Adventure classics – Billions of tiny architects of the reefs

Voyage of HMS Beagle

by Charles Darwin

***

One of the questions on Charles Darwin’s mind even before the Royal Navy’s survey ship, HMS Beagle, began its multi-year, round the world mission in 1831 was the origin of coral reefs, the only geological features formed by animals. Exactly how coral polyps by the billions formed their structures was a question of importance both for geological theorists and for the British naval strategists.

Among the instructions issued for the Beagle’s voyage was that “the circularly-formed Coral Islands in the Pacific occasionally afford excellent land-locked harbours. . . and would be well adapted to any nice astronomical observations which might require to be carried on in undisturbed tranquility. While these are quietly proceeding. . . a very interesting inquiry might be instituted respecting the formation of these coral reefs, ” including examination of the “modern and very plausible theory” that such reefs have been raised from the summits of extinct volcanoes.

Oceanic coral reefs often formed a circular island or group of islands with a central open area, or lagoon. Scottish geologist Charles Lyell, whose books Darwin studied, and to whom he would dedicate his own book about the Beagle’s voyage, believed that coral reefs grew around the summits of underwater volcanoes, with the circular atolls reflecting the opening of the volcanic crater.

Acting on the Admiralty’s orders, Beagle Captain Robert Fitzroy stopped at the Keeling (now Cocos) Islands in the Indian Ocean. Among other activities, he took soundings of the area to determine the depth at which coral formations began. These gave the depth limit for living corals as only about 20 fathoms (37 meters), but the island in question, rising more than 7200 feet (2100 meters) Darwin wrote, “forms a lofty submarine mountain, with sides steeper even than those of the most abrupt volcanic cone. . . and every single atom, from the least particle to the largest fragment of rock in this great pile, which however is small compared with very many other lagoon-islands, bears the stamp of having been subjected to organic arrangement. We feel surprise when travelers tell us of the vast dimensions of the Pyramids and other great ruins, but how utterly insignificant are the greatest of these, when compared to these mountains of stone accumulated by the agency of various minute and tender animals!”

If the sea depth had remained constant, corals could never have built their reefs on the bottom, far below their habitable limit. On the other hand, besides the fact that the Keeling Islands soundings showed they were not formed around volcanoes, how did coral reefs come into existence?

Although it pained Darwin to disagree with Lyell, he proposed a new theory: that reefs formed around the fringes of existing land, and that it was the subsequent subsidence or sinking of that land below sea level that formed the open, circular interior lagoon of atolls. (At the time, no one was aware of the additional role played by rising and lowering sea levels in the formation of coral reefs.)

Darwin would end by writing an entire book, The Structure and Distribution of Coral Reefs, to state his new theory of the place of the humble coral polyps in the formation of the Earth. Far from being offended by Darwin, Charles Lyell was delighted.

(Next Friday, Adventure classics begins a September of young adventurers with William Golding’s Lord of the Flies. And you thought your schooldays were bad. . . )

Adventure classics – The mystery of the vanished giants

The Voyage of HMS Beagle

by Charles Darwin

***

As the Royal Navy’s HMS Beagle made its way around South America in the mid-1830’s, frequently dropping off its on-board geologist, Charles Darwin, to make inland excursions inland, Darwin had time to wonder about the strange fossils he was collecting along river banks. What were they? And what had happened to sweep them from the earth?

The sheer numbers of fossils troubled him. He struggled to accommodate the huge quantities of bones, seemingly produced by the catastrophic deaths of large numbers of animals, with his enthusiasm for Scottish geologist Charles Lyell’s theory of gradual change over time.

“The number of the remains embedded in the grand estuary deposit which forms the Pampas and covers the granitic rocks of Banda Oriental, must be extraordinarily great. . . Besides those which I found during my short excursions, I heard of many others, and the origin of such names as ‘the stream of the animal’, ‘the hill of the giant’, is obvious,” he wrote in his journal of the voyage of the Beagle.

Darwin published his records in two editions, in 1839 and 1845. The 1957 collectible edition I’m working from is based on the 1845 edition, when Darwin was well on his way to puzzling out his theory of evolution based on natural selection. Some of the South American fossils, such as the extinct giant sloths, were obviously related to still living relatives. But what to make of creatures such as the Macrauchenia, looking like a humpless camel with a long snout, whose 1913 reconstruction illustrates this post? Or the Toxodon, with a rhinocerous-like body and rodent-like teeth, whose skull he bought for 18 pence, which when found “was quite perfect; but the boys knocked out some of the teeth with stones, and then set up the head as a mark to throw at.”

It would require 21^st century science to place the giant grazers in the family trees Darwin was starting to create, Scientific American magazine reported March 20, 2015, in “Mystery of Darwin’s ‘Strange Animals’Solved.”

Initially researchers trying to place the animals’ relationships were stumped by a lack of DNA, which degrades quickly in the South American climate. Instead, an international team used a different tactic, extracting collagen protein, to at last give two of Darwin’s mysterious mammals their place in the family tree of mammals.

“Part of a group of more than 250 mammals known as the South American ungulates,” Scientific American reported, “the creatures lived on the continent for around 60 million years before disappearing around 12,000 years ago.”

Purely on the basis of their fossils’ physical appearance, some had been suspected them of being related to elephants. Darwin himself suggested a possible link between the camel-like Macrauchenia and surviving llama-relatives, the wild guanacos of the pampas. But after sequencing proteins from two museum specimens of Toxodon and two of Macrauchenia, scientists finally placed them in a group that includes horses, tapirs and rhinos.

And what caused their disappearance after millions of years of survival? I’m going to suggest a catastrophic cause undreamed of by Darwin – the arrival of humans on the South American continent.

(Next Friday, Adventure classics puts Darwin back on board ship, to continue an August of adventures at sea.)