Sharks’ Teeth

I chose​ the perfect place to read Martin Rudwick’s book: the Isle of Islay, off the coast of Western Scotland. The archaeology of Islay is a long-standing interest of mine, especially the earliest traces of human settlement, which my excavations suggest took place 12,000 years ago or very soon afterwards. That’s nothing compared to the age of the bedrock of the island, much of which is Precambrian, dating to 1.8 billion years ago. For years I’ve walked across that bedrock with my mind fixed on the human past, neglectful of the rocks and the way they came to be dated and embedded in the history of the Earth. And so my New Year’s walk across the Lewisian Gneiss, the sandstones and the dolerite dykes of the island, was enriched by Rudwick’s demonstration that the science of such rocks is every bit as important as archaeology in defining who and what we are. Rudwick is the pre-eminent historian of earth sciences, and Earth’s Deep History, a grand sweep from the 17th to the 21st century, is a thrilling story of discovery and debate, insight and interpretation.

Archbishop Ussher is the starting point. In his Annals of the Old Covenant (Annales Veteris Testamenti, 1650-54), Ussher drew on the genealogies in the Bible to trace the date of Creation to 4004 bc. I recall my university tutor’s ridicule: Ussher had even specified a date in mid-October. But Rudwick offers a more considered view, explaining that Ussher was simply deploying the best scholarly practice of his time. Today’s earth scientists may use radiometric dating, but they are driven by the same motive as Ussher: the quest for an accurate and detailed chronology.

Rudwick emphasises the positive, or at least unobstructive, role of religion since Ussher, correcting the received idea that the 19th century in particular saw a great ideological clash between science and religion. The Bible was not a barrier to scientific thought: instead, Rudwick argues, the coherent sequence of events described in Genesis pre-adapted European culture to think about Earth and life on Earth in a similar historical way. As early as the 17th century it was recognised that each ‘day’ in the Creation story might represent something far longer, preparing the way for the notion of geological epochs. Many scholars were aware of the difficulties of interpreting texts written in ancient languages. Rather than seeking to demonstrate the literal truth of Genesis, geology would amplify or clarify the biblical account. In this way, one could be both a devout Christian and a scientist, as William Buckland and Adam Sedgwick, the two leading British geologists of the 19th century, were. Of course, biblical literalism resurged periodically, notably in the US. But the majority of scientists just got on with their geology, leaving others to worry about its implications for religion. What they were revealing about the sheer scale and unanticipated strangeness of the Earth’s long history was often treated as welcome new evidence for God’s Creation.

Ussher had been entirely reliant on written texts, but those who followed him began to study fossils, confused though they were by these strange objects. Was their resemblance to living things a clue to their origin or were they the product of a process of mineralisation underground? Robert Hooke made early use of microscopy to identify cells in what could thereby be identified as fossilised wood, and Nicolas Steno dissected a shark’s head to demonstrate that the well known fossils called glossopetrae, ‘tongue-stones’, found embedded in rocks on land, had once been sharks’ teeth, and must have come from sharks much larger than any currently alive. The biblical Flood provided a convenient explanation for the discovery of fossil shells on mountains, but the Flood was itself thought of in a variety of ways. Writing in 1695, John Woodward, an English physician and fossil collector, proposed that it resulted from a temporary suspension of gravity, during which all the materials of the Earth were churned up into a thick suspension. When gravity returned, the materials settled into the successive layers of rock strata within which fossils were embedded. Although fossils provided evidence for life before the Flood, in Woodward’s model they would be randomly distributed between the strata, and could offer no evidence for past environments.

In describing the work of Hooke and other 17th-century scholars, Rudwick introduces a second key theme, the influence of historical scholarship in shaping the discipline of geology. In a nice reversal of the common wisdom that the flow of ideas is from the sciences to the humanities, he shows how historians’ methods were transposed for use in the study of the natural world. The collection and study of human antiquities provided a ready source of analogies for natural relics: fossils were nature’s coins, stratigraphy was nature’s archive and the bones of extinct mammoths nature’s monuments. All such antiquities, whether from the human past or from nature, required deciphering. Nicolas Desmarest described his explorations of extinct volcanoes in central France as analogous to the excavations at Herculaneum. The 17th and 18th centuries saw a series of coups d’état and revolutions, some that swept governments from power and some that didn’t. Was the history of the Earth, too, a sequence of unpredictable events rather than a steady state or a predictable evolution in one particular direction?

