The endosymbiotic theory proposes that eukaryotic cells evolved from a symbiotic relationship between different types of prokaryotic cells. This theory was first proposed by biologist Lynn Margulis in the 1960s and has since become widely accepted in the scientific community.

What is the Endosymbiotic Theory?

According to the endosymbiotic theory, eukaryotic cells evolved from a symbiotic relationship between different types of prokaryotic cells. Specifically, it suggests that mitochondria and chloroplasts, two organelles found in eukaryotic cells, were once free-living bacteria that were engulfed by another type of cell.

Under this theory, the host cell provided protection and nutrients to the engulfed bacteria, while the bacteria provided additional energy to the host cell through processes like cellular respiration (in the case of mitochondria) or photosynthesis (in the case of chloroplasts). Over time, these relationships became permanent and gave rise to modern-day eukaryotic cells with their characteristic organelles.

Who Was Lynn Margulis?

Lynn Margulis was an American biologist who first proposed the endosymbiotic theory in 1967. Born in Chicago in 1938, she earned her bachelor’s degree from the University of Chicago before completing a PhD at UC Berkeley.

Throughout her career, Margulis was known for her work on symbiosis and evolution. In addition to proposing the endosymbiotic theory, she also developed other theories related to symbiosis and collaborated with other scientists on research related to topics like oceanography and environmental conservation.

Margulis received numerous awards for her work during her lifetime, including a National Medal of Science in 1999. She passed away in 2011 but continues to be remembered as one of the most important biologists of her generation.


The endosymbiotic theory has become widely accepted as an explanation for how eukaryotic cells evolved. By proposing that mitochondria and chloroplasts were once free-living bacteria, Lynn Margulis changed the way scientists think about the origins of life on Earth. Her work continues to inspire new research into symbiosis, evolution, and the relationships between different types of organisms.