Science is a key advance in the free game C-evo, inaugurating the Industrial Age.

Of the three ways of reaching it, Philosophy+Theology is the one most chosen by AI nations. Humans with an eye on Ballistics (for Artillery) and Rocketry (for air unit speed) or concerned about enemy warships may instead research Metallurgy and hope to acquire partial knowledge of Philosophy or the more likely Theology (which allows Bach's Cathedral) by exchange with a friend.

Beyond Science, provided that you have Engineering, if defensive strength needs a little boost go for Chemistry then Explosives (which gets your ground unit defensive strength to 40 and also allows Engineers and Canals) but if it needs a bigger boost you may prefer Steam Engine > Railroad > Steel first so as to design a 16/48/2. With Railroad and Banking you can research Industrialization and each resulting Factory will (after construction) let you produce units and buildings faster.

Science, in C-evo a Key Technology, prerequisite to a whole new era

Any two of: Metallurgy, Philosophy, Theology
Allows: Nothing

Has military or other value: Some bonus tiles display new icons and start producing extra material or trade. See C-evo HOWTO: Appendix
Further advances
that require it:

Chemistry; Physics; Steam Engine; University