A hypothesis believed by many as the way we came to be as we are.
States that life began in a cell, evolved up the chain and eventually, from ape to man.
Debated by Fundamental Christians and some other religions mainly because of problems finding accurate missing links (many links have been disproved as falsified information.. don't just trust what your science teacher OR sunday school teacher told you.)
Creation- a main alternative to this idea is also a hypothesis, from a scientific standpoint. Unless you can prove something with observations and experimentation, it will ALWAYS be just a theory.
Scientific Method states:
1. collect data
2. form hypothesis
3. make observations/experiment
4. revise hypothesis
5. repeat until you have enough proof to
6. form a theory (generally accepted explanation based on qualitive and quantitive data)
7. after the test of time, it may be judged to be scientific law.