The significance of the endosymbiotic theory in biology lies in its explanation for the origin of eukaryotic cells. This theory proposes that certain organelles, particularly mitochondria and chloroplasts, originated as free-living prokaryotes that entered into a symbiotic relationship with early eukaryotic ancestors. Over time, these prokaryotes became integral to the host cell, evolving into organelles that perform essential functions such as ATP production and photosynthesis. The theory supports the diversity of life and highlights evolutionary processes, demonstrating how complex cells can arise from simpler forms through mutualism and natural selection.