Find the large collection of useful biology calculators from our site! Biology is a broad scientific field that studies the interactions among living organisms. It has several unifying themes that link it all together. Evolution is also a major theme, which explains the unity of life. Also, it shows that organisms are capable of reproducing and controlling their environment. Biology has multiple subdisciplines, each of which is defined by the nature of the questions that they answer and the tools they use. Earth's diversity is astonishing. There are more than 400 known species of organisms living on Earth, and some of them are prokaryotic and eukaryotic. Word biology comes from the Ancient Greek words for the terms life and biologa.