de Verd group resemble, in all their physical conditions, far more closely the Galápagos Islands, than the latter physically resemble the coast of America, yet the aboriginal inhabitants of the two groups are totally unlike; those of the Cape de Verd Islands bearing the impress of Africa, as the inhabitants of the Galápagos Archipelago are stamped with that of America.4
Why, Darwin asked himself in his journal, would this be? Why were the taxa of Galápagos "created on American types of organization?"5
Darwin, at the time, had already been exposed to certain concepts employed in evolution. His creative grandfather Erasmus, who had died before his birth, had believed that all species were related, and the French scientist Jean-Baptiste Lamarck had argued that species were mutable, changing from one form to another. But neither had provided persuasive evidence for the view.
On his trip around the world, Darwin realized that the only way to answer the biogeographical questions with which he was confronted was not with natural theology but through a theory of evolution. Two species of the same genus that appear so much alike are related through a recent common ancestor, and this ties them to a common birthplace. The dominant distributional pattern of life on Earth, which suggests a general proportional relationship between taxonomic and geographical distance, is precisely what you would expect if all organisms were descended from the same ancestor. But the theological viewpoint, unconstrained by the unifying and co-locating processes of heredity, could not explain this distributional pattern. In On the Origin of Species, Darwin returned to the question of Galápagos and Cape Verde organisms and answered the problem he had stated many years earlier in his journal:
The inhabitants of the Cape de Verde Islands are related to those of Africa, like those of the Galápagos to America. I believe this grand fact can receive no sort of explanation on the ordinary view of independent creation; whereas on the view here maintained, it is obvious that the Galápagos Islands would be likely to receive colonists, whether by occasional means of transport or by formerly continuous land, from America; and the Cape de Verde Islands from Africa; and that such colonists would be liable to modifications; the principle of inheritance still betraying their original birthplace.6
While this observation and explanation may seem prosaic today, a strong expectation remains, even among many educated people, that certain types of habitats—swamps, deserts, rainforests, oceanic islands, frozen tundra—should usually contain the same types of plants and animals, no matter where they occur on the globe. For example, a well-known commercial, shown in America every year at Christmas, uses the magic of computer animation to depict a baby emperor penguin offering a drink to a polar bear cub. This fictional mingling of polar bears with emperor penguins is actually quite common in the American media, often appearing in cartoons, Christmas specials, billboards, or magazine advertisements. In reality, polar bears live within or near the Arctic Circle while emperor penguins live on the opposite side of the Earth in Antarctica, and the two species have never naturally interacted. Both polar bears and emperor penguins do boast superb adaptations that equip them for a harsh and freezing existence,7 but that does not mean they should appear everywhere icy. Polar bears are descended from the Northern Hemisphere brown bears, which include the North American grizzlies and Kodiak bears of the islands of Alaska. The domain of grizzlies extends so far north that grizzlies and polar bears have successfully mated in the wild, a union that produced an odd-looking bear, mostly white with brown patches, which fell victim to a hunter in the Canadian Arctic in 2006. Again, we confirm Darwin's observation of one species replacing an extremely similar species from an adjacent region.
Likewise, emperor penguins descend from a Southern Hemisphere penguin-like ancestor, and other kinds of penguins are widely distributed throughout the southern end of the world, with some occurring as far north as Galápagos. The king penguin, the closest relative of the emperor penguin, lives nearby on a number of surrounding Sub-Antarctic Islands. Penguin-like fossils have been found, as one would expect, in the same southern regions as their current distribution: Antarctica, New Zealand, Southern Australia, Southern South America, and South Africa. Given their moderate but not exceptional dispersal abilities and the tropical climates separating the poles, neither identical species of polar bears nor identical species of emperor penguins could occupy both the Arctic and Antarctica.
All moderate and poor dispersing taxa that have adapted to such severe high-latitude regions near the North or South Poles must have evolved from closely related organisms from the more temperate, lower latitudinal regions nearby. So Antarctica and the Arctic, given the current climatic barriers of the Earth, could not exclusively share the same species or even the two most closely related species of a genus unless its individuals were peculiarly proficient at long-distance travel.
