When Pigs Ruled the Earth
- When Life Nearly Died: The Greatest Mass Extinction of All Time by Michael Benton
Thames and Hudson, 336 pp, £16.95, March 2003, ISBN 0 500 05116 X
Some years ago, a National Enquirer headline announced that Martians had killed off the dinosaurs while visiting Earth to do some big-game hunting. It is hard to imagine such an explanation for the extinction of the trilobites – hard-shelled creatures that looked like giant woodlice. Compared with T. rex and triceratops, they lack the trophy quality. But the episode that led to the trilobites’ demise was actually the greatest threat there has ever been to life on Earth. It occurred at the end of the Permian period, 251 million years ago. Over a very short time, a huge number of hitherto dominant forms of life disappeared: not only trilobites, which were already on the way out, but most classes of organism on both land and sea; all were gone for ever.
Massive and repeated volcanic eruptions resulted in poison gases and dust pouring into the atmosphere, triggering a drizzle of acid rain that ate away forests and destroyed animal habitats. The system that keeps the atmosphere in balance spiralled out of control in a runaway greenhouse effect. Eventually, only mushrooms and slime moulds flourished. In the final disaster, huge bubbles of methane and carbon dioxide burst through the oceans and spurted seawater high into the atmosphere. Ninety-five per cent of species died, starved of oxygen on a planet that had become almost uninhabitable.
This is how many scientists picture what happened between the end of the Permian period and the beginning of the Triassic. Rather than a meteorite impact of the kind that would kill off the dinosaurs millions of years later, this threat to the Earth’s life-support system seems to have been the result of a succession of events that fed into one another until the final collapse. In When Life Nearly Died, Michael Benton, a palaeontologist, describes the decades of research that led to the current consensus about what happened, and argues against an extraterrestrial cause for the catastrophe.
It is astonishing that scientists have acknowledged the existence of this devastating episode only in the past decade, although geologists first recorded a major changeover in the Earth’s flora and fauna nearly 15o years ago. The reasons for this lie in the origins of geology as a science. The study of the Earth emerged in the 18th and early 19th centuries from a combination of cosmology, natural historical description and theological commentary. Some geologists, most notably Charles Lyell in his Principles of Geology (1830-33), played down the role of catastrophes in the Earth’s history, hoping ‘to free the science from Moses’ and other forms of what he condemned as imaginative excess. Reliance on smaller-scale changes, such as those we can observe in the present day, became the guarantee of philosophical rigour. Many authorities, especially outside the English-speaking world, took a more flexible view of the range of forces that might be involved in Earth history; but there is no doubt that Lyell’s denial of catastrophes was influential. By the middle of the 20th century, most experts viewed meteorite impacts and global extinctions as the province of superannuated theorists, science fiction writers and cranks.
Opinion changed in a remarkably short period of time. From the 1960s, the rise of planetary science meant that the Earth came to be seen as a dynamic body in space, and so as potentially subject to extraterrestrial influences. This was a very different kind of geology from the cautious, empirical programme of research followed by Lyell and his contemporaries. The refusal to countenance global catastrophes continued until the early 1980s, however, when Luis Alvarez, a physicist, won geologists over with his bold claim that a meteorite impact had caused the Cretaceous-Tertiary extinction and hence the death of the dinosaurs. Geologists now had every reason to search for evidence of other catastrophes.
Proving the suddenness of a mass extinction was not easy, however. As Lyell and other Victorian geologists never tired of saying, the Earth’s strata are like a book. But the book is badly preserved, with whole pages and chapters missing. It is often impossible to tell whether something was lost as a result of erosion, say, or whether life forms actually died out. Because of accidents of preservation, fossils tend to peter out slowly in any strata sequence, with the result that the general record suggests the gradual disappearance of species rather than sudden extinction. Only by carrying out a dogged search close to the boundary can the final disappearance of a species be pinned down.
The strata record for the late Permian era is especially poor, and sequences that bridge the gap are rare. The Permian had been named in 1841 by Lyell’s fellow geologist Roderick Murchison, from sequences found near the town of Perm, on the western edge of the Urals. Following the usual practice in early Victorian geology, Murchison worked fast: afraid of being pre-empted, he raced for thousands of miles across the countryside on a tarantass drawn by six horses. By the standards of late 20th-century research, his haste meant that the Permian system’s upper limits were poorly defined, although scientists had soon filled in some of the missing bits.
The most important sequences were found in the Karoo Basin of Southern Africa. In the mid-19th century, Andrew Bain, a Scottish road engineer, organised a huge network of collectors at the Cape, who sent vertebrate fossils back to London – some of them were as large as rhinos, others mouse-sized. Many of the specimens were in poor condition, and some had been smashed by evangelical Boers, who feared that they might be used to prove the antiquity of the world. The naturalist Richard Owen, who worked in London at the centre of an imperial network, identified them as the remains of mammal-like reptiles that he called dicynodonts (from the two canine teeth that these animals used to break up their diet of plants). Pinpointing the position of these fossils in the geological record proved extremely difficult – the rocks were a mass of red mudstones and sandstones – but has been vital to determining what happened in the late Permian era.
