Fossil secrets of the da Vinci codex

Did Leonardo decipher traces of ancient life centuries before Darwin?
It was to be Leonardo da Vinci's most impressive work yet. In 1483, the Duke of Milan, Ludovico Sforza, commissioned the up-and-coming artist to create a huge bronze statue of a horse, standing over 7 metres tall. Da Vinci spent the next 10 years perfecting a full-size clay model. Sadly, it was never cast in bronze. Tonnes of the metal were needed, and Sforza ended up using the earmarked supplies to make weapons for use against invading French troops. When the French army took Milan in 1499, its archers used da Vinci's clay horse for target practice.
Those years in Milan were nevertheless important for da Vinci, and not only for the many masterpieces he painted in that time. The polymath was also working on a very different project inspired by an intriguing feature of the surrounding countryside: embedded in the rocks there appeared to be a multitude of small stone sea creatures. "The hills around Parma and Piacenza show abundant molluscs and bored corals still attached to the rocks," da Vinci wrote a few years later. "When I was working on the great horse in Milan, certain peasants brought me a huge bagful of them."
Da Vinci recorded his observations of these and other fossils in a secret notebook now known as the Codex Leicester. His findings have long been known to be ahead of their time, but a new analysis suggests that the work was even more advanced than previously thought, with da Vinci correctly deciphering not only the origin of body fossils, which are the direct remains of an animal, but also trace fossils - the tracks and burrows left behind by ancient creatures. Such ideas would only be matched by modern naturalists hundreds of years later (see timeline).
The origin of body fossils was the subject of fierce debate in da Vinci's time. The ancient Greeks understood that seashells embedded in rocks were the remains of prehistoric creatures deposited on the floor of a sea that had once covered the land. But the idea had been lost with the collapse of their civilisation, and by the time of the Renaissance, most scholars were confounded by the shells. There were two main theories to explain them: one claimed that the shells were inorganic structures that had grown spontaneously within the rock. The fact that they mimicked living creatures wasn't considered too strange - it was simply thought to reflect the harmony that existed between the various realms of nature. The other theory claimed that the shells were the remains of sea creatures that had been deposited on mountaintops during the great flood described in the Bible.
Curious discoveries
Indeed, it took until the late 18th century for scholars to accept the idea that body fossils were in fact the remains of living creatures preserved in sediment. The study of trace fossils, a field called ichnology, came even later, with the first scientific observations of fossil footprints. In a letter written in 1845, Charles Darwin described the identification of such footprints as "one of the most curious discoveries of the present century and highly important in its several bearings". The ambiguous burrows and trails left by invertebrates crawling through the sediment were even more difficult to interpret. Naturalists initially thought they represented fossilised seaweed, and it was only in the late 19th century and early 20th century that their true origin was elucidated.
Da Vinci had worked all this out long ago. He was inspired by a geological formation between Milan and Florence known as the Apennine foredeep deposit. Ten million years ago, this region was a marine basin that formed next to the Apennine mountains as clashing tectonic plates caused the entire region to rise. Seismic shocks triggered currents that carried huge volumes of sand down to the seabed, which subsequently turned into sedimentary rock.
Today this area is characterised by picturesque hills laden with olive groves and vineyards, with the Apennine peaks visible in the distance. Rivers have cut deep into the landscape, creating rocky cliffs of alternating layers of sandstone and marl. Da Vinci is known to have studied bird flight here, but it is also the perfect place for fossil hunting.
Da Vinci used the observations he made in these hills to counter both of the prevailing theories for how fossils came about. He refuted the flood theory by observing that the fossils tend to be found in several superposed layers throughout the rock. From this, he inferred that they must have been deposited at several different times, rather than in one catastrophic event.
Da Vinci was positively withering when he considered the inorganic theory. He pointed out that marine fossils are only ever found in sedimentary rocks that appear to be of oceanic origin. Yet if the fossils were inorganic, they would presumably grow in all types of rocks. And if the stony shells had grown from seeds within the rocks, how could they expand each year, as indicated by growth bands on their shells, without fracturing the material that surrounded them? Only "ignoramuses" would believe such a thing, he wrote.
Crucial to da Vinci's case against the two theories, though, would have been his understanding of trace fossils, says Andrea Baucon, a palaeontologist based at UNESCO's Naturtejo Geopark in Idanha-a-Nova, Portugal. This discovery, had it been reported publicly, would have been the crowning glory of his evidence against the inorganic and flood theories of fossil origins. Instead, it was overlooked for centuries.
Baucon started reading the Codex Leicester as part of a project on geology and art. The codex consists of 18 yellowed sheets of paper, each densely covered on both sides with da Vinci's back-to-front mirror writing, along with copious messy sketches. It became the most expensive book in the world when Bill Gates bought it for over $30 million in 1994.
Although he couldn't access the Codex Leicester directly, Baucon studied scanned images of the pages, and was amazed to discover what could best be described as an ichnological observation. On the reverse side of the tenth page, da Vinci had written: "Among one and another rock layers, there are the traces of the worms that crawled in them when they were not yet dry."
Baucon describes this as his "eureka moment". Scholars had read the line before, but none had fully grasped its implications. From this one sentence, Baucon realised that centuries before other scientists, da Vinci had correctly interpreted both the biological origin of the fossil traces and the process by which they had become fossilised (Palaios, vol 25, p 361). "The modern interpretation is that these 'rock scribbles' were produced by worm-like organisms in marine environments, when the sediment was still loose," says Baucon. "This corresponds to da Vinci's observations."