Popular biogeographical mythology, like the fictional grouping of emperor penguins with polar bears, often springs from this same impulse to associate taxa with a certain kind of environment rather than a particular place. Even after I was familiar with the theory of evolution, I used to have similar distributional misconceptions regarding the Hawaiian Islands. Glimpsing pictures and videos of its rainforests and waterfalls, I had a hazy idea that the Hawaiian biota was vaguely Amazonian, peppered with an exotic and colorful assortment of lizards, frogs, and large branch-hugging snakes. Most of all, I suspected the islands were plagued with an intolerable number of mosquitoes and ants. I remember how surprised I was when I discovered that, in reality, Hawaii has no native ants, mosquitoes, frogs, lizards, or snakes at all. Hawaii, in fact, completely lacks all native terrestrial vertebrates – no amphibians, no reptiles, and no mammals except for one species of bat. Those few vertebrates or ants or mosquitoes that do appear on Hawaii today are the result of recent human introduction. This should not really have been surprising as the Hawaiian Islands formed in the middle of the Pacific, 2400 miles (3900 kilometers) from the nearest continent, which is far too distant to permit natural immigration of all but the most adept over-water travelers. Yet I still remember my astonishment that such a rich ecological system, so spectacularly diverse in its birds and flora, could be so poor in things that walk and crawl.
Darwin, as his writings make clear, suffered similar biogeographical expectations and was greatly amazed to find these expectations wanting. In his journal, Darwin noted that Galápagos strangely lacked frogs—as in fact, did most of the islands he had visited—even though island environments seemed peculiarly suited for the amphibian life. Darwin knew that on some islands where frogs had been introduced, they almost always flourished. So why wouldn't frogs naturally occur on islands? Once again, Darwin returned to a question he had asked himself in his journal and answered it in On the Origin of Species: all frogs descend from a common mainland fresh-water ancestor, and nearly all species die very quickly when immersed in salt water due to their water-permeable skin. Frogs do not appear on remote oceanic islands like Galápagos or Hawaii because they are particularly confined by large marine barriers, and the same is true, as Darwin wrote, for terrestrial mammals:
Why, it may be asked, has the supposed creative force produced bats and no other mammals on remote islands? On my view this question can easily be answered; for no terrestrial mammal can be transported across a wide space of sea, but bats can fly across.8
The types of plants and animals that Darwin found on islands – and those he did not find—made it clear that taxa were not merely placed there by an invisible hand.9 All island flora and fauna had to reach their oceanic residences under their own power, leaving the nearest continental region and their closest relatives behind. This is why all of the birds, insects, spiders, and plants that inhabit remote ocean islands exhibit certain traits that make them particularly amenable to long-distance overseas travel. Palm trees so frequently adorn islands because coconuts float and are extremely resistant to seawater. Wind carries the dust-like spores of ferns into the remotest interiors of the oceans. Long-jawed, orb-weaving spiders, which have the extraordinary distinction of occurring on every habitable land mass, can take to the air, gliding for great distances on their slender silk strands, drifting like plankton of the skies.10 Given enough time, those organisms well-suited for such trans-marine jaunts eventually reach newly forming islands and successfully colonize them. Volcanic rocks that sprout from beneath the waves, at first lifeless and forbidding, soon begin to green and buzz and chirp. While these Magellans of the organic world continue to populate oceanic islands, those not disposed to transport by wings, wind, or waves must remain confined to the continents.
Consider, now, how compelling—how informative—these facts of distribution would be to someone trying to decide between the natural theology of pre-Victorian biologists and some other, less supernatural explanation for the origin of species. Island distributions led Darwin to realize that the extraordinary species of the Galápagos Islands—the bizarre and "hideous" marine iguanas, the giant tortoises, the peculiar finches—must be the result of a certain number of individuals reaching the islands from nearby regions like Central or South America or the Caribbean. He also knew that these Galápagan species all possessed unusual adaptations that differentiated them from the American type. Clearly, something had happened to the American species after they had reached the islands that had transformed them into the unique and curious forms observed on Galápagos today.
Moreover, each of these Galápagan groups comprised multiple species, like the thirteen species of finches or the two groups of iguanas—both land and marine. Why did none of these species also appear in South America? Was it conceivable that individuals from all thirteen species of finches managed to reach these isolated islands from South America due to different random events, miraculously keeping all members of these similar groups in proximity? Was it reasonable to believe that after the thirteen similar groups of ancestors reached Galápagos from South America, all the other members of their species then died out on the continent? Certainly, the only reasonable way to explain this distribution would be if individuals from just one American finch species colonized the islands, and the thirteen resident species are all descendants of this first and only group of finch pioneers. Species, Darwin had started to realize even before he had finished his voyage, must have evolved.
