Deep Time Discovered

Putting Dates to the History of Life

Geologists have their own mecca, a stretch of the Acasta River deep in the Northwest Territories in Canada. You can get there only by canoeing for days up the river or by flying a float plane north from the town of Yellowknife, over an expanse of half land, half water. The water takes the form of thousands of lakes and ponds, some strung together into blobby rivers, in every shape that Ice Age glaciers could possibly carve. You ski to a landing near a spindly island in the middle of the river. The shore is covered with black spruce, reindeer moss, heather, and lichens. Chirping plovers cut the silence, and blackflies and mosquitoes drill your skin.

A wall of exposed rock tumbles down to the water, and you can clamber down among the boulders. The rocks here are granite, dark gray hunks flecked with bits of feldspar that look pretty much like any other piece of granite you may have encountered. They are exceptional in only one way: some of them are more than 4 billion years old, which makes them the oldest known rocks on Earth. From our planet’s infancy the minerals that made them up have held together, as continents have been torn apart and fused back together.

Their age is so vast that it’s almost impossible to comprehend. Think of a year as equaling the length of your outstretched arms. To equal the age of the Acasta rocks, you’d have to hold hands in a line of people circling the Earth 200 times. But as hard as it may be to imagine, it is a picture that would have made Darwin very happy.

When Darwin first proposed his theory of evolution, he could not have known the great age of the Acasta rocks. The physics required to determine their antiquity was still 50 years away. Darwin suspected that the world was spectacularly old, which certainly would agree well with his theory of gradual, generation‑upon‑generation evolution. But it was only during the twentieth century that paleontologists and geologists precisely mapped the terra incognita of Earth’s history. They discovered a way to establish not just the order in which new life‑forms appeared on Earth, but their actual dates in history, from the earliest signs of life more than 3.85 billion years ago, to the earliest animals 600 million years ago, to the first members of our own species 150,000 years ago.

Too Warm to be Old

Of all the objections that were raised against Darwin, be they religious, biological, or geological, one of the most troubling to him was over the age of the planet. It came not from a bishop, a biologist, or a geologist, but from an unexpected source: a physicist.

William Thomson (better known as Lord Kelvin) was one of the world’s leading physicists when Darwin first published Origin of Species. For Kelvin, the universe was a swirl of energy, electricity, and heat. He demonstrated how electricity acted like a fluid, just like water. He also showed how entropy dominates the universe: everything goes from order to disorder unless it receives energy to keep it organized. Burn a candle down to its stump, and the soot, gases, and heat that it releases will never spontaneously join back together into a candle.

As esoteric as Kelvin’s work might seem, it made him rich when he applied it to designing the transatlantic cables that joined Europe to North America by telegraph. And sometimes, when Kelvin was bobbing on a cable ship in the middle of the Atlantic, he would think about how old Earth was. Kelvin was a devout man, but he didn’t accept that Earth was a few thousand years old simply because someone decided that the Bible said so. He thought that it should be possible instead to put an upper bound on the age of the Earth scientifically, by studying its heat.

Miners, Kelvin knew, found that the deeper they dug, the hotter the rocks became. To account for this heat, Kelvin speculated that Earth formed from the collision of miniature failed planets, and the energy of their impact created a molten blob (a speculation that later proved to be right). Kelvin assumed that once the impacts ended, there would be no way for the planet to receive any new heat, so it would gradually cool like a dying ember. Its surface cooled off the fastest, while its interior remained warm to this day. Only at some point in the distant future would Earth’s core become as cold as its surface.

Kelvin and other physicists had worked out equations to accurately predict how objects cool, and he applied them to the entire planet. By measuring how fast heat escaped from rocks, and how hot the deepest mine shafts became, he came up with an estimate for the age of the planet. He concluded in 1862 that Earth had been cooling no more than 100 million years.

Kelvin’s original motive had been to show how sloppy geology was compared to physics. But after reading Origin of Species, he was happy to use his results to attack Darwin. Steeped as Darwin was in Lyell’s ancient geology, he assumed the gradual changes of natural selection could take as long as they needed to alter life. But Kelvin’s results wouldn’t allow him enough time. Kelvin himself wasn’t a rabid anti‑evolutionist–for all he knew, all of life might have started as a germ–but he saw life today as evidence of design, of the handiwork of God. He used his estimate for Earth’s age to cut Darwin down with a single scimitar slice.

Huxley tried to defend Darwin by making a compromise–something Huxley rarely did with critics. He said that biologists had to accept an age for Earth that geologists and physicists decided on and figure out how evolution could work in that span of time. If Earth was only 100 million years old, then evolution must be able to work at high speed. Wallace went further, suggesting that at times evolution could work far faster than it does today. As Earth wobbles on its axis, the planet might experience harsh climates that would make evolution run at high speed.

Darwin wasn’t satisfied. “I am greatly troubled at the short duration of the world according to Sir W. Thomson,” he wrote in a letter. Kelvin, meanwhile, was getting new reports on the planet’s temperature, and kept revising his estimate–each time shortening Earth’s life span. By the time he was done, he had winnowed it down to only 20 million years. All the while, Darwin could do nothing but grit his teeth. As he struggled to flesh out his theory of evolution, “then comes Sir W. Thomson like an odious specter.”