This question pervaded scholarship two hundred years ago, and remains pertinent today in debates about the extent and significance of comet strikes on Earth’s development. In the 17th century, Descartes and James Hutton supported the idea of a steady-state system, or at least that the Earth goes through regular patterns of change such that its future state can be predicted, while others – such as Jean-André Deluc, who first introduced the term ‘geology’ – believed the Earth’s past and future were as unpredictable as human history. By this time, there was growing recognition that almost all of the Earth’s history occurred before humans came along; Georges Leclerc suggested a sudden origin for the Earth 75,000 years ago, thanks to a close encounter between a comet and the sun, since when the Earth has been gradually cooling – an idea that persisted throughout much of the 18th and 19th centuries.

During the later 18th century, the focus turned away from grand and ultimately untestable theories towards fieldwork, helped by the development of the mining industry and governments’ need to know what was underground. A series of discoveries, not least of the diversity and thickness of the rock formations, suggested much longer timescales for the Earth’s history. The fossil record was now growing rapidly, and large mammals were becoming important. Georges Cuvier, the leading expert, drew on comparative anatomy to reconstruct a large bestiary of extinct species. Both he and William Buckland assumed that a major catastrophe – a deluge – had killed off these species, forming a sharp boundary between the human and the antediluvian world.

Cuvier worked with Alexandre Brongniart, director of the state porcelain factory at Sèvres, just outside Paris, to create a three-dimensional map of the rock formations of the Paris Basin. They saw that different formations were characterised not only by the minerals they contained but also by particular fossils. One of Cuvier’s key contributions was to establish that many of the bones found in the lower geological layers came from reptiles not mammals; he showed, for instance, that what had been classified as a toothed whale found in chalk deposits near Maastricht was in fact a huge marine lizard, later named Mosasaur; he was also the first to identify flying creatures in geological deposits as neither birds nor bats but flying reptiles that he named ptéro-dactyle, ‘wing-fingered’.

The recognition that there had been a succession of animals from reptiles to mammals indicated a directional history of life on Earth. By the early 19th century, stratigraphy was the staple work of geologists, and the familiar terms for geological periods were introduced: the Cretaceous, Jurassic, Triassic, Permian and Carboniferous, underlain by the Silurian, Cambrian, Ordovician and Devonian. By the end of the century a consensus had emerged: the Earth had a long history of gradual cooling; life forms were adapted to changing environments and would appear and disappear accordingly, but life was linear and progressive, though punctuated occasionally by periods of sudden and violent change; humans made their appearance at the very end.

In 1837, Louis Agassiz, taking a break from his specialism in fish fossils, proposed that the Earth had relatively recently been in the grip of an ice age. A static ice sheet had once covered the whole of the northern hemisphere, he argued, which explained such phenomena as ‘erratics’, boulders found far from their geological source. Agassiz suggested that erratics found in northern France had once slid along an ice sheet descending from the Alps; others, notably Jean de Charpentier, saw that in fact they had been dragged below moving glaciers, causing the deeply scratched rock surfaces documented in northern Europe and the ridges of rock debris (moraines) that marked where such glaciers had come to an end.

The notion of an ice age was initially met with much scepticism. The consensus was that the Earth had been cooling slowly and steadily throughout its long history: it was difficult to adjust to the idea that there had been a sudden cold period and then a return to comparative warmth. But the idea gradually gained ground, helped along by the results of polar exploration. Where once the Alps had been the reference point when thinking about what North America and northern Europe may once have looked like, now it was Greenland and Antarctica. Eventually it was accepted that there hadn’t been just one ice age, but a sequence of them, raising questions about their possible role in what appeared to be mass extinctions of mammoths and other large mammals, and in the origin and early history of human beings.

Increasingly reliable evidence of human fossils accumulated during the early 19th century but was given a rough reception, not least by Cuvier. In 1833, Philippe-Charles Schmerling found, buried deep in the floor of a cave in Belgium, what he took to be two human skulls, close to a mammoth tooth and intermixed with stone artefacts and the bones of extinct animals. His case was rejected even by such distinguished scientists as the geologist Charles Lyell, who should have appreciated Schmerling’s careful fieldwork and attention to stratigraphy. Ultimately, however, the evidence became irresistible: in the 1840s Jacques Boucher de Perthes discovered stone tools buried in the gravel of the Somme valley alongside the bones of extinct animals, and in 1858 stone artefacts intermingled with the bones of hyena and rhinoceros were found sealed below a crust of stalagmites in Brixham Cave near Torquay. The Brixham Cave excavation was overseen by Lyell himself, who then, together with Boucher de Perthes, began a co-ordinated campaign to improve scientific opinion regarding human antiquity.