Unfortunately, such powerful biogeographical evidence for evolution rarely appears in textbooks or renewed debates over Creationism, now repackaged and stamped with the label, "intelligent design." This is particularly troubling given how much more powerful and explanatory the theory of evolution becomes within a biogeographical framework. While, like most people, I had grasped the basic principles of evolution in middle-school biology, I never had a truly profound and complete view of how evolutionary processes operate in the real world until I also understood it biogeographically—until, that is, I had a glimpse of the rich, global picture that impelled Darwin toward his discovery. In my view, biology teachers first describing the theory of evolution should place such evidence at the top of the syllabus, for once a student understands why there is no such thing as a Tahitian raccoon or why Koala fossils can only occur in Australia or why Arctic foxes cannot simultaneously inhabit both polar regions, he or she may begin to sense the full power of the theory of evolution.
The British geneticist J.B.S. Haldane once famously said that he would give up the theory of evolution if a fossil rabbit were found in the Precambrian. Haldane's quote shows, in an understated way, just how constraining the predictions of the evolutionary view really are. The theory requires the general emergence of organic complexity from simpler systems in a relatively predictable way—single-celled organisms, multi-cellular creatures, then aquatic vertebrates, all followed by the first lungfish-like creatures that crawled onto land. This, in turn, had to precede the appearance of primitive four-legged terrestrial vertebrates and their eventual division into amphibians, reptiles, and mammals. Only after the first appearance of mammals could we expect to see the emergence of primitive fossil rabbit forms and finally modern rabbits. That the chronological order of fossils, implied by stratigraphic layering and reconfirmed by radiometric dating, should so precisely match the chronological order necessitated by evolution can be no coincidence.
Just as evolutionary processes require a continuous genetic flow through time, with no inexplicable disjunctions, those same processes also require a continuous genetic flow through space. For a botanical metaphor that may help enliven this perspective, we may imagine, instead of the tree of life drawn alone on a blank page, an intricate system of creeping ivy steadily extending and branching over the surface of the globe, tracing with its myriad stems the divergent lines of descent through continents and oceans. Two stems of this unifying global ivy, corresponding to a couple of primitive marsupial lines, would extend into Australia and divide into nearly 200 branches representing kangaroos, koalas, bandicoots, and all the pouched animals down under. In South America, we could trace a lone stem for Darwin's finches running from the coast into the Galápagos Archipelago and there splitting into thirteen branches that extend among the different islands. As all the stems and branches of this symbolic plant must remain whole and return to the same root, we can get an idea of what is and is not biogeographically feasible, helping provide distributional analogues to that impossible Precambrian rabbit.
To cite a few examples, biogeographers could say, with similar Haldane-like boldness, that we would give up the theory of evolution if anyone discovered giraffes in Hawaii or fossil kangaroos in Spain or another group of Galápagan iguanas in Lithuania, each of which would represent an unacceptable spatial disjunction – a rend in the evolutionary creeping ivy that would be every bit as problematic as the temporal disjunction described by Haldane. As with the chronological order of fossils, the continuous genetic flow of evolution so constrains the possible locations of plants and animals and allows so many opportunities for falsification that we may disregard any theory that cannot explain why organic distributions, all over the globe, should so persistently conform to such a faultless hereditary structure. The fact is that no theory other than evolution can describe how the creeping ivy of organic descent could so envelop the planet yet always remain whole.11
The Autobiography of Charles Darwin, ed. by Nora Barlow, Barnes and Noble Publishing Inc., (2005, originally published in 1887) p. 18. All quotes from Darwin's autobiography come from this edition.
