Final answer:
The endosymbiotic theory is not a law because laws in science describe consistent observations and are often expressed mathematically, whereas the endosymbiotic theory explains the specific evolutionary event of eukaryotic cell development from prokaryotic cells, supported by strong biological evidence.
Step-by-step explanation:
The endosymbiotic theory is not a law in science because scientific laws describe universal and consistent observations under certain natural conditions, usually formulated by mathematical equations. Meanwhile, theories are well-substantiated explanations for various aspects of the natural world that incorporate facts, laws, and tested hypotheses. The endosymbiotic theory suggests eukaryotes originated from prokaryotic cells engaging in a symbiotic relationship, eventually evolving into the complex cells we see today. This theory is supported by a wealth of evidence, such as the presence of DNA within mitochondria and chloroplasts that is similar to bacterial DNA, their double-membrane structures, and the fact that they reproduce through a process similar to bacteria.
Despite the overwhelming evidence and widespread acceptance in the scientific community – with figures such as biologist Lynn Margulis making significant contributions – the process of categorizing it as a law is not applicable because it doesn't describe a consistent observation but rather an evolutionary event. Furthermore, the theory is continuously refined as new evidence comes to light, which is a natural part of the scientific process.