Friday, July 6, 2018

A Beautiful Scientific Mind: Charles Darwin and His Fossils

I recently skimmed through Adrian Lister’s book Darwin’s Fossils: The Collection that Shaped the Theory of Evolution (Smithsonian Books, 2018). I am not aware that any previous book has gone through all of the specimens that Darwin collected while traveling around the world on H.M.S. Beagle in 1832-1835. Nearly every science-literate person has heard about the finches Darwin saw on the Galápagos Islands, and how these observations eventually led him to think of natural selection. But what about the fossils that he collected and sent back to England?

Previous authors such as Niles Eldredge have noted that Darwin’s fossils included the bones of numerous large extinct mammals from South America, and that it was from this that Darwin concluded what could be called the succession of forms. That is, in the past, the kinds of fossilized mammals lived in the same locations that they are currently found. Glyptodonts (giant armadillos) lived in South America; armadillos lived there today. Giant llamas lived there in the past; llamas live there today. Giant sloths lived there in the past; sloths live there today. Darwin concluded, and scientists widely agreed, that when species become extinct, they are succeeded by similar species in the same location. Today, we can hardly force ourselves to avoid saying that modern species live in the same places that their ancestors lived. But “ancestors” means “evolutionary ancestors” and this is the very thing that Darwin eventually concluded from his observations. But he had to gather the evidence first.

Darwin collected a lot of other fossils as well. He collected petrified wood. He found a petrified forest in Patagonia. He noted that the bases of the trunks were inclined away from the Andes, and so were the sedimentary layers in the rocks, which implied that the Andes had been pushed up out of the Earth since those sedimentary layers had been formed. He also found a few carbonized leaves such as those that can today be found near Clarkia, Idaho, about which I wrote in 2014.

Darwin also found fossilized seashells far above the high tide line, some of them even high in the Andes. The conclusion that was obvious to him, as to us, is that land that was once below the sea has risen. But Darwin was not satisfied with drawing the obvious conclusion. He wanted to eliminate other possible explanations, and to do so before his critics might attempt it in print. Suppose, for example, that the fossilized seashells above high tide were not actually fossils, but were modern middens? That is, what if fishermen hauled seashells up onto land (which they did in fact do) and that what Darwin was seeing was just a pile of leftover shells? Darwin observed midden heaps that were produced by fishermen, and found that the shells were in piles, while the putative fossil shells were individually spaced out, not in piles. In fact, when Darwin asked local fishermen if the putative fossils could have put the shells there, they laughed at the idea.

Darwin suspected that the fossil seashells far above high tide were not deposited in the places where they had lived but had been dead at the time the waves deposited them on an ancient beach. He needed evidence for this. He saw that some of the shells had dead barnacles on the inner surface, which means the shells had been dead for a while before being buried in sediments. If Darwin had not looked for this evidence, he might not later have been able to distinguish fossil shells from an ancient seashore vs. fossil shells from an ancient shell bed.

Darwin also thought that earthquakes caused the successive stepwise uplift of land in South America. But he wanted evidence for this. He got it. In 1835, he witnessed a severe earthquake in Concepción on the Pacific coast of Chile. This one earthquake lifted the land surface at least eight feet. Such earthquakes happen about every century; these earthquakes could easily have produced the Andes. If Darwin had not seen one of these earthquakes for himself, he would have had to depend on second-hand accounts. In fact, an earthquake at least as strong as the 1835 quake, in almost exactly the same place, occurred in 2010.

My point is that while he was on his voyage, Darwin was not just collecting fossils but testing scientific hypotheses about them while he was there rather than wishing he had done so when he got back home. Until I looked through Lister’s book, I had not known this.

Lister’s book also told the story of Darwin’s discovery that coral atolls formed as volcanoes subsided in the ocean floor. This would explain not only their roughly circular shape but also why the atolls were not perfectly circular, like volcanoes, and why they were much larger than volcanoes. The new corals built their reefs on top of old reefs, each new reef being in a slightly different location from the old reef. (A delightful story: When prominent geologist Charles Lyell read Darwin’s theory of coral reefs, he danced around with wild contortions. I’d always thought Lyell was kind of a stick-in-the-mud but I was wrong.) Darwin could not actually prove that there were volcanoes at the bases of coral atolls, however. This was first done in 1952 when geologists drilled a hole down into a coral reef, piercing through thousands of feet of coral limestone, until they hit volcanic rock.

I would not recommend buying this book, unless you are a paleontologist, because it is heavy reading for the rest of us. But it is well illustrated and taught me some new things that, even though I have read dozens of books about Darwin, I’d never known. Get it from your library.

Darwin had a beautiful mind. He was always questioning everything, including his own assumptions. This made him a happy man, because he noticed so many beautiful details of the natural world that he might otherwise have overlooked. The natural world is full of surprises, but only to the person who looks closely and thinks carefully.

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