Some of the most engaging chapters of Benton’s book concern his experiences in the other major bridging sequence of strata, along the River Sakmara in the Urals. In the mid 1990s Russian geologists took Benton and some colleagues to locations that had been closed to foreign researchers for years. Touchy issues about intellectual property had to be worked out, as the English team quickly measured sequences that their Russian colleagues had been studying for years. The beautiful Russian maps, which had taken years to produce and were available only in limited numbers, were no longer marked secret, but the visitors were told they could not copy them. Benton subsequently edited a large collection of articles based on these expeditions, with contributions from Western and Russian palaeontologists; research hidden for decades by the Cold War now became available to specialists all over the world.
The display of these geological wonders has a fascinating history of its own. The progressive sequence of life, familiar to any visitor to the fossil galleries at a natural history museum, was first displayed to the public in the late Georgian and early Victorian London theatre. In John Martin’s apocalyptic paintings of prehistoric life from the 1830s, gruesome monsters tear at one another in the slime. They appeared as mezzotints at the front of popular geological books, and in a folio work on the bones of sea-dragons by Thomas Hawkins, whose fossil collection is now in the Natural History Museum. Traces of the Victorian geological spectacular can also still be seen in the extraordinary models of extinct monsters built for the grounds of the Crystal Palace when it moved to Sydenham in 1854. Here, iguanodon and other giant dinosaurs, made out of bricks, iron and cement, appear as the lords of reptilian creation. More than a million visitors a year saw these displays in the 19th century, and they provided a new middle-class audience with what Lyell called a ‘geological raree show’ akin to Leicester Square panoramas, travelling circuses and theatrical extravaganzas.
Victorian geologists avoided catastrophic explanations not only because they wanted to distance themselves from spectacles, but because of their association with popular millenarianism and theological disputes about the end of the world. While Murchison and Lyell were conducting their researches, vast congregations were coming to hear Edward Irving, John Cumming and others preach on the signs of the last days. The rise of infidel philosophy (including the doctrines of geology) was said to signal the nearness of the end. Cumming thought David Hume was the arch-frog that creeps from the mouth of the dragon in the Book of Revelation; and geologists were the offspring of the frog.
Benton is at considerable pains to show that his particular disaster is the ‘big one’, ‘the greatest catastrophe of all time’; but the workmanlike tone of his book makes its engagement with the spectacle of oblivion all the more telling. In encouraging a fascination with cosmic apocalypse, scientists take on the mantle of secular prophets, and Benton’s opening chapter claims that the study of ancient extinctions might help scientists and policy-makers understand the modern biodiversity crisis, allowing them ‘to work with real facts and figures’. By the end of the book, though, the facile notion that geologists can give clear advice about what to do to counter it begins to recede. The situations are too different, and the data too scanty, to make a case for using the late Permian events as a model for the modern rush to environmental catastrophe. Unlike the hapless dicynodonts and trilobites, we are presumably in a position to do something about it – and it comes as a relief to find that the book’s last section is entitled ‘Unanswered Questions’.
The real fascination of this story lies in its strangeness. Even the death of the dinosaurs, for all its violence, seems to fit more comfortably into the traditional picture of progress towards the present. Their extinction is seen as the start of the ascendancy of mammals and ultimately of humans. In contrast, the animals that survived the late Permian disaster seem as weird as those that died out. The larger ones were worst hit, and of the profusion of mammal-like reptiles, only one genus lived through the disaster – Lystrosaurus, a medium-sized generalist that could, pig-like, eat many kinds of food. These creatures rapidly filled the vacant ecological niches around them, so that for a brief period their bones are almost the only ones that seem to be preserved. As Benton says, this was ‘When Pigs Ruled the Earth’. We could be reading about a different planet, not this world a quarter of a billion years ago.
Yet the narrative remains firmly centred on the present, and Lystrosaurus’s importance is clearly its crucial role in the history that culminates in the evolution of human intelligence. As the sole remaining representative of the mammal-like reptiles, it is on the narrow path that leads to us. If Lystrosaurus had not survived, we wouldn’t be here. Nowhere is the poignancy of this story more apparent than in the book’s black and white illustrations by John Sibbick. Most reconstructions of ancient life, such as the BBC’s Walking with Dinosaurs, picture the past in hyperreal technicolor, using digital technology or acrylic. Sibbick’s pen and ink drawings are both vivid and anatomically accurate, inviting one to imagine scenes without forgetting the scientific work that has made re-creating them possible. In one of the most striking, the last of a dying race of dicynodonts gazes forlornly on a devastated landscape. He has come to the water’s edge in the hope of finding food, but the water is poisoned and lifeless.