The theory that the ‘seeds of life’ (in the form of microorganisms or their dormant spores) exist throughout the universe and are transported by meteorites or other small celestial bodies. According to this strong panspermia theory, when such a meteorite collides with a planet the microorganisms are released, potentially enabling them to colonize new sites where conditions are suitable for life. Proponents argue that this could be how life on Earth originated. A so-called weak panspermia theory posits that merely some of the essential building blocks of life, such as amino acids or nucleobases, were delivered in this way, and that these contributed to the emergence of the first life forms uniquely on earth. Our planet experienced intense meteor bombardment about 4 to 3.8 billion years ago and there is now strong evidence that meteorites can contain organic molecules; but whether and how these enabled life to begin nearly 3.5 billion years ago is contentious. See also astrobiology.