The puzzle of the origin of the eukaryotic cell is extremely complicated. ~ Dutch microbiologist Thijs Ettema
To foster communication among the likeminded and thereby take colonial form, archaic bacteria evolved the genic production of kinases: enzymes which mediate chemical energy flows. This innovation was an essential pre-adaptation to cellular complexity, and to multicellularity. After all, bonding begins with communicating shared interests.
Going through life alone is a struggle which may be eased by partnership with another. Life as a cell isn’t any different.
~2.5 BYA, an archaean host and an endosymbiotic bacterium committed to an everlasting partnership via irreversible specialization; thus arose single-celled eukaryotes, around the time that continents were emerging, and oxygen levels beginning to rise.
As a pre-adaptation, host archaea had invented membrane technology that afforded keeping guests within their own quarters. As a lure for endosymbiosis, archaea were dining on fatty acids and butane – a diet of carbon-based molecules that would have generated nourishing byproducts for partner bacteria.
The concept caught on. A wily single-celled protist endosymbiotically retained a photosynthetic bacterium, giving rise to the plastid which afforded autotrophy. Whence came plants: from green algae which grew determined to make a living on land.
A variety of eukaryotic cell types arose over several hundred million years via waves of endosymbiotic inspiration. All relied upon extensive intracellular communication to be able to live and work together.
Another cell type arose: a complex consumer of other life, requiring a diet of fresh prefabricated amino acids and vitamins. From petite worms all animals evolved.
Protists arose which straddled the demarcation between animals and plants. Protist is more a leftover grouping than a designation.
Diverging from animals 1 BYA, the forebearer of fungus got a strong cell wall and gained great metabolic versatility. It ate ready-made foodstuffs from other organisms, dead or alive.
Many of the major events in the diversification of life owe to species interactions. The consequences: mitochondria and the origin of the eukaryotic cell; chloroplasts and the origin of plants; dinoflagellates and the origin of coral reefs; lichens, mycorrhizae, and rhizobia and the process of terrestrial plant succession; gut symbionts and animal digestion. ~ American evolutionary biologist John Thompson
One impetus to eukaryotic evolution was efficiency. That division of labor is more productive than solitary toil is a universal fact in all social realms, from biofilms to the societies of creatures.
A communication system was necessary for eukaryotic emergence. This required a shared foundation. The answer was DNA.
Viruses were the vehicle for disseminating DNA to all other life and making it the universal standard. The employment of DNA was positively infectious.
Viruses modulate the function and evolution of all living things. ~ American viral ecologist Joshua Weitz et al
It is also quite likely that early pathogens were provocative in the emergence of eukaryotes. From the earliest life to present day, predation drives evolutionary responses.
Viruses have contributed enormously to the communication between cells, and to the appearance of multicellular organisms on Earth. ~ French virologist Felix Rey
At least one virus was instrumental in the origin of fungi. The virus hijacked the ancestor of fungi, seeking to turn it into a virus factory. The fungus that survived selectively incorporated vital viral bits that afforded pliability in development and gave it greater cooperative and parasitic abilities.
Once organisms have a shared communication system, and are thus able to optimize operations, specialization arises, followed inexorably by interdependence; whereupon eukaryotes evolved.
Many unicellular organisms have a colonial form (pluricellularity), often when food becomes scarce, or as a defense. Single-celled green algae exposed to unicellular predators are easy prey. Clumping makes a difference. Small cell colonies offer an ideal trade-off between security from predation and maintaining sufficient surface area for nutrient uptake.
Then next step was labor specialization. Prokaryotic colonial cells are not differentiated, but cells often coordinate to perform different tasks.
Capsaspora is an amoeba with 3 distinct life stages. The demands of solitude and solidarity with others at different stages calls for flexibility. The communication and coordination needed for multicellularity has similar demands. Though single-celled, Capsaspora has capabilities essential for multicellularity. Some of the same genic tricks have been seen in other single-celled organisms.