Multiple origins of multicellularity and biological cooperation

Multicellularity has more than one history

Multicellular organisation evolved in several lineages, including animals, land plants, fungi and different algal groups. The details differ: cells may remain together after division or come together during part of a life cycle, and their dependence on the group can vary. A definition based only on 'more than one cell' captures different arrangements from a definition requiring tightly integrated development and division of labour.

Grosberg and Strathmann's review compares such routes and the conditions for cooperation. Shared ancestry, repeated group reproduction and control of within-group conflict are useful distinctions. Clonal assembly can make initial interests more similar, but later mutation can still introduce differences. Aggregation can join cells with distinct ancestry, yet it does not automatically prevent an organised collective life cycle.

The first model is deliberately simpler than any whole natural history. One C founder undergoes three rounds of binary division, with no mutation: one becomes two, then four, then eight C cells. The second panel joins four C and four D cells that were already separate. There is no division after they join. Both groups contain eight cells, but only the first supplied history derives them all from the same immediate founder.

Cooperation concerns consequences

Cell labels C and D denote assigned genotypes, not moral character. To call a behaviour cooperative, examine how it affects the persistence or reproduction of other cells or of the group. To identify a conflict, specify the competing consequences. A cell that divides more can contribute to a larger group in one setting and disrupt its coordinated development in another.

Imagine a group that requires some cells to collect usable resources and others to produce dispersing offspring. An offspring may begin the next group, making the group's arrangement heritable through its developmental programme. A cell's contribution then matters partly because of what happens to later groups, not solely because of how many times that particular cell divides now.

Ratcliff and colleagues' 2012 experimental evolution study provides one bounded case: under selected laboratory conditions, yeast populations evolved multicellular clusters with group-level propagation and features associated with a multicellular life cycle. This is evidence that a transition can be studied experimentally. It does not reproduce all independent natural origins or establish their universal timetable.

Two requirements can limit the same output

The invented group always contains eight cells. W worker cells collect and process external material into two usable resource units each. The remaining R=8−W cells can each make at most two offspring. Each offspring costs two resource units. Workers do not create matter or energy; the model assumes the necessary external resources are available.

Resources can therefore support at most W offspring, while reproductive cells can make at most 2R. The group's output is the smaller limit: min(W,2R). This rule makes the trade-off visible. Too few workers restrict supply. Too few reproductive cells restrict production even when usable resources remain.

The plot has two capacity lines and the actual output beneath their lower limit. At W=5, resource support is five and reproductive capacity is six, giving five offspring. At W=6, resource support rises to six but reproductive capacity falls to four, giving four offspring. More resource-gathering cells do not improve output indefinitely.

A specialised cell can contribute without reproducing

In the model, roles are regulated differences within one inherited group programme. They need not represent different genotypes. A worker's contribution can help offspring carrying the programme to form later groups, even if that worker does not itself produce an offspring in the interval. This separates cell-level reproduction from reproduction of an organised group.

Suppose one of five workers switches to the reproductive role. There are now four workers and four reproducers. Resource support falls to four, so group output falls from five to four despite increased reproductive capacity. The example shows a possible conflict between immediate cell roles and collective output. It has not assigned which genotype benefits, whether the switch is heritable or how frequent it becomes.

Real systems also face maintenance costs, spatial constraints, developmental reliability and changing resources. Their specialised roles cannot be derived from this single rule. What transfers is the reasoning: name the level of reproduction, identify the limiting processes and test whether a proposed change helps one component while damaging another.

Worked example

Check your understanding

1. Why do two eight-cell groups in the assembly diagram not have the same supplied history?

   Show reasoning

   The clonal group follows three binary-division rounds from one C founder without mutation. The aggregative group joins four pre-existing C and four D cells without subsequent division. Equal current cell count does not imply equal immediate ancestry or genotype composition.

2. Compare the original output of seven workers with that of five workers.

   Show reasoning

   Seven workers collect fourteen units, enough for seven offspring, but the one remaining reproductive cell can make only two. Output is min(7,2)=2. Five workers and three reproducers give min(5,6)=5. The extra resource supply cannot remove the shortage of reproductive capacity.

3. Do the five-worker maximum and the C/D labels identify the universal origin of cooperation or prove that D cells cheat?

   Show reasoning

   Neither does. Five workers is a result of an assigned numerical rule, not a natural optimum across organisms. C and D are genotype labels with no behaviour or fitness specified. Claims about cooperation or conflict need consequences, inheritance and group reproduction to be examined separately.

What to carry forward

Multicellularity involves several routes and levels of reproduction. Useful explanations follow cell ancestry, coordinated functions and conflicts rather than equating contact, clonality or size with complete integration.