NPR's Krulwich celebrates his ignorance
As a long-time supporter of National Public Radio, I was disappointed by Robert Krulwich's incorrect claim that "we don't know" why it took so long for single-celled life to "join with another" and become multicellular.
Multicellularity probably didn't evolve that way. When we exposed unicellular yeast to selection for larger size, clusters descended from single cells (reproducing but not separating) out-competed clusters formed by single cells coming together.
As for why it apparently took so long, the simplest explanations are:
1) multicellularity requires so many simultaneous mutations that the combination only arises every billion years or so, or
2) during the first 2-3 billion years of life on earth, conditions were such that there was little or no benefit to multicellularity.
Our experimental results favor the second hypothesis. With strong selection, simple multicellularity evolved in less than a month from unicellular yeast and in less than a year in other species (not yet published).