American Scientist

Two centuries before Charles Darwin wrote On the Origin of Species, Bishop James Ussher calculated the age of the Earth. To do so, the Primate of All Ireland (time has given his title a certain irony) carefully mined the Old and New Testaments for genealogical information that might lead him back to the date of Creation. In so doing, he concluded that the Earth was only about 5,600 years old. It’s easy to ridicule the bishop and his date, but to do so misses a larger point. Ussher’s approach was rigorous—even elegant—and he seemed to understand that the Earth’s age could only be inferred through careful observation of evidence. What doomed his result—and, indeed, many a conclusion in science—were faulty assumptions. The bishop, who took as an axiom that the biblical account of Creation was literal, painstakingly did the math attendant on the notorious biblical begats. He then dutifully added to his tally the five days that separated the creation of the Earth from the creation of human beings, and arrived at the exact date of Creation: October 23, 4004 B.C. Of course, he was wrong.

A Subtle Blow

By the 18th century, scientists in various disciplines had used diverse approaches to calculate the planet’s age and reached different conclusions. As they did, the Earth got older. Still, by the mid-19th century, when Darwin was writing the Origin, the age of the planet was a contentious issue. Although the 4004 B.C. date had fallen from favor, estimates still varied wildly and tended to err on the side of youth.

What Darwin realized was that a youthful Earth was appealing not only because it adhered to the biblical time line, but also because it was simply easier to imagine. He knew that his own argument for natural selection depended on vast conceptions of time, and he also understood that the time spans required would be nearly impossible to comprehend. In a section of the Origin entitled “On the Lapse of Time,” he wrote:

It is hardly possible for me even to recall to the reader, who may not be a practical geologist, the facts leading the mind feebly to comprehend the lapse of time. He who can read Sir Charles Lyell’s grand work on the Principles of Geology … yet does not admit how incomprehensibly vast have been the past periods of time, may at once close this volume.

Darwin feared that his readers would be unable to understand the deep time over which natural selection acts, and that their failure would be problematic for his argument. Those with limited imaginations might as well put away his book at once.

From the time he began writing in his notebooks after returning from his voyage on the HMS Beagle, Darwin knew that his was a radical theory, time spans aside. In 1837, 22 years before the publication of the Origin, he wrote “cuidado” (caution) in his journal as he thought about the implications of his ideas. As many have noted before, in the new Darwinian reality, human beings were still a unique species—but no more so than any other species, living or extinct. Gone forever was the notion that we were special, that our origins were unlike those of any other organism on Earth. Darwin carefully avoided mentioning human beings in the Origin, writing, in a masterpiece of understatement buried on page 488 of the first edition: “Much light will be thrown on the origin of man and his history.” Indeed.

Still, when I recently reread the Origin, I was struck by a subtler blow the book delivers to human hubris. The Origin remains, even in the 21st century, a radical work, which argues that the fundamental forces driving life on this planet occur on timescales that render the span of a human life insignificant. Furthermore, although the effects of natural selection are there for all to see, its daily operation is almost completely hidden from view. Both our life spans and our five senses are inadequate to the task of comprehension: The most powerful mechanism of organic change lies well beyond our everyday experience.

The Dangers of Extrapolation

By Darwin’s time, 19th-century geologists and paleontologists were well aware of the fossil record—and they knew that fossils were old. Naturalists recognized the characteristic appearance and extinction of faunas in the geologic record; by the mid-1800s, they had named the periods and even established their order of succession. But the implications of the fossil record for the age of the Earth remained controversial. One group of scholars constructed increasingly elaborate theories of successive catastrophes that reconciled the fossil record with a young Earth. Lyell and Darwin belonged to another camp, pioneered by James Hutton, which argued that the fossil history was evidence of a far more ancient Earth. As Hutton put it in a 1785 paper presented to the Royal Society of Edinburgh, Earth’s geology revealed “no vestige of a beginning, no prospect of an end.”

The matter was far from settled. Indeed, the assignment of accurate absolute dates to the geologic timescale would have to wait for the discovery of radioactive decay at the turn of the 20th century. That’s not to say no one tried; clever calculations were certainly attempted during Darwin’s lifetime. Most notably, the influential physicist William Thomson—better known as Lord Kelvin—argued for the power of physical approaches to the question of time spans. He used the rate of cooling of an originally molten Earth to conclude that our planet was likely 100 million years old.

Here again, as with Bishop Ussher, the flaw was not in the method, but in its underlying assumptions. Kelvin knew nothing about radioactivity and the constant heat it supplied. His model of a cooling Earth failed to account for new heat contributed by radioactive decay. Nor did he consider the transfer of heat from Earth’s deep interior to its crust by convection. These omissions led to a serious underestimate of the planet’s age.

At the same time that Kelvin was making the case for a 100-million-year-old Earth, geologists championed a completely different approach. Their argument depended on careful observation of the speed of deposition and erosion in contemporary landscapes. Geologists in the 19th century understood that these processes, writ large, could carve canyons, build up or erase thick sedimentary layers and reshape topography. By looking at these large-scale phenomena in terms of slow—but measurable—current rates of deposition and erosion, they could estimate how long those processes had been at work to generate contemporary geologic features. This approach, sometimes called uniformitarianism, would lead to significantly greater estimates of the age of the Earth.

