It is well known that the term טבע doesn't appear in tanach. The truth is that it hardly appears in chazal. The Ramban in Hilchos Deios definitely uses the term to mean nature in describing the nature of fire. Our gemara is one of the few places that we find the term being used in chazal - טבעא דבבל גרמא לי דלא חזאי דמא. The ya'avetz points out that the term was well used in his time by scientists to refer to nature, and the source seems to be from here where it refers to the biological nature of the babylonians. Perhaps the root of the word is based on the gemara in Sanhedrin 37b where it says that Hashem is different than a normal craftsman. When human being who טובע כמה מטבעות בחותם אחד - makes many coins with the same stamp, they will all look the same. However, Hashem uses the same stamp to be מטבע all man kind and everyone looks different. The term טבע really means the stamp of Hashem which he used to stamp the world and set it in motion allowing it to take its natural course.