Single-celled organisms are a supremely successful life form, easily accounting for more than half the living biomass on the Earth’s surface; a proportion that rises precipitously when considering subterranean lives.
It took 4 times longer for a eukaryotic cell to evolve from prokaryotes than it did for life to arise on Earth from inanimate matter. But duration does not account for what seems inevitable when viewed from the perspective of energy efficiency, an elemental evolutionary driver.
The evolution of cells permitted potentially competing metabolic processes to be localized and an energy-efficient metabolism to emerge. Increasingly complex metabolism evolved with intracellular compartments, courtesy of membranes which allow electrochemical potentials to develop and drive electrons between compartments in controlled processes. Semi-porous eukaryotic membranes act as filters and conduits; an evolutionary orchestration involving both exchange and defense.
Multicellular life forms evolved over a billion years ago. The transition from single cell to multicellular life is best understood as separate organisms in a shared ambient coming into closer interdependence, to fuse into a single, yet differentiated, being.
Difficult living conditions spur evolutionary adaptation. It is likely that environmental demands and opportunities drove eukaryotic union. The availability of oxygen as respiratory fuel may have sponsored the evolution of eukaryotes.
Eukaryotic cells have an important advantage over prokaryotes: size. Larger cells afford greater internal complexity, when coupled with compartmentalization that allows specialized functioning, as with organelles.
At one time chloroplasts were free-agent, light-gobbling cells: ancestral cyanobacteria. Their photosynthetic skills were so appreciated that they were incorporated by early eukaryotes.
In all algae and green plants living today, chloroplasts retain portions of their circular, prokaryote DNA, as well as other trappings of their ancient heritage. Chloroplasts divide by splitting, quite independently of cell division where they reside. But chloroplasts long ago forfeited genes essential for independence: genes which now reside in the cell nucleus. Chloroplasts gained a comfy home, while the host cell acquired a power plant.