The theory of spontaneous generation (or
abiogenesis) proposes that living organisms can arise from nonliving matter. It was proposed by Aristotle (384–322 BC) and was widely accepted for almost 2000 years, until experiments by Francesco Redi (1626–1697) challenged it. In the 18th century, British scientist John T. Needham (1713–1781) conducted experiments suggesting that perhaps spontaneous generation of microscopic life was indeed possible, but in 1799, experiments by Italian scientist Lazzaro Spallanzani (1729–1799) reported results that contradicted Needham’s findings. The debate continued until experiments by French scientist Louis Pasteur (1822–1895), using swan-necked flasks that remained free of microbes, disproved the theory definitively.
The debate over spontaneous generation led in part to the development of a generalized scientific method by which questions are answered through observations of the outcomes of carefully controlled experiments. It consists of four steps:
1. A group of observations leads a scientist to ask a question about some phenomenon.
2. The scientist generates a hypothesis—a potential answer to the question.
3. The scientist designs and conducts an experiment to test the hypothesis.
4. Based on the observed results of the experiment, the scientist either accepts, rejects, or modifies the hypothesis.
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