Lynn Margulis made defining achievements in biology by proposing the endosymbiotic theory, which revolutionized the understanding of cell evolution. Her work demonstrated that mitochondria and chloroplasts originated as free-living bacteria that formed symbiotic relationships with host cells. This theory challenged the conventional view of evolution and emphasized the importance of collaboration in natural selection. Margulis also contributed significantly to the study of microbial evolution and multicellularity. Her advocacy for holistic approaches in environmental science highlighted the interdependency of life forms, further influencing the field of symbiosis.