[Tribune7]

You could see through the microscope that individual cells of Volvox looked very similar to single-celled algae like Euglena. It wasn't hard, therefore, to see how multicellularity could have evolved.

So how did they? :-)

[Right Wing Professor]

Volvox evolution: there are whole books written about it. Alas, I haven't read them:

David L. Kirk
VOLVOX, Molecular-Genetic Origins of Multicellularity and Cellular Differentiation

There's also this and this. I have access to the second article; I'll read it and post a summary in the next few days.