Explain how combining observations and facts to construct the theory of evolution which explains that life all originates from one source doesn't involve guess work. They took a puzzle with tons of pieces missing and put forth an explanation to explain the bits that they do know. We know evolution happens. Do we know everything about why, how, when, at what pace, origins??? No we do not. The explanation is a best guess on how to explain what we have observed thus it is a THEORY.I think scientists would normally ditch 'educated guess' for 'hypothesis,' which is more neatly defined.