"Until the second half of the nineteenth century, it was generally believed that life could arise spontaneously from nonliving matter, a process known as spontaneous generation. An early opponent of spontaneous generation, Francesco Redi, demonstrated in 1668 that maggots, the larvae of flies, do not arise spontaneously from decaying meat." (see source) In other words, life comes from life, cells come from cells, and it is generally thought that living matter cannot come from unliving matter.