Last spring, the geobiologist Dominic Papineau and colleagues reported that fossilised microorganisms were identified in 3.77-4.28-billion-year-old iron-rich rock in Quebec: hematite tubes and filaments whose appearance is similar to microorganisms that today live in hydrothermal vents. Others dismissed their findings as ‘dubiofossils’, a term the geologist Hans Hofmann coined in 1972 to describe controversial fossils. ‘Fossils,’ Hofmann wrote, were proven biological; ‘pseudofossils’ resembled life but were inorganic; ‘dubiofossils’ (also known as Problematica or Miscellanea) were equivocal. No one is sure whether either of these findings has discovered ancient petrified organisms or not.
For centuries, long before ‘biology’ coalesced as a discipline in the early 19th century, scientists struggled to understand what constitutes ‘life’. Millennia ago, Aristotle’s Scala Naturae (ladder of being) described arrayed nature on a continuum that advanced in perfection from rocks to humans (the Catholic Church crowned that ladder with God and the angels; Carl Linnaeus quietly removed them from his own taxonomy). The 17th-century naturalist Athanasius Kircher believed that vitalism was impressed in different substances, with mineral forces forming some fossils in an attenuated process similar to the growth of plants and animals. German Romantics of the 19th century such as Novalis and Johann Wolfgang von Goethe were captivated by caves, and thought the same individuating forces (Triebkraft) generating crystals climaxed in humans. ‘Life itself’ did not exist. All was organic.
Today’s textbooks teach high-school students that life is marked by specific capacities – reproduction, metabolism, adaptation, self-organisation, growth. But biologists and other scientists are less resolute about what makes life unique. In 1943, Erwin Schrödinger answered the query ‘What is Life?’ as a physicist – it is a negative entropic system, like any other. Such thinking influenced mid-century molecular biologists, who borrowed cybernetic theory to think of life as signalling servomechanisms and homeostats made up of molecular ‘information’. DNA is still called a ‘code’ for a reason. At the close of the 20th century, different scientists also defined life according to chaos theory, thermodynamics and other physical processes. Computer scientists believed that they could generate artificial life on a computer.
Geologists examining fossils in rocks help us to gain purchase on the conundrum of what constitutes life by identifying its remains. For relatively recent fossils – think dinosaurs – the answer is straightforward. Though extinct, their bodies look much like extant organisms: bilaterally symmetrical, bearing notable features such as skeletons, teeth and tails. But life was altogether different before the Cambrian Explosion 539-541 million years ago when, in an evolutionary paroxysm, most of the animal body forms we recognise today suddenly appeared. What, then, of the remains of first life-forms, those that lived and died on an Earth almost entirely unlike our own, at a time before continents accreted, when sulphurous seas stretched across a young planet beneath a pale Sun in an atmosphere devoid of oxygen, when tides surged and months lasted a mere 20 days? How would one recognise fossils that are 2.5 to 3.9 billion years old?
This is one of the questions driving geobiology, a discipline that originated in the mid-20th century with precursors in older fields of palaeontology, geology and the life sciences. The big presumption of geobiology is the notion that Earth and life are mutually informing forces, and that our planet has changed in concert with the evolution of life. Searching for fossils on early Earth, a planet in many ways profoundly different from Earth today, resembles efforts to figure out what life might look like on Mars, icy moons and exoplanets. Such problems are not new. In the mid-19th century, naturalists debated whether a strange entity named Eozoön might be the first lifeform; mid-century geobiologists setting the guidelines for research in their new field hotly debated how to sort ‘real’ fossils from lifelike imprints in stone. Today, geobiologists’ work also helps us to think over how life on other planets might be identified.
Though little known, a series of geological discoveries in the mid-19th century first suggested that there might be a longer history of palaeontology, one that reached deep into Earth’s antiquity, even to the origins of life itself. In 1858, the rock beneath the Cambrian stratum was not called pre-Cambrian; it was simply ‘Azoic’, because no one believed that life could be found there. Yet that same year, a collector for the Geological Survey of Canada found something curiously lifelike in Azoic limestone in the Laurentian stratum: ‘peculiar laminated forms, consisting of alternate layers of carbonate of lime and serpentine, or of carbonate of lime and white pyroxene’. William Dawson, a former student of Charles Lyell and then principal of McGill University in Montreal, examined the rocks. In On the Origin of Species (1859), Charles Darwin noted (in what is now known as ‘Darwin’s Dilemma’) that, if his theory of natural selection were correct, the fossil record should show organisms ‘before the lowest Silurian stratum was deposited’. Following Origin’s publication, scientific debate over the existence of originary fossils was volatile. Yet finding them proved difficult – if Darwin was right, then where were they?
Dawson was an anti-Darwinist, but he recognised the patterns he saw as undoubtedly organic – he believed them to be the skeletal remains of giant foraminifera (single-celled organisms that grow hard external shells). In arguing that this rock was truly of organic origin, Dawson focused on what he termed the fossil’s ‘beauty and complexity’, noting a series of microtubules he was certain could not have been formed by purely physico-chemical means. In a patriotic flourish, standing before the Natural History Society of Montreal in 1865, he named the organism, which he believed to be the progenitor of all life on Earth, the ‘Dawn Animal of Canada’, or Eozoön canadense.
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