What does the endosymbiosis theory explain regarding eukaryotic cells?

The endosymbiosis theory is an important concept in evolutionary biology that specifically addresses how eukaryotic cells originated from prokaryotic cells. This theory suggests that eukaryotic cells arose through a symbiotic relationship between different species of prokaryotes. According to the theory, certain prokaryotic cells were engulfed by ancestral eukaryotic cells and established a mutually beneficial relationship. Over time, these engulfed prokaryotes, which were likely the ancestors of mitochondria and chloroplasts, became integral components of the eukaryotic cell, eventually losing much of their independence and functionality.

This theory provides a framework for understanding the complex cellular structures and organelles found in eukaryotic cells, contrasting significantly with the simpler structure of prokaryotic cells. By explaining this crucial evolutionary step, the endosymbiosis theory helps clarify the progression from single-celled organisms to more complex life forms that include plants, animals, and fungi.