Eukaryotes evolved in a world filled with bacteria and throughout their shared history these two branches of life have developed a complex set of ways to compete and cooperate with each other. ~ American cytologist Rosanna Alegado et al
Multicellularity arose by 2.1 BYA, and independently evolved at least 46 times in all domains of life. The genetic capability was in place well beforehand. Multicellularity was an adaptive response.
Cooperation is wired into cells. ~ American cytologist Thomas Zwaka
Multicellularity was an evolutionary déjà vu to the emergence of eukaryotes. Both were spurred by cellular cooperation and the promise of greater productivity via division of labor. The primary challenge of multicellularity was coordination among cells to optimize efficacy. Shared chemical languages begat the specialization that characterizes and benefits multicellular organisms.
Multicellularity has its challenges, most notably renegades. Cells in a collective must live and die cooperatively.
When cells die in a group, they could poison each other. Instead, dead animal cells provide recyclable raw materials for those still going (autophagy): a tremendous savings for a heterotroph.
Another peril posed by multicellularity is a single cell growing at the expense of others. That hazard still looms with cancer, where cells refuse to play by the rules.
Early multicellular organisms evolved defenses against miscreants. The green algae Volvox programmatically limits the number of times a cell can divide.
That helps reduce the potential for cells to become renegade. ~ American evolutionary biologist Richard Michod
Choanoflagellates are flagellate eukaryotes, existing as free-living single cells or in rosette-shaped colonies. They are the closest cellular relative to metazoa (animals).
Choanoflagellates evolved not long before the origin of animals. ~ American cytologist Nicole King
The bacterium Algoriphagus machipongonensis inspires a unicellular choanoflagellate to divide into a multicellular organism. The individual cells interact as a coordinated cluster. A specific sulfonolipid in the bacterium’s cell membrane provides the biochemical inducement for choanoflagellate division. The process is reminiscent of animal embryology.
Gliding bacteria employ sulfonolipids in their cell membranes to move about. In multicellular eukaryotes, sulfonolipids regulate cell migration and differentiation.
It is likely that endosymbiotic bacteria were instrumental in the evolution of multicellular eukaryotes. This is consistent with the ongoing importance that microbiota play in eukaryotic life.
Pluricellular clusters, such as biofilms, readily form when conditions are favorable. When the situation changes and single cells are again advantageous, these pluricellular congregations go back to more solitary lifestyles.
Things had to change to enforce multicellularity. One of those things was apoptosis: programmed cell death. It seems odd that cellular suicide is beneficial, but higher rates of apoptosis – with dying cells acting as weak links in cellular connectivity – allow cellular clusters to circumvent growth constraints imposed by physical volume and nutrient flow limitations.
Once apoptosis evolved, along with other traits that engendered and enforced cooperative cellular ventures, such as language development and the trust inherent in specialization, the benefits of multicellularity both ratcheted growth prospects and precluded reversibility to single-cell living.
Multicellularity has arisen independently many times. ~ Michael Benton