Why Evolution Is True

by Jerry A. Coyne

You can find religions without creationism, but you never find creationism without religion.

And—the ultimate irony—creationism has even established a foothold on the Galapagos archipelago. There, on the very land that symbolizes evolution, the iconic islands that inspired Darwin, a Seventh-day Adventist school dispenses undiluted creationist biology to children of all faiths.

FIGURE 2. A phylogeny (evolutionary tree) of vertebrates, showing how evolution produces a hierarchical grouping of features, and thus of species containing these features. The dots indicate where on the tree each trait arose.

Natural selection is not a master engineer, but a tinkerer. It doesn’t produce the absolute perfection achievable by a designer starting from scratch, but merely the best it can do with what it has to work with.

Taking into account all of these requirements, it’s clear that the fossil record must be incomplete. How incomplete? The total number of species that ever lived on earth has been estimated to range between 17 million (probably a drastic underestimate given that at least 10 million species are alive today) and 4 billion. Since we have discovered around 250,000 different fossil species, we can estimate that we have fossil evidence of only 0.1 percent to 1 percent of all species—hardly a good sample of the history of life! Many amazing creatures must have existed that are forever lost to us. Nevertheless, we have enough fossils to give us a good idea of how evolution proceeded, and to discern how major groups split off from one another.

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The first organisms, simple photosynthetic bacteria, appear in sediments about 3.5 billion years old, only about a billion years after the planet was formed. These single cells were all that occupied the earth for the next 2 billion years, after which we see the first simple “eukaryotes”: organisms having true cells with nuclei and chromosomes. Then, around 600 million years ago, a whole gamut of relatively simple but multicelled organisms arise, including worms, jellyfish, and sponges. These groups diversify over the next several million years, with terrestrial plants and tetrapods (four-legged animals, the earliest of which were lobe-finned fish) appearing about 400 million years ago. Earlier groups, of course, often persisted: photosynthetic bacteria, sponges, and worms appear in the early fossil record, and are still with us. Fifty million years later we find the first true amphibians, and after another 50 million years reptiles come along. The first mammals show up around 250 million years ago (arising, as predicted, from reptilian ancestors), and the first birds, also descended from reptiles, show up 50 million years later. After the earliest mammals appear, they, along with insects and land plants, become ever more diverse, and as we approach the shallowest rocks, the fossils increasingly come to resemble living species. Humans are newcomers on the scene—our lineage branches off from that of other primates only about 7 million years ago, the merest sliver of evoluti...

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In medieval Europe, before there was paper, manuscripts were made by writing on parchment and vellum, thin sheets of dried animal skin. Because these were hard to produce, many medieval writers simply reused earlier texts by scraping off the old words and writing on the newly cleaned pages. These recycled manuscripts are called palimpsests, from the Greek palimpsestos, meaning “scraped again.”

Like these ancient texts, organisms are palimpsests of history-evolutionary history. Within the bodies of animals and plants lie clues to their ancestry, clues that are testimony to evolution. And they are many.

But these minor benefits are surely outweighed by the severe problems that come with the human appendix. Its narrowness makes it easily clogged, which can lead to its infection and inflammation, otherwise known as appendicitis. If not treated, a ruptured appendix can kill you. You have about one chance in fifteen of getting appendicitis in your lifetime. Fortunately, thanks to the evolutionarily recent practice of surgery, the chance of dying when you get appendicitis is only 1 percent. But before doctors began to remove inflamed appendixes in the late nineteenth century, mortality may have exceeded 20 percent. In other words, before the days of surgical removal, more than one person in a hundred died of appendicitis. That’s pretty strong natural selection.

