Nicolas Steno is rarely heard of outside Intro to Geology, but anyone hoping to understand life on Earth should see how Steno expanded and connected those very concepts: Earth, life, and understanding. Born Niels Stensen in 1638 Denmark, son of a goldsmith, he was a sickly kid whose school chums died of plague. He survived to cut up corpses as an anatomist, studying organs shared across species. He found a duct in animal skulls that sends saliva to the mouth. He refuted Descartes' idea that only humans had a pineal gland, proving it wasn't the seat of the soul, arguably, the debut of neuroscience. Most remarkable for the time was his method. Steno never let ancient texts, Aristotelian metaphysics, or Cartesian deductions overrule empirical, experimental evidence. His vision, uncluttered by speculation or rationalization, went deep. Steno had seen how gallstones form in wet organs by accretion. They obeyed molding principles he knew from the goldsmith trade, rules useful across disciplines for understanding solids by their structural relationships. Later, the Grand Duke of Tuscany had him dissect a shark. Its teeth resembled tongue stones, odd rocks seen inside other rocks in Malta and the mountains near Florence. Pliny the Elder, old Roman naturalist, said these fell from the sky. In the Dark Ages, folks said they were snake tongues, petrified by Saint Paul. Steno saw that tongue stones were shark teeth and vice versa, with the same signs of structural growth. Figuring similar things are made in similar ways, he argued the ancient teeth came from ancient sharks in waters that formed rock around the teeth and became mountains. Rock layers were once layers of watery sediment, which would lay out horizontally, one atop another, oldest up to newest. If layers were deformed, tilted, cut by a fault or a canyon, that change came after the layer formed. Sounds simple today; back then, revolutionary. He'd invented stratigraphy and laid geology's ground work. By finding one origin for shark teeth from two eras by stating natural laws ruling the present also ruled the past, Steno planted seeds for uniformitarianism, the idea that the past was shaped by processes observable today. In the 18th and 19th centuries, English uniformitarian geologists, James Hutton and Charles Lyell, studied current, very slow rates of erosion and sedimentation and realized the Earth had to be way older than the biblical guestimate, 6000 years. Out of their work came the rock cycle, which combined with plate tectonics in the mid-twentieth century to give us the great molten-crusting, quaking, all-encircling theory of the Earth, from a gallstone to a 4.5 billion-year-old planet. Now think bigger, take it to biology. Say you see shark teeth in one layer and a fossil of an organism you've never seen under that. The deeper fossil's older, yes? You now have evidence of the origin and extinction of species over time. Get uniformitarian. Maybe a process still active today caused changes not just in rocks but in life. It might also explain similarities and differences between species found by anatomists like Steno. It's a lot to ponder, but Charles Darwin had the time on a long trip to the Galapagos, reading a copy of his friend Charles Lyell's "Principles of Geology," which Steno sort of founded. Sometimes giants stand on the shoulders of curious little people. Nicolas Steno helped evolve evolution, broke ground for geology, and showed how unbiased, empirical observation can cut across intellectual borders to deepen our perspective. His finest accomplishment, though, may be his maxim, casting the search for truth beyond our senses and our current understanding as the pursuit of the beauty of the as yet unknown. Beautiful is what we see, more beautiful is what we know, most beautiful, by far, is what we don't.