The theory of biogenesis states that all living things arise from pre-existing living things. This means that life cannot spontaneously generate from non-living matter, but instead must come from existing life.

Here's a breakdown of the theory:

* Spontaneous generation (also known as abiogenesis) was a long-held belief that living organisms could arise from non-living matter, like maggots from rotting meat or mice from grain.

* Biogenesis challenges this idea, suggesting that life only arises from existing life.

* Experiments like those conducted by Louis Pasteur in the 19th century provided strong evidence for biogenesis, demonstrating that life only arises from pre-existing life.

While biogenesis explains how life continues to exist and reproduce, it doesn't explain the origin of life itself. Scientists are still exploring the conditions and processes that led to the first living organism on Earth, but biogenesis tells us that all life we see today can be traced back to that initial source.