Petrifying Juices

When​ Virgin Galactic’s spaceplane VSS Unity made its third commercial flight on 8 September 2023, its three crew members were accompanied by three paying customers, ‘private astronauts’ who had bought their tickets as long ago as 2004 (when they were a bit less than half the current asking price of $450,000). Ken Baxter, a Las Vegas real-estate investor; the British racing car designer Adrian Reynard; and Timothy Nash, a South African millionaire, experienced a few moments of zero-gravity before returning to Earth.

The flight would have been uncontroversial, except that Nash had taken with him a small carbon-fibre tube containing two hominin fossils, one of them two million years old: a collarbone from Australopithecus sediba, the type specimen for an extinct species. It was found in 2008 at an archaeological site known as Malapa, in the Cradle of Humankind World Heritage park north-west of Johannesburg; the other fossil, a 250,000-year-old Homo naledi metacarpal, was found in 2013 at another site in the park. In recent years Nash has been buying up sites in the area with the aim of creating a ‘world-class palaeotourism hub’. The fossils were borrowed from the collection at the University of the Witwatersrand. The permit application made some mention of experiments that might be conducted to see whether the fossils were affected by radiation in the high atmosphere, but it was explicit that loaning Nash the fossils – one of which, it pointed out, came from a site ‘on his property’ – was chiefly a publicity stunt, and the South African Heritage Resources Agency signed off on that basis.

Palaeontologists, unsurprisingly, hit the roof. There were concerns about the lack of scientific justification for sending the fossils on the trip; about the cavalier attitude towards ancestral remains; about the ‘neocolonial’ exploitation of African resources; and, of course, about the risk that if something were to go wrong, the fossils could be lost for ever. Bernhard Zipfel, the curator of collections at Witwatersrand, claimed that these particular relics had been chosen because ‘they are among the most documented fossils of hominids in existence, with casts, scans and images available across the world.’ But a cast is only a facsimile. As Dale Greenwalt makes clear in Remnants of Ancient Life, there’s no substitute for the real thing.

For hundreds of years the study of fossils was the province of comparative anatomy. Experts examined different specimens as if they were sculptures, meticulously reconstructing the changing morphologies of the distant past from appearance alone. But in the last few decades technological advances have made it possible to analyse not just the form of fossils, but also their contents: molecules that once functioned inside the living organism and have remained there, preserved in situ. Fossils are no longer mere shapes or outlines, but potential repositories for these ‘remnants of ancient life’. Greenwalt himself is a palaeoentomologist. For eleven months of the year he curates a collection of fossil insects at the Smithsonian Institution, but every summer he goes hunting for specimens in the remote landscape of northern Montana, where the icy Flathead River carves steep banks through the landscape of shale and siltstone. To reach his favourite site, he has to ford the fast current twice a day – it’s manageable in the morning, but much harder in the afternoon with a backpack full of ‘compressed’ mosquitoes.

In the early modern era fossils weren’t a well-defined category. Athanasius Kircher speculated that these strange objects were produced by ‘juices’ flowing underground, which not only hardened animals into stone but also conjured images to enchant and educate. His Mundus subterraneus (1664) included fossilised fish together with ghostly engravings of owls, a geode that split open to reveal a crystalline Virgin and Child, and an object resembling a hairy man – ‘some say a hermit, others think John the Baptist.’

Kircher’s contemporaries puzzled over fossilised animals and their distribution. The Walloon mathematician René-François de Sluse wrote to the Royal Society in London enclosing a sketch of stones resembling shellfish: ‘It is strange that such little creatures should be found, hardened by a petrifying juice, so far from the sea.’ The natural philosopher Nicolas Steno, contemplating stratified rocks in the hills of Tuscany, boldly concluded that they had been formed by the layering of sediment over time. The presence of marine fossils on hillsides wasn’t simply ‘strange’, it was a clue: the hills had once been the seabed. Steno was careful to make his radical new history compatible with Christian scripture, but eventually lost faith in geology and abandoned his studies to become a bishop. He told a friend in a letter in 1671 that, in truth, nothing was really known about the origins of natural objects such as fossils.