In the first edition of the Origin, Darwin employed this method to calculate the age of the Weald, a sandstone formation in Southern England. This feature, once covered by a layer of chalk (still visible in nearby formations), had been completely denuded. Based on estimates of the rate of erosion, Darwin calculated that it would have taken at least 300 million years to remove all of the sediments covering the Weald. This estimate was three times greater than even the most generous prevailing notions of the age of the Earth. Kelvin was unimpressed with both the method and the result that Darwin obtained. And, as it turns out, Darwin was wrong, too. The Weald, in an unusual turn, was younger than he thought. Shortly after its publication in 1859, Darwin acknowledged that his estimate was likely incorrect, and the episode continued to disturb him. In an 1860 letter to his friend Asa Gray, he complained that “in fact geologists have no means of gauging the infinitude of past time.” A few months later, he wrote to his friend Lyell about the dangers of extrapolation:

Having burnt my own fingers so consumedly with the Wealden, I am fearful for you.… for Heaven-sake take care of your fingers; to burn them severely, as I have done, is very unpleasant.

By the third edition of the Origin, published in 1861, the discussion of the age of the Weald was entirely gone. Telling time was proving trickier than Darwin had thought.

Slipping the Bonds

Despite his misstep with the Weald, Darwin was profoundly influenced by his training in geology and his deep friendships with many of the leading geologists of his time. Natural selection, in effect, was a force directly analogous to erosion and deposition. Like erosion, it was capable of destruction, eliminating the less fit in the struggle for existence. And, like sedimentation, natural selection could also act as a creative force, building new forms through the slow accretion of imperceptible changes.

But if geology was slow, natural selection was even slower. Lyell, in a metaphor that Darwin later admired as “capital,” made the point in his book Principles of Geology:

In a word, the movement of the inorganic world is obvious and palpable, and might be likened to the minute-hand of a clock, the progress of which can be seen and heard, whereas the fluctuations of the living creation are nearly invisible, and resemble the motion of the hour-hand of the time-piece.

Natural selection needed time, and lots of it—time immeasurable in human life spans.

Aware of the challenge involved in communicating such a vast expanse of time, Darwin began the Origin in a beautifully calculating way. The first chapter is a compendium of examples of artificial selection brought about over a few generations of selective breeding. These reassuring instances demonstrate selection at the human scale, with human agency as the driving force. Many of Darwin’s readers, country gentlemen of means, would have been intimately familiar with the breeds he discussed. An avid pigeon fancier himself (he boasted of belonging to two London pigeon clubs), Darwin marshalled the English carrier and the short-faced tumbler, the runt and the barb, the turbit, the pouter and the fantail—all the while alluding to the power of pigeon fanciers to generate these breeds from a single rock-pigeon ancestor. Easing slowly into the scope of time, Darwin reassured his readers that these many breeds had been brought about in the course of human history.

This first chapter, “Variation under Domestication,” had, as its name suggests, an explicit agenda. In it, Darwin established the importance and ubiquity of variation. By emphasizing the consequences of artificial selection, he struck an important first blow against the purported immutability of species. And by alluding to the common ancestry of so many radically different contemporary variations, he set the cornerstone of the argument for descent with modification. But behind these explicit goals, this chapter also bolstered readers’ confidence in their ability to understand vast stretches of time and to apprehend nature through their senses. Darwin wrote, for example, that:

the earliest known record of pigeons is in the fifth Ægyptian dynasty, about 3000 B.C. … but Mr. Birch informs me that pigeons are given in a bill of fare in the previous dynasty.

He thus anchored his argument in the realm of human history. When he spoke of how “King Charles’s spaniel has been unconsciously modified to a large extent since the time of that monarch,” he reassured his readers that the forces at play are within the scope of human experience. He knew, I believe, that if we could understand his examples—which he explained with extraordinary care—then we might be ready for a glimpse of deep time.

By offering that glimpse, the Origin ushers us into the world of contemporary science. It asks us to acknowledge the riotous diversity of living forms and then accept that they are the result of material forces we cannot directly observe, acting over time spans we cannot imagine. It reminds us that chance operates at scales both large and small, and that selection sifts inexorably through life’s variety. We live our lives in three dimensions for our threescore and ten allotted years. Yet every branch of contemporary science, from statistics to cosmology, alludes to processes that operate on scales outside of human experience: the millisecond and the nanometer, the eon and the light-year. For some, this is cause, if not for despair, then for strong suspicion and perhaps even rejection of the scientific enterprise. But there is grandeur in the notion that we are part of nature, and not above it. And there is comfort in the realization that, although our senses and brains constrain us, we can still understand phenomena far beyond those we can witness. In the end, it may be this ability to slip the bonds of our own experience—those of time, those of our senses—that makes us human.

So it’s worth taking a moment to contemplate what we now know: The Earth is 4,540,000,000 years old. Life has been around for at least 3,500,000,000 of those years.