Niels Steensen or Nicolaus Steno (1639–86) was a Danish scientist and theologian who is famous for his significant breakthroughs in especially anatomy and geology. He was the first to study the human heart scientifically and to discover that it is a muscle, and he examined the human brain and criticized earlier descriptions of its parts and their functions.
In the field of geology, he formulated several pioneering theories relating to the study of strata, i.e. layers in soil or rock, and described how such layers would form chronologically through sedimentation. He also studyed fossils, in particular suggesting a theory that could describe the process of fossilization through chemical changes.
However, in the late 1660s, while studying in Florence, Niels Steensen converted from Protestantism to Catholicism. While he continued to study and teach scientific subjects for some years, by the mid-1670s he had dedicated himself to fully to theology, becoming an important figure in the Counter-Reformation. He was made Bishop of Schwerin in 1676, working to promote Catholicism in the predominantly Protestant Northern Germany until his death ten years later.
Niels Steensen was originally buried in the Basilica di San Lorenzo in Florence, but his body was exhumed in 1953 and reburied in the "Capella Stenoniana" in the same basilica. He was beatified by the Catholic Church in 1987. As of 2008, his case for canonization is still being considered by the Holy See.