A recent visit to Teotihuacán — the ancient Mesoamerican city in present-day Mexico, built by earlier cultures around 600 BCE and later rediscovered by the Aztecs — left me wonder-smitten by the see-saw of our search for truth and our search for meaning, by a peculiar confluence of chemistry, culture, and chance that unrinds the layers of reality to put us face to face with the mystery at its core.
Situated at the foot of a dormant volcano, Teotihuacán stunned the Toltec settlers with the discovery of a lustrous black material partway between stone and glass, brittle yet hard, breathlessly beautiful. Soon, they were laboring in obsidian workshops by the thousands, making from it delicate beaded jewelry and deadly weapons, household tools and ritual figurines, mirrors and surgical instruments, which traveled along trade routes to become the pillar of the Toltec economy. Its abundance and versatility may be why they never arrived at metallurgy, but obsidian became as important to the development of their civilization as steel has been to ours.
It would also become the ouroboros of their civilization — the source of prosperity by which they would flourish for centuries and the ominous overlord by which they would perish.
Not a mineral but a volcanic glass made of igneous rock, obsidian forms as lava cools too rapidly for mineral crystals to nucleate. It is composed primarily of silicon dioxide, with trace amounts of various oxides — mostly aluminum, iron, potassium, sodium, and calcium — the ratio of which varies by the circumstances of each eruption, creating a particular chemical fingerprint, so that each piece of obsidian can now be traced to its original source using nuclear and X-ray analyses.
As if volcanic glass weren’t already miraculous enough, the discovery of a special kind of obsidian — iridescent, with a green-gold sheen — catapulted Teotihuacán to the status of an ancient metropolis. Rainbow obsidian soon became the most valuable kind of obsidian in Mesoamerica, attracting people from faraway lands in search of wealth, much as the Gold Rush changed the demographics of nineteenth-century North America.
With the discovery of this doubly dazzling obsidian, Teotihuacán became home to people from different cultures with no common language and no common rituals. And yet they lived together harmoniously in the fertile valley, sharing its riches — it is hard to fight while flourishing — until the eruption of a different volcano in present-day Ecuador induced regional climate change that sent entire ecosystems into a protracted draught and left Teotihuacán on the brink of famine. Suddenly, the bedrock of this composite society began fissuring along class lines as the nobles feasted and the starving laborers clashed over resources. A kind of civil war broke out, from which Teotihuacán never recovered. The survivors abandoned the city, but not before burning the dwellings of the ruling class to the ground. Only its pyramids — Toltec temples to the Sun and the Moon — stood intact by the time the Aztecs came upon it nearly a thousand years later and named it “City of the Gods.”
One of the geochemical wonders of this Earth, iridescent obsidian occurs when nanoparticles of magnetite — an iron oxide present in most obsidian — form a thin film that reflects light waves at the upper and lower boundaries of the material in such a way that they interfere with one another, magnifying the reflection at some wavelengths and diminishing it at others. This process, known as thin-film interference, is what produces the colorful luster of oil spills and soap bubbles.
Magnetite gave Teotihuacán its rare rainbow obsidian, but it also fomented the destruction of Mesoamerican civilization by the Spaniards. Humans discovered the property of magnetism through naturally magnetized pieces of rock containing magnetite, known as lodestones, which became the first magnetic compasses, revolutionizing navigation. Without magnetite, Columbus may have ended up another anonymous sailor shipwrecked on an anonymous shore.
A seeming triumph of human nature’s ingenuity, the invention of the compass turned out to be a mere refraction of nature’s own imagination: Magnetite crystals have been found in the upper beaks of homing pigeons and many migratory birds — a kind of built-in internal compass that allows them to orient by Earth’s magnetic fields in their staggering feats of navigation. (Small amounts of magnetite are also found in various regions of the human brain, including the hippocampus — the crucible of our autonoeic consciousness; my friend Lia is convinced that my homing-pigeon sense of direction, which overcompensates for the mediocrity of my other senses, is due to abnormal amounts of magnetite in my brain.)
A built-in compass explains why, for instance, bar-tailed godwits — some of the longest-distance migrants on Earth — can leave their nesting grounds in Alaska and head for their breeding grounds in New Zealand not along the continental arc of Asia and the rim of Australia, where they can easily orient by visual landmarks like mountains and cities, but over the open Pacific Ocean. Across the immense monotony of blue, where a mistake by even a fraction of a degree would take them to a wholly different destination, they have found their way year after year, eon after eon.
Geologist and geophysicist Joe Kirschvink discovered magnetite while studying honeybees and homing pigeons as a graduate student at Princeton University in the 1970s. The idea that some animals navigate by magnetism was not new. At the dawn of the century, the Belgian playwright and amateur apiarist Maurice Maeterlinck had observed that bees navigate by “senses and properties of matter wholly unknown to ourselves,” which he termed “magnetic intuition.” A generation before him, and a decade before Darwin staggered the world with his evolutionary theory, the Russian zoologist and explorer Alexander Theodor von Middendorff had speculated:
The amazing steadfastness of migratory birds — despite wind and weather, despite night and fog — may be due to the fact that the birds are constantly aware of the direction of the magnetic pole and therefore know exactly how to keep to their direction of migration.
To have located the basis of biomagnetism in magnetite seemed like a triumph of science over mystery. But in the decades since, as our instruments have become more sophisticated and our theories more testable, research has revealed the presence of a protein in the retinal cells of birds — cryptochrome — that may be making use of quantum entanglement to provide a whole other mechanism of magnetoreception. More knowledge has only unlatched more mystery: The total system may involve multiple build-in instruments interacting with multiple fundamental laws and forces. I think of Henry Beston, who wrote a century ago that “in a world older and more complete than ours,” other animals “move finished and complete, gifted with extensions of the senses we have lost or never attained, living by voices we shall never hear.” I think of the difference between science and civilization: Science knows it is unfinished, a perennial process, whereas every civilization mistakes itself for the end point of progress.
Walking down Teotihuacán’s central promenade and watching the Sun pyramid gradually eclipse the volcano, the evolutionary triumph of my peripheral vision registers a flash of yellow. I turn to see a small bird aglow against the ruins, perched on a stone ledge above a man in a sombrero selling obsidian souvenirs. The warblers — godless, tradeless, needful only of sky and song — are among the most regular border-crossers between North and South America, their migratory routes stretching from Alaska to the Amazon. Older than the Toltecs, older than the sediment deposits that separated the Atlantic and Pacific Oceans to bridge the Americas, older than our oldest myths, they have seen civilizations rise and crumble, and will one day see Hollywood overgrown with poppies and Manhattan returned to the sea. And when they fly over the ruins of the Sistine Chapel and Silicon Valley, they will be guided by the same mysterious forces that guided the first of their kind.
“From the basic biological perspective,” concluded a team of scientists studying the magnetic compass of warblers, “the perception of the magnetic field remains the only sense for which the sensory mechanism and its location still remain unknown.”
It is salutary for us to have regular reminders that we don’t understand many of nature’s mysteries because we don’t, and may never, understand ourselves; that all of our creative restlessness, everything of beauty and substance we have ever made — our temples and our theorems, the Moonlight Sonata and general relativity — has sprung from our confrontation with the mystery of which we are a part. The Toltecs and the Aztecs gave shape to the mystery in Quetzalcoatl — their feathered god of creation and knowledge — staring at me from the base of the pyramid with the stony serenity of the centuries, knowing everything and knowing nothing.
