Final answer:
The endosymbiotic theory explains the origin of eukaryotic cells, suggesting that organelles like mitochondria were once independent prokaryotes that entered into a symbiotic relationship with a larger host cell, eventually evolving into the organelles we see today.
Step-by-step explanation:
The Endosymbiotic Theory
The endosymbiotic theory is a well-supported explanation that elucidates the origin of eukaryotic cells, which are characterized by having a nucleus and membrane-bound organelles such as mitochondria and chloroplasts. This theory hypothesizes that these organelles originated from free-living prokaryotes that were engulfed by larger host cells. The engulfed cells, which survived instead of being digested, entered into a mutually beneficial symbiotic relationship with their host. Over time, they became permanent residents, evolving into the organelles we see in eukaryotic cells today. Mitochondria, for example, have their own DNA and replicate independently within the cell, which supports the idea that they were once independent prokaryotic organisms.
Scientist Lynn Margulis significantly contributed to the acceptance of this theory in the 1960s, by providing evidence that eukaryotic cell organelles such as mitochondria and chloroplasts were once free-living bacteria. These organelles provide essential functions, such as energy production and photosynthesis, and carry their own genetic material, which is distinct from the nuclear DNA of the eukaryotic cells they reside in.
Endosymbiosis has allowed eukaryotic cells to develop complex functions and gave rise to the diverse forms of eukaryotic life we observe today. By understanding this evolutionary relationship, scientists can continue to uncover how life has evolved over the billions of years since eukaryotic cells first appeared.