As one group evolves from another, it often adds its development program on top of the old one. Noting this principle, Ernst Haeckel, a German evolutionist and Darwin’s contemporary, formulated a “biogenetic law” in 1866, famously summarized as “Ontogeny recapitulates phylogeny.” This means that the development of an organism simply replays its evolutionary history. But this notion is true in only a limited sense. Embryonic stages don’t look like the adult forms of their ancestors, as Haeckel claimed, but like the embryonic forms of ancestors. Human fetuses, for example, never resemble adult fish or reptiles, but in certain ways they do resemble embryonic fish and reptiles. Also, the recapitulation is neither strict nor inevitable: not every feature of an ancestor’s embryo appears in its descendants, nor do all stages of development unfold in a strict evolutionary order. Further, some species, like plants, have dispensed with nearly all traces of their ancestry during development. Haeckel’s law has fallen into disrepute not only because it wasn’t strictly true, but also because Haeckel was accused, largely unjustly, of fudging some drawings of early embryos to make them look more similar than they really are.19 Yet we shouldn’t throw out the baby with the bathwater. Embryos still show a form of recapitulation: features that arose earlier in evolution often appear earlier in development. And this makes sense only if species have an evolutionary history.

The larynx and its nerve remained connected during this process, but the sixth aortic arch on the left side of the body moved down into the chest to become a nonfunctional remnant, the ligamentum arteriosum.

During our evolution, the blood vessel from the fifth arch disappeared, and the vessels from the fourth and sixth arches moved downward into the future torso so that they could become the aorta and a ligament connecting the aorta to the pulmonary artery. But the laryngeal nerve, still behind the sixth arch, had to remain connected to the embryonic structures that become the larynx, structures that remained near the brain. As the future aorta evolved backward toward the heart, the laryngeal nerve was forced to evolve backward along with it.

And would an intelligent designer have created the small gap between the human ovary and Fallopian tube, so that an egg must cross this gap before it can travel through the tube and implant in the uterus? Occasionally a fertilized egg doesn’t make the leap successfully and implants in the abdomen. This produces an “abdominal pregnancy,” almost invariably fatal to the baby and, without surgery, to the mother. The gap is a remnant of our fish and reptilian ancestors, who shed eggs directly from the ovary to the outside of their bodies. The Fallopian tube is an imperfect connection because it evolved later as an add-on in mammals.20

REALIZING THAT THE DISTRIBUTION of species on islands provides conclusive proof of evolution was one of the greatest pieces of sleuthing in the history of biology.

As the zoologist George Gaylord Simpson remarked, “Any event that is not absolutely impossible ... becomes probable if enough time passes.”

Richard Dawkins provided the most concise definition of natural selection: it is “the non-random survival of random variants.”

The non random survival of randomly varying replicators.

And so, especially in small populations, the proportion of different alleles can change over time entirely by chance. And new mutations may enter the fray and themselves rise or fall in frequency due to this random sampling. Eventually the resulting “random walk” can even cause genes to become fixed in the population (that is, rise to 100 percent frequency) or, alternatively, get completely lost. Such random change in the frequency of genes over time is called genetic drift. It is a legitimate type of evolution, since it involves changes in the frequencies of alleles over time, but it doesn’t arise from natural selection.

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FIGURE 24. Fifteen hominin species, the periods over which they occur as fossils, and the nature of their brain, teeth, and locomotion. Fossils designated by open boxes are too fragmentary to draw conclusions about locomotion and brain size.

But around 28,000 years ago, the Neanderthal fossils vanish. When I was a student, I was taught that they simply evolved into modern humans. This idea now seems incorrect. What really happened to them is arguably the biggest unknown about human evolution. Their disappearance may have been associated with the spread of another form originating in Africa: Homo sapiens.

After sleeping through a hundred million centuries we have finally opened our eyes on a sumptuous planet, sparkling with color, bountiful with life. Within decades we must close our eyes again. Isn’t it a noble, an enlightened way of spending our brief time in the sun, to work at understanding the universe and how we have come to wake up in it? This is how I answer when I am asked—as I am surprisingly often—why I bother to get up in the mornings. -Richard Dawkins

But there is something even more wondrous. We are the one creature to whom natural selection has bequeathed a brain complex enough to comprehend the laws that govern the universe. And we should be proud that we are the only species that has figured out how we came to be.