Despite Steno’s pessimism, fossils became widely accepted as evidence of the past existence of organisms now extinct. As such evidence mounted, scientists had to become more comfortable, as Darwin put it, ‘drawing large cheques upon the Bank of Time’. The earth is 4.5 billion years old and life about 3.5 billion – or so the earliest fossils suggest. We think multicellular life emerged within the last billion years, but the ‘traditional’ fossil record of recognisably animal forms only begins around 574 million years ago, during the Ediacaran period. Palaeontologists haven’t always been polite about the ages before that: one particular stretch of time is known as the Boring Billion. Early fossils derive largely from the realm of the microbes – rafts, mats and microfossils, the actual walls of ancient cells.

The further back you go, the harder studying fossils becomes. Time deepens like a coastal shelf, and the organisms descend into a murky taxonomic gloom. As Greenwalt shows, molecular evidence offers some illumination. One of his examples is Dickinsonia, an Ediacaran organism that floats enigmatically outside taxonomic bounds. Dickinsonia fossils resemble ribbed oval leaves and can be over a metre in diameter. From their shape alone, it has been proposed that they were either gigantic single cells, multicellular animals, or perhaps some sort of lichen. But though their cells are long gone, at the molecular level traces of their original bodies remain: one fossil recovered from north-eastern Russia was coated with a thin black layer less than a thousandth of a millimetre thick. In 2018, scientists used mass spectrometry to analyse the layer, revealing the carbon skeletons of cholesterol-like molecules. If these conclusions are correct, Dickinsonia would count among the earliest animals.

What natural philosophers called ‘petrifying juice’ we would now call permineralisation, a process whereby minerals crystallise out of water into the empty spaces inside a dead body. But permineralisation isn’t always a complete process, and even fossils that seem to be entirely stone can contain traces of original molecules. Some preservation methods are more effective still. In 1982, the entomologist George Poinar and electron microscopist Roberta Hess published a paper on a 40-million-year-old fossil fly stuck in a glob of amber. Exquisite images taken with Hess’s instruments reveal individual cells in the fly’s abdomen, frozen in death, a microscopic Pompeii. Zoom in further and you can see structures straight out of a biology textbook: mitochondria, bubble-shaped fat reservoirs, even cellular nuclei. Poinar and Hess later speculated that if a mosquito were preserved in amber just after it had sucked blood from a dinosaur, it might be possible to recover the dinosaur’s DNA from blood cells in the mosquito’s stomach.

A young Michael Crichton visited Poinar and Hess and peppered them with questions. His first attempt to use what he’d learned was a screenplay about a graduate student who recreated a pterosaur; after several redrafts he ended up with Jurassic Park (1990), in which a bioengineering billionaire populates an island with a host of dinosaurs, all generated from ancient DNA extracted from mosquitoes fossilised in amber. Spielberg’s film followed three years later. It isn’t crazy to think that the shaky start of ancient DNA studies was at least partly due to Crichton’s success in making the conceit seem plausible. Jurassic Park was followed by what Greenwalt calls ‘a period of dilettantism’, as researchers reported sensational findings of DNA from millions of years ago, extracted from samples without proper controls against contamination. Researchers who tried to reproduce these results comprehensively failed, and within a few years there were decisive rebuttals: one paper, in the Journal of Molecular Evolution in 2002, was given the unimprovable title ‘Curiously Modern DNA for a “250-Million-Year-Old” Bacterium’.

It wasn’t until 2013 that an example emerged of a blood-engorged mosquito fossil. It wasn’t in amber but was one of the squished insects from the Flathead River. Greenwalt noticed the distended abdomen while photographing the specimen, and marvelled at the improbable sequence of events required to bring such a thing about. The insect would have had to ‘take a blood meal, be blown to the water’s surface … sink to the bottom of a pond [and become] embedded in fine anaerobic sediment’, all without disrupting its fragile abdomen. Greenwalt colleagues bombarded the fossil with atoms of bismuth (the heaviest of the non-radioactive metals), ripping up fragments of ancient molecules from its surface for further analysis. They found molecular fragments derived from haemoglobin, fulfilling at least one of the prerequisites of Crichton’s dream.

