As his experiment, Redi prepared six containers with meat inside of them. Two of the containers were left open, two were sealed tightly, and the last two were covered with a gauze. Then, his hypothesis was proved when maggots developed in the uncovered jars, but no maggots appeared in the containers covered with a gauze or in the tightly sealed jars. He concluded that maggots could only form when flies were able to lay eggs in the meat, and that the maggots were the offspring of flies, not the product of spontaneous generation.

Lazzaro Spallanzani got wind of the experiments Needham had done, and he did not agree with it's conclusions. He then performed hundreds of carefully executed experiments using heated broth just like Needham did. He prepared broth in sealed jars and unsealed jars that were infused with plant and animal matter. Spallanzani’s results contradicted the findings of Needham: Heated but sealed flasks remained clean, without any signs of spontaneous growth, unless the flasks were opened to the air. This suggested that microbes were introduced into these flasks from the air, not spontanously generated.

Needham, defeated and as a last minute struggle proposed that Lazzaro had destroyed a "life force" from which life comes from thanks to the extended boiling processes.

Louis Pasteur, a prominent French chemist accepted the challenge.

In 1858, Pasteur filtered air through a gun-cotton filter, examined the cotton, and found it full of microorganisms. Suggesting that exposure of a broth to air was not introducing a “life force” to the broth but rather airborne microorganisms.

Later, Pasteur made a series of flasks with long, twisted necks (“swan-neck” flasks), in which he boiled broth to sterilize it. The design allowed air inside the flasks to be exchanged with air from the outside, but prevented the introduction of any airborne microorganisms, which would get caught in the twists and bends of the flasks’ necks. If a life force besides the airborne microorganisms were responsible for microbial growth within the sterilized flasks, it would have access to the broth, whereas the microorganisms would not. He correctly predicted that sterilized broth in his swan-neck flasks would remain sterile as long as the swan necks remained intact.

Pasteur’s set of experiments disproved the theory of spontaneous generation and earned him the prestigious Alhumbert Prize from the Paris Academy of Sciences in 1862.

The spontaneous formation of organic molecules was first demonstrated experimentally in the 1950s, when Stanley Miller and Harold Urey showed that the discharge of electric sparks into a mixture of H₂, CH₄, and NH₃, in the presence of water, led to the formation of a variety of organic molecules. Although Miller's experiments did not precisely reproduce the conditions of primitive Earth, they clearly demonstrated the plausibility of the synthesis of organic molecules, providing the basic materials from which the first living organisms arose.

Some of these organisms ingested prokaryotic cells that then survived within the organism and developed a symbiotic relationship. Mitochondria formed when bacteria capable of aerobic respiration were ingested; chloroplasts formed when photosynthetic bacteria were ingested, which then eventually lost their cell wall and much of their DNA because they were not of benefit within the host cell. Mitochondria and chloroplasts cannot grow outside their host cell.

Evidence for this is based on the following:

1. Chloroplasts are the same size as prokaryotic cells, divided by binary fission, and, like bacteria, have FTS proteins at their division plane. The mitochondria are the same size as prokaryotic cells, divide by binary fission, and the mitochondria of some protists have FTS homologs at their division plane.

2. Mitochondria and chloroplasts have their own DNA that is circular, not linear.

3. Mitochondria and chloroplasts have their own ribosomes that have 30S and 50S subunits, not 40S and 60S.

4. Several more primitive eukaryotic microbes, such as Giardia and Trichomonas have a nuclear membrane but no mitochondria.

Libretexts. (2023, August 31). 7.8: The Endosymbiotic Theory. Biology LibreTexts. https://bio.libretexts.org/Bookshelves/Microbiology/Microbiology_(Kaiser)/Unit_4%3A_Eukaryotic_Microorganisms_and_Viruses/07%3A_The_Eukaryotic_Cell/7.8%3A_The_Endosymbiotic_Theory

Other theories that some scientists support are the "RNA world" hypothesis. This hypothesis suggests that the first life was self-replicating RNA. Others favor the metabolism-first hypothesis, placing metabolic networks before DNA or RNA.

And lastly, the Panspermian theory, this hypothesis was first given life by Greek philosopher Anaxagoras who asserted that life traveled in the universe in the form of seed. Nowadays, the theory states that spores from other planets came to other recipient planets on board meteorites.