Final answer:
Lazzaro Spallanzani observed that sealed flasks of heated broth did not show signs of life unless opened to air, challenging the belief in spontaneous generation. His findings contradicted those of John Needham and supported the principle that life comes from life.
Step-by-step explanation:
People once believed in the theory of spontaneous generation, which suggested that living organisms could arise from nonliving matter. Lazzaro Spallanzani, however, observed through his experiments that this was not the case. He used heated broth in sealed and unsealed jars, infused with plant and animal matter, and found that when the broth was sealed properly after boiling, no microbial life appeared unless the flasks were later exposed to air, which suggested that life came from preexisting life rather than spontaneously generating.
In contrast to Spallanzani's findings, John Needham believed that life came from a "life force" and that Spallanzani's methods destroyed this force, thereby preventing spontaneous generation. Both Spallanzani's and Francesco Redi's experiments were early scientific rebuttals to spontaneous generation, which would later be conclusively disproven by Louis Pasteur. While the dispute over spontaneous generation continued, the evidence collected by Spallanzani and others laid the groundwork for the eventual acceptance of the principle that life only comes from existing life.