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Stumbled across a somewhat interesting paper:
https://www.nature.com/articles/s41598-019-39558-8
Short version: a mobile microbe shifted to a colonial form in under 1,000 generations after being subjected to a predatory microbe that can't ingest the multicellular form.
The work that led to the paper involved setting up multiple cultures of an algae called Chlamydomonas, which is single celled and mobile - it has two very prominent flagella. Those cultures were then exposed to a single-celled predator that you may remember from school: paramecium. It has a groove in which it ingests smaller organisms for digestion. Critically for this work, the groove has a finite size; get big enough, and you can't be eaten.
Over about 700 generations, two of the cultures evolved so that the lived as larger clusters of cells; the mobile form was only used as a type of reproduction. One of the two formed random, disordered aggregates, but the second typically formed an 8 cell cluster. It appears that the clusters are the product of cell divisions - instead of budding off to form an independent living organism, cells stayed stuck to their partners after division, at least until the cluster got sufficiently large.
The paper hasn't reported any of the DNA changes that are involved with this. And there's still a big step (one might, in fact, call it a "gap") between this and having specialized cell types. But having a life cycle that invariably includes a multicellular form is likely to be a prerequisite for evolving specialized cell types.
https://www.nature.com/articles/s41598-019-39558-8
Short version: a mobile microbe shifted to a colonial form in under 1,000 generations after being subjected to a predatory microbe that can't ingest the multicellular form.
The work that led to the paper involved setting up multiple cultures of an algae called Chlamydomonas, which is single celled and mobile - it has two very prominent flagella. Those cultures were then exposed to a single-celled predator that you may remember from school: paramecium. It has a groove in which it ingests smaller organisms for digestion. Critically for this work, the groove has a finite size; get big enough, and you can't be eaten.
Over about 700 generations, two of the cultures evolved so that the lived as larger clusters of cells; the mobile form was only used as a type of reproduction. One of the two formed random, disordered aggregates, but the second typically formed an 8 cell cluster. It appears that the clusters are the product of cell divisions - instead of budding off to form an independent living organism, cells stayed stuck to their partners after division, at least until the cluster got sufficiently large.
The paper hasn't reported any of the DNA changes that are involved with this. And there's still a big step (one might, in fact, call it a "gap") between this and having specialized cell types. But having a life cycle that invariably includes a multicellular form is likely to be a prerequisite for evolving specialized cell types.
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