But there was no question of recovering any genetic material. DNA is a large, fragile molecule, and our records of it barely scratch the surface of deep time: the oldest reliable reports are from around two million years ago. Proteins such as haemoglobin last longer, though how much longer isn’t entirely clear. Some of the most controversial claims have been made by the American palaeontologist Mary Schweitzer; she has reported, for example, recovering collagen from a 78-million-year-old Tyrannosaurus rex. Greenwalt is diplomatic, but points out that this could easily have been the result of contamination from a scientist’s skin cream or a flake of dandruff. Schweitzer continues to publish intriguing research on dinosaur fossils, but the current consensus is that proteins don’t last longer than three or four million years. As Greenwalt puts it, ‘protein sequences from really deep time may occasionally make it into the pages of the New York Times, but they do not appear in textbooks.’

It may not be possible to ‘de-extinct’ whole organisms, but researchers have been able to resurrect individual proteins from relatively recent fossils. By drilling into the femur of a 43,000-year-old mammoth to get thirty grams of bone powder, scientists were able to extract DNA and then sequence it for the genes encoding haemoglobin. The mammoth’s haemoglobin was almost identical to modern-day elephant haemoglobin – but not quite. To explore the differences, the researchers used a standard genetic engineering technique to construct a circular loop of DNA containing the genes, then inserted that loop into E. coli. The bacteria dutifully used their cellular machinery on the haemoglobin genes, converting the information that had been ossified in the mammoth’s bone into an abundant supply of the ancient protein. Tests showed that the mammoth haemoglobin was better at performing its role at colder temperatures.

Where proteins and DNA falter, molecular pigments can survive. At a quarry in Idaho, Greenwalt watches as a piece of damp shale is split open to reveal a 16-million-year-old fossil leaf. After covering it with acid for several minutes then rinsing with water, the intact three-dimensional leaf can be lifted off the surface with a knife blade: it could be one of last year’s leaves. When the right conditions are in place for preservation, leaves can retain their chlorophyll for millions of years – though Greenwalt confesses that the examples he’s seen look more brown than green, with only a hint of ‘something you might call sage’.

Colour has played an important role in elaborating what we know about the dinosaurs. Not only might many dinosaurs have been feathered, but some palaeobiologists think their colours can be inferred too.* Melanosomes, the globular packets of pigment inside cells, can be shaped and distributed in a variety of ways, resulting in differences of colour. Some believe it may be possible to calibrate fossilised melanosome distributions against present-day bird feathers and so ‘de-extinct’ colour. Artist’s impressions of these ‘new’ dinosaurs often seem slightly garish, like colourised black-and-white films.

Our visions of the past exert a powerful aesthetic influence. We know, for example, from the analysis of microscopic flecks of Greco-Roman marble sculptures that they were often painted in gaudy colours, but that the layers of paint have been almost entirely stripped away by the passage of time. (‘Thank God and every other god there is/That time is an aesthete,’ Clive James wrote in response to this ‘polychromatic crap’.) In the 18th and 19th centuries, statues were routinely ‘restored’ with sulphuric acid. And in the late 1930s, as if more than two thousand years of weathering hadn’t been enough, the Parthenon marbles were scrubbed with copper wire brushes because Lord Duveen, who was funding a new gallery for them at the British Museum, was obsessed with returning them to perfect whiteness. When the news got out, there was a scandal – much to the bemusement of the museum’s chief restorer, who patiently explained to the Daily Express that he’d been using the same methods for years, under four different directors.

Like sculptures, fossils need curators. A raw lump of stone must be prepared and cleaned before it can be studied as a fossil; scientists of the past may well have inadvertently destroyed interesting surface layers. Still, most of its molecular secrets will have remained locked inside – as long as a fossil isn’t blasted into space, it can be handed down to future generations to explore. The overlap between curatorial and scientific interests does sometimes give rise to difficulties. When Greenwalt hears in a seminar that modern-day ants use zinc to harden the edges of their mandibles, he has a colleague at the Smithsonian use a powerful X-ray machine to confirm that the jaws of a 46-million-year-old fossil beetle are still chock-full of zinc. But what to do next? Greenwalt’s many follow-up questions could be answered only by using techniques that would require grinding the beetle’s mandibles into powder – something he can’t countenance. The idea of ‘destructive’ sampling makes palaeobiologists wince. When Greenwalt poses the dilemma to a museum audience, everyone agrees they’d rather keep the fossil intact, in the hope that newer, non-destructive technologies will eventually come along. Extracting new information from old fossils is a question of knowing what to look for – but it’s also a question of knowing when to stop.