Question:

How did multicellularity originate?

Claim:

Single celled organisms evolved into multicellular organisms because of a combination of cell arggegation and cell proliferation.

Initial Model:

bubbl_thing_1.jpg


Evidence:

from J Wiley online library:

Multicellular organisms probably originated as groups of cells formed in several ways, including cell proliferation from a group of founder cells and aggregation. Cooperation among cells benefits the group, but may be costly (altruistic) or beneficial (synergistic) to individual cooperating cells. In this paper, we study conflict mediation, the process by which genetic modifiers evolve that enhance cooperation by altering the parameters of development or rules of formation of cell groups. We are particularly interested in the conditions under which these modifiers lead to a new higher-level unit of selection with increased cooperation among group members and heritable variation in fitness at the group level. By sculpting the fitness variation and opportunity for selection at the two levels, conflict modifiers create new functions at the organism level. An organism is more than a group of cooperating cells related by common descent; organisms require adaptations that regulate conflict within. Otherwise their continued evolution is frustrated by the creation of within-organism variation and conflict between levels of selection. The evolution of conflict modifiers is a necessary prerequisite to the emergence of individuality and the continued well being of the organism. Conflict leads — through the evolution of adaptations that reduce it — to greater individuality and harmony for the organism.

from cambridge journal:

While early eukaryotic life must have been unicellular, multicellular lifeforms evolved multiple times from protistan ancestors in diverse eukaryotic lineages. The origins of multicellularity are of special interest because they require evolutionary transitions towards increased levels of complexity.
. We have generated new sequence data from the nuclear large subunit ribosomal DNA gene (LSU rDNA) and the SSU rDNA gene of several unicellular opisthokont protists – a nucleariid amoeba (Nuclearia simplex) and four choanoflagellates (Codosiga gracilis, Choanoeca perplexa,Proterospongia choanojuncta and Stephanoeca diplocostata) to provide the basis for re-examining relationships among several unicellular lineages and their multicellular relatives (animals and fungi). Our data indicate that: (1) choanoflagellates are a monophyletic rather than a paraphyletic assemblage that independently gave rise to animals and fungi as suggested by some authors and (2) the nucleariid filose amoebae are the likely sister group to Fungi

Final Reasoning:

The phenomenon of multicellularity evolved from cell aggregation and cell proliferation. These two concepts are both ways that cells seem to naturally congregate, creating cell colonies. These cell colonies would either become synergistic (beneficial to each individual cell) or altruistic (not beneficial for each individual cell, but beneficial as a group). Cell groups would experience exclusive benefits and continue to multiply due to natural selection and their ability to survive, then natural selection occurs and these cell groups evolve. Three major eukaryotic groups (fungi, plants, animals) come from these multicellular colonies based on adaptation, chlorophyll, and sources of nutrients. Thus, multicellularity evolved

Final Model:

zz.jpg