Final answer:
The probability of endosymbiosis occurring cannot be easily quantified because it represents a singular and complex event in life's evolutionary history. The endosymbiotic theory explains the emergence of organelles like mitochondria and chloroplasts and is supported by genetic and biological evidence, though modern instances of endosymbiosis can still be observed.
Step-by-step explanation:
The probability of endosymbiosis occurring is not a straightforward numerical value that can be easily quantified. The concept of endosymbiosis is deeply rooted in the evolutionary history of life and refers to an event where one organism lives symbiotically inside another organism. According to the endosymbiotic theory, such events have been pivotal in the evolution of eukaryotic cells, leading to the development of organelles such as mitochondria and chloroplasts from previously free-living prokaryotes.
Lynn Margulis was instrumental in developing and popularizing the endosymbiotic theory in the 1960s. The evidence supporting this theory includes the fact that mitochondria and chloroplasts contained their own DNA, which is similar to bacterial DNA, and that these organelles replicate through a process similar to bacterial binary fission. Additionally, modern examples of endosymbiosis also support the validity of this theory. For instance, we can see endosymbiotic relationships in various protists, where bacteria-like organelles exist within the cell, benefiting both the host and the endosymbiont.
However, attaching arbitrary probabilities to such singular and complex historical events is not feasible in the study of evolution. The endosymbiotic events that led to eukaryotic cell evolution are understood to be unique and even rare when considering the vast timescale of life's history on Earth. Therefore, it is more appropriate to focus on the mechanisms and evidence of endosymbiosis rather than trying to calculate its probability.