Its formal acceptance coincided with the publication of Darwin’s On the Origin of Species in 1859. Rudwick firmly places the debates about the evolution of species in historical context: the issue could be raised only once it had been established that the Earth itself had a history. Darwin’s great challenge was to explain why there was no evidence in the fossil record for the process of gradual change within species that he proposed. He was fully persuaded by Lyell’s geological uniformitarianism: a belief in the slow and steady pace of change driven by the same natural laws and processes observable in the world today. As such, the discovery of Archaeopteryx, which seemed to represent a missing link between reptiles and birds and therefore suggested that evolution happened by sudden macro-evolutionary leaps rather than tiny steps, was unhelpful to Darwin’s cause. The spectre of human evolution haunted the debate. For Lyell and other savants the problem was not so much Darwin’s proposal that the physical form of human beings had evolved but that so too had their moral sense. That there had indeed been human evolution became evident when the first Neanderthal fossils were discovered, closely followed by ‘Java Man’ in the Far East, which prompted questions that haven’t gone away about the evolutionary relationships between such species but also about their intelligence, language and ways of life.

As​ the 19th century came to an end, there was no longer any doubt that the Earth was many millions of years old, though just how many millions was unclear. Darwin had suggested 300 million, John Phillips a mere 96 million; the physicist Lord Kelvin initially suggested a billion years based on a projected rate of cooling, but later reduced his estimate to forty or fifty thousand. The discovery of radioactivity in the early 20th century put an end to the speculation. It was recognised that heat was being generated by radioactive decay inside the Earth, and radiometric dating methods soon established that the Earth was an order of magnitude older than anyone had supposed. In 1953 the American physicist Clair Patterson used such methods to derive an estimate of 4.5 billion years, a figure still thought reliable.

This great age for the Earth made it plausible to assume that its history included a greater range and diversity of events than would otherwise have been possible. The idea had been around for a long time that mountain ranges were the result of movements of sections of the Earth’s crust, though the assumption was that the movements had been purely vertical. In the 1870s, geologists in India proposed that Africa, Australia and India had once been part of a single massive landmass, Gondwana Land. In tracing the development of the theory of plate tectonics, Rudwick notes that it wasn’t helped by its early characterisation as ‘continental drift’: the key physical units – tectonic plates – are not continents, and they don’t drift randomly but are dragged by huge underground convection currents. He also notes an intriguing collective volte-face on the part of US geologists, who were at first opposed to tectonic theory (‘the Americans are about the toughest isolationists in existence, geologically as well as politically,’ a South African geologist remarked), but turned into its most ardent supporters when the theory became universally accepted from the 1960s onwards.

The fossil discoveries made in the late 20th century were no less thrilling than those made by Hooke looking down his microscope or by Buckland discovering a hyena den in southern England. But the new finds were fossils of much older and much smaller creatures: microfossils and Ediacara, soft-bodied fish, found in Precambrian rocks. These showed that the so-called Cambrian explosion was not of life itself but merely of animals with large bodies and then with hard shells. Fossils of life-forms even older than the Ediacara were discovered subsequently: stromatolites, literally ‘rocky pillows’, dating back 3.5 billion years. These appear to have been formed by microscopic life secreting or trapping mineral material and slowly growing upwards to form large mounds. Modern equivalents have been discovered that generate oxygen as a waste product, suggesting the possibility that ancient stromatolites were responsible for what has become known as the ‘great oxygenation event’, which allowed larger-bodied organisms, and eventually humans, to evolve.

One of Rudwick’s achievements in Earth’s Deep History is to show that much of our current understanding of causal process strongly resonates with ideas that emerged long before there was any hard evidence. There has, for instance, been a constant dialogue between catastrophism and uniformitarianism; the former had a late 20th-century revival in the talk of meteorite impacts and mass extinctions, echoing Cuvier’s suggestion of a catastrophic end to the antediluvian world. In 1875 James Croll argued that glaciation took place in cycles determined by minor variations in the Earth’s orbit around the sun; this idea was dismissed but then revived by Milutin Milanković in 1930 and finally shown to be correct in a paper by two American climate scientists and a geophysicist in 1976. Equally striking is the foreshadowing of the modern theory of plate tectonics in Athanasius Kircher’s view in the 17th century that large parts of the continents had once been under the sea, and by Deluc’s suggestion in the 18th century of an interchange between continents and oceans.

Finally in the late 20th century earth scientists began to spend more time looking upwards, at the moon. Although evidence had been accumulating that comets had occasionally struck the Earth, it was only when the pockmarked surface of the moon was studied that the likely extent of such bombardment was appreciated. By the 1980s more than two hundred terrestrial impact sites or ‘astroblemes’ had been identified on Earth. There is substantial support for the theory that the impacts were responsible both for the origin of life on Earth, by delivering the constituents of DNA, and for one or more of the mass extinction events. By the early 21st century, the history of the Earth was conceived as just one of a divergent set of possible planetary histories – one in which a complex set of contingencies had resulted in living organisms and then intelligent life.