Lynn Margulis was an American evolutionary biologist who profoundly altered the understanding of how complex life originated on Earth and how evolution proceeds. Her work fundamentally challenged the prevailing view that competition and gradual genetic mutation were the sole forces driving biological change. Instead, Margulis championed the role of cooperation and intimate association between different organisms. This perspective forced the scientific community to reconsider the nature of biological individuality and the history of life, shifting the focus to the foundational role of microorganisms.
The Endosymbiotic Theory
Margulis’s most recognized contribution is the modern revival and powerful substantiation of the endosymbiotic theory, which explains the origin of the complex cell structure known as the eukaryote. This theory posits that certain organelles within eukaryotic cells, such as those that manage energy production and photosynthesis, were once free-living bacteria. This concept proposes that the complex cells that make up all animals, plants, fungi, and protists arose from a series of mergers between distinct, simpler cells called prokaryotes.
The theory details a specific engulfment event where a large, single-celled organism absorbed a smaller, oxygen-respiring bacterium, but did not digest it. Instead, the smaller bacterium survived within the larger host, establishing a mutually beneficial relationship that eventually became permanent. This smaller cell evolved into the mitochondrion, the organelle responsible for generating the cell’s energy currency through aerobic respiration.
A similar, subsequent event occurred in the lineage leading to plants and algae, where a cell containing a mitochondrion engulfed a photosynthetic cyanobacterium, which then evolved into the chloroplast.
Molecular and structural evidence collected since Margulis first published her ideas strongly supports this ancient merger. The key evidence includes:
- Mitochondria and chloroplasts possess their own separate, circular DNA molecules, similar to the genomes found in bacteria.
- Both organelles reproduce independently within the cell by binary fission, the same method used by free-living bacteria.
- The ribosomes found within these organelles resemble those of bacteria and differ from the ribosomes in the surrounding host cell cytoplasm.
- The presence of a double membrane surrounding both organelles, where the inner membrane represents the original bacterial cell membrane.
Evolution Driven by Symbiosis
Margulis extended the endosymbiotic theory into a broader concept called symbiogenesis, proposing that the merging of different organisms to form a new, single entity is a major mechanism of evolutionary innovation. She argued that the most significant leaps in the history of life, such as the emergence of the eukaryotic cell, did not result from the gradual accumulation of minor random gene mutations, but from sudden, intimate, and permanent partnerships. This perspective contrasted sharply with the prevailing neo-Darwinian emphasis on competition and individual struggle for survival as the primary evolutionary engine.
Symbiogenesis suggests that large-scale novelty often arises through the acquisition of an entire new genome or a set of genes from a different species through a symbiotic event. Margulis speculated that other cellular components, such as the microtubules that form the internal scaffolding and movement structures of the cell, might have originated from an ancient association with spirochete bacteria. While the evidence for this specific proposal is less conclusive than for the organelles, it illustrates her broader view that cooperation, long-term cohabitation, and genetic sharing between species are the true drivers of macroevolutionary change.
Reclassifying Cellular Life
To organize the immense diversity of life, Margulis strongly advocated for a five-kingdom classification system. This system organizes life into Monera, Protista, Fungi, Plantae, and Animalia, a structure that became widely adopted in biological education. The foundational element of this classification is the clear separation of the prokaryotes—the Monera kingdom, which includes all bacteria and archaea lacking a nucleus—from the eukaryotes, which are all cells possessing a true nucleus.
This distinction was necessary for understanding endosymbiosis, clarifying which cell was the host (a proto-eukaryote) and which was the engulfed symbiont (a prokaryote). Within the eukaryotic domain, Margulis heavily emphasized the kingdom Protista, grouping together all single-celled eukaryotes and their immediate descendants. She viewed the Protista as the “catch-all” kingdom where the complex eukaryotic cell structure diversified before the evolution of multicellular life.
Collaboration on the Gaia Hypothesis
Margulis’s interest in global, large-scale cooperation led to her long-standing collaboration with chemist James Lovelock on the Gaia Hypothesis. This hypothesis proposes that the Earth’s biosphere, atmosphere, hydrosphere, and lithosphere interact as a self-regulating system that maintains the conditions necessary for life to thrive. The Earth, in this view, is not merely a passive planet, but an actively regulated system where living organisms control the global environment to maintain a stable, habitable state.
Margulis’s specific contribution was integrating the role of microorganisms into this global model. She stressed that bacteria and other microbes, which have dominated the planet for billions of years, are powerful regulators of global biogeochemical cycles. For example, she highlighted how microbial metabolism processes and circulates atmospheric components like nitrogen, sulfur, and methane, maintaining the chemical composition of the atmosphere and oceans.