Nicolaus Steno
Steno, Nicolaus (1638-1686) was a Danish anatomist and geologist whose work provided a deeper understanding of anatomy, geology, paleontology, and crystallography. He proposed a scientific explanation for fossils and geological strata long before geology was recognized as a legitimate science.
Steno was born Niels Stensen, but is better known by the Latinized form of his name, Nicolaus Steno. He converted from his native Lutheran religion to Catholicism in 1667. He received his M.D. degree from the University of Leiden, in the Netherlands, in 1664 and spent most of his life in Italy due to his religious beliefs.
Settling in Tuscany in 1666, Steno served as house physician to Grand Duke Ferdinand II of Tuscany. Over the next 10 years he published several important treatises detailing his findings in both anatomy and geology. He was ordained as a priest in 1675 and, after becoming a bishop in 1677, abandoned science to spend the rest of his life serving his faith.
Steno's contributions as an anatomist included discoving the “duct of Steno,” part of the salivary gland system, and proving the heart to be composed primarily of a muscle. He also provided early details of the lachrymal gland and explained the function of the brain and the ovaries.
As a geologist he recognized, as did a few of his contemporaries, that fossils had once been living organisms that were petrified after death, and his work with fossils led him to try to understand the more general question of how any solid object (for example, a fossil) could come to be found inside another solid object, such as a rock or a layer of rock. Steno reasoned that rock strata and similar deposits were generally formed in horizontal layers. This reasoning led to his most important contribution to geology: Steno's law of superposition. It states that the earth's strata are layered, with the oldest layer on the bottom and the youngest layer at the top, unless later processes disturbed this arrangement. Thus, he was essentially the first to identify the evolution of the earth over time by means of geology.