6 ideas
The nucleated cell is not a descendant of one lineage but a merger of formerly free-living bacteria acquired in sequence. Mitochondria descend from engulfed oxygen-respiring bacteria and chloroplasts from engulfed cyanobacteria, each retaining its own DNA and dividing on its own. Margulis also argued that undulipodia (cilia and flagella) came from spirochetes, but that third merger is disputed and has not won acceptance.
Symbiogenesis is the origin of new tissues, organs, or species through the long-term merging of different organisms into one. Selection alone cannot create major innovations. Heritable novelty comes from organisms fusing their whole genomes and metabolic abilities, not only from gradually piling up random point mutations within one lineage.
Treat any individual plant or animal as a stable consortium of once-separate beings, not as a single self. Seen this way, the boundaries of 'individual' blur. A human, a cow, or a lichen is an ecosystem of cooperating lineages whose joint persistence is what evolution actually works on.
Mainstream zoology-centered theory treats random mutation plus competitive selection among individuals as enough to explain life's history. Margulis argues this ignores microbes, which make up most of evolutionary history, and ignores lateral genome acquisition. The result is a framework that describes change within species but cannot explain how new kinds of organisms appear.
A lichen is a fungus and an alga or cyanobacterium fused into a new form that neither partner could produce alone. Lichens were long classified as single organisms. They show plainly how integrating two lineages yields a novel organism with new capacities, such as colonizing bare rock.
Earth's surface environment, including atmospheric composition, temperature, and chemistry, is regulated by the combined metabolism of its living organisms, mostly microbes. This regulation is not purposeful. It emerges from countless interacting organisms whose waste products and uptakes form feedback loops that keep conditions habitable.