Darwin would write in his autobiography that he expected it would surprise many people to discover how religious he had been, having remained "orthodox" even during the first years of his voyage. As Darwin would later write: "Whilst on board the Beagle I was quite orthodox, and I remember being heartily laughed at by several of the officers (though themselves orthodox) for quoting the Bible as an unanswerable authority on some point of morality" (Autobiography, p. 66). Still, even as a young man, Darwin had met with other views that almost certainly made him more amenable to the unconventional discoveries that awaited. He came from a family of free thinkers, and his grandfather, Erasmus, had suggested in his writings that species may have come from simpler forms. Darwin also was familiar with Larmarck's ideas on the mutability of species, and he brought on his voyage a copy of Lyell's Principles of Geology, which described important evidence for the ancient history of the Earth.
Fortunately for scientific historians, there is no need to speculate on the observations that motivated Darwin because no scientist has ever been more detailed or forthcoming—in journal entries, in private letters, in his autobiography—about the conceptual development of an idea. We can follow the emergence of the theory of evolution from its roots in his "Journal of Researches," kept during his five-year trip around the world and usually referred to today as The Voyage of the Beagle. Or we can retrace it through his autobiographical reminiscences, written near the end of his life, about his progression from a young natural theologian, likely destined for a quiet existence at a county parish, into the most controversial scientist of the Victorian era. It would not be overreaching to say that we know more about the conceptual evolution of evolution—about the origin of Origin—than we do about the development of any other scientific viewpoint. And we know from Darwin's own statements on the matter precisely what facts compelled him, what observations confirmed in his mind the vast interconnectedness of life. As Darwin makes clear, time and again, it was biogeography, above all else, that led him to the theory of evolution.
In his autobiography, Darwin details some of those observations:
During the voyage of the Beagle I had been deeply impressed by discovering in the Pampean formation great fossil animals covered with armour like that on the existing armadillos; secondly, by the manner in which closely allied animals replace one another in proceeding southwards over the Continent; and thirdly by the South American character of most of the productions of the Galápagos archipelago, and more especially by the manner in which they differ slightly on each island of the group; none of these islands appearing to be very ancient in a geological sense.
It was evident that facts such as these, as well as many others, could be explained on the supposition that species gradually become modified; and the subject haunted me. (p. 44.)
Charles Darwin, The Voyage of the Beagle, in From So Simple a Beginning; The Four Great Books by Charles Darwin, edited by E.O. Wilson. W.W. Norton & Company, New York, 2006, p. 341. All quotes from The Voyage of the Beagle and On the Origin of Species will come from this edition.
The Voyage of the Beagle, p. 341.
On the Origin of Species, pp. 702-3.
For example, the hairs of a polar bear's white fur are hollow and filled with air providing both insulation and buoyancy. It is a common, though false, scientific legend that the hollow hairs act like fiber optic wires and reflect sunlight toward their black skin, which then absorbs it. Instead, the polar bear hair is peculiarly well suited to keep heat from radiating outward – not for trying to transmit it inward. Beneath the skin is yet another layer of insulation, thick layers of fat.
On the Origin of Species, 700.
A few Creationists of today try to explain island distributions by resorting to Noah's Ark. Such animals could not reach these islands after the flood. But the tale of the flood leads to even more biogeographical problems. Imagine each one of the 13 different species of Darwin's finches flying from the Ark through Eurasia into North America, then into South America, and then to Galápagos, without any individuals of the group ever ending up anywhere else. Now imagine the same problem with all 140 species of Australian marsupials—from the marsupial mouse to the Kangaroo.
The distribution of these spiders extends to so many remote regions it has led to what may be the only known biogeographical joke: As Neal Armstrong once said, "One small step for man..., hey, is that a long-jawed orb-weaver?"
This is an adaptation of a chapter from my “Here Be Dragons: How the Study of Animal and Plant Distributions Revolutionized Our Views of Life and Earth.”
By Huxley this is an excellent post, Dennis.
Darwin’s epiphany — as important historically as Saul’s conversion on the road to Damascus — forced him to confront the limits of his natural theology, but as a true man of science, he let the evidence lead him rather than his preconceptions.
As a result, the incredible body of biological knowledge that has developed truly demonstrates, as @richarddawkins wrote so eloquently, one crucial aspect of the Magic of Reality.
If only the cdesign proponentsists would adopt a modicum of intellectual integrity, they too could experience that *real* magic and replace their Stone Age beliefs with ones that have tangible value (medical, ecological, etc.); alas, societal progress will come on this, as it does in physics, one funeral at a time.
Be well, and keep sharing that Magic of Reality!
Great post, thanks