As Baucon explored the rest of the codex, he found further clues that da Vinci was thinking about trace fossils. When da Vinci wrote about the bagful of corals that the peasants had brought him, he used the adjective "tarlati", derived from the Italian word for "woodworm", to describe the holes that permeate the coral's skeleton. That suggests he knew the holes had been created by living creatures, says Baucon. You can also find descriptions which suggest that like modern scientists, da Vinci used observations of living animals to decipher the fossilised marks. For example, in notes preceding the comment about worm traces in the rock, da Vinci describes a species of mollusc that "does not swim, but makes a furrow in the sand and proceeds along the furrow's edges".
These findings alone were unique for his time, and not matched until at least the 19th century, says Baucon. But da Vinci went further. The Codex Leicester shows that da Vinci's interpretation of trace fossils was crucial in enabling him to prove his ideas about the nature of fossilised shells and the sedimentary origin of the rocks that held them.
The burrows left by worm-like organisms feeding between layers of mud, for example, proved that the rocky mountain top was once a soft sea bottom, providing compelling evidence against the flood theory. And the presence of worm holes bored into the shells and corals helped to dismiss the inorganic theory. The fossils were clearly biological, da Vinci said, "because there remains a trace of the [animal's] movements in the shell which [it] consumed in the same manner as a woodworm in wood."
"Leonardo understood that these borings were the result of past biological activity," says Baucon. "He also recognised the biological interaction between the bored shell and the boring organism, at a time when scholars misinterpreted even fossil shells."
Although these ideas about fossil origins were distinctly modern, da Vinci used them as evidence to support a world view, inspired partly by ancient Greek philosophies, that would seem ludicrous by today's standards. Da Vinci's interest in fossils stemmed from his theory that the human body was a microcosm of the Earth (the macrocosm), and as such the two were unified on a metaphysical level and mirrored each other in form and function. He described it as follows in the Codex Leicester: "We might say that the earth has a spirit of growth, and its flesh is the soil, its bones the arrangement and connection of the rocks of which the mountains are composed, its cartilage the tufa [porous rock], and its blood the springs of the water."
To prove his point, da Vinci wished to show that earth and water circulate around the planet in the same way that he believed liquids and nutrients move around the body - hence his conviction that sediment on the soft seabed could eventually end up on top of mountains.
Neither the fossil studies nor the microcosm-macrocosm theory had the slightest impact on da Vinci's contemporaries, however. Da Vinci rarely published his findings, and with his revolutionary insights hidden away in his notebooks, both the inorganic and flood theories retained currency until the end of the 18th century, when they were gradually discarded in favour of the modern view.
Sketching wormholes
The findings have impressed Adrienne Mayor of Stanford University, California, a historian of science and folklorist who has studied interpretations of fossils in "pre-scientific" cultures. She describes Baucon's paper as "wonderful". "I had no idea that Leonardo could also put this in his honours," she says. Mayor notes that although the ancient Greeks had already correctly interpreted body fossils as the remains of ancient creatures, da Vinci was probably the first person ever to draw on trace fossils such as worm tracks and furrows to understand the fossilisation process. "That is revolutionary, very convincing," she says. "We can say that Leonardo is the father of ichnology."
Da Vinci was probably the first person to draw on trace fossils to understand fossilisation
Domenico Laurenza, a Renaissance science specialist at the Galileo Museum in Florence, Italy, who has been commissioned by Gates to produce a new translation of the Codex Leicester, is more cautious, particularly about lumping da Vinci's research in with more modern ichnological work. "Da Vinci's general conception is pre-Darwinian," he says. "That doesn't mean that his studies are less scientific than those by Darwin, they are just different."
Da Vinci is most famous for his paintings and sketches, of course. Might his ideas about fossils have influenced these too? He often included geological features in his paintings, depicting sedimentary structures and weathering processes in elaborate detail. According to Martin Kemp, an expert in da Vinci's art based at the University of Oxford, the microcosm-macrocosm theory is represented in the Mona Lisa. The flowing waters in the background are echoed in the woman's figure, Kemp says, in the "delicate cascades of her hair" and the "little rivulets of drapery falling from her gathered neckline".
Could trace fossils have also been depicted in the great man's paintings? Baucon thinks so. He has identified two potential candidates. One is a pale undulating trace on the surface of a sedimentary rock layer in the Virgin of the Rocks. The other is the presence of several winding lines on a rock layer in the Madonna of the Yarnwinder.
Kemp is sceptical. He says that the traces in the Madonna were originally red-coloured, and simply represent threads of yarn. Although he doesn't rule out the Virgin example, he says he would need to study the painting under magnification in order to check Baucon's theory.
But there is one other depiction that seems almost certain to be the first known illustration of a trace fossil: a roughly drawn honeycomb structure in another of da Vinci's notebooks, Codex I. This mysterious pattern is sketched on a page without any caption, alongside a series of images of body fossils. Baucon believes that it represents Palaeodictyon, one of the most common and characteristic trace fossils of the Apennine foredeep deposits.
This hexagonal fossil (pictured above) is thought to be a burrow system that allows its inhabitant to farm bacteria, but the creator of this crafty den is unknown. Researchers even sent the deep-sea submersible Alvin to locate modern-day versions of the burrows at a vent system 3500 metres beneath the ocean surface, but no creatures were found inside.
Scholars may argue over whether da Vinci deserves to be called the first ichnologist. But in drawing Palaeodictyon, he set a puzzle that today's ichnologists would still love to solve.
Jo Marchant is a consultant for New Scientist based in London and the author of Decoding the Heavens: Solving the mystery of the world's first computer
New Scientist