Earliest multicellular life? Maybe not.
This week's paper is "Large colonial organisms with coordinated growth in oxygenated environments 2.1 Gyr ago", published in Nature by Abderrazak El Albani and colleagues. These two-billion-year-old fossils are clearly the remains of something living, based on appearance, different carbon isotope composition from the surrounding rock. But are they colonies of unicellular organisms, or something more organized?
The authors note the resemblance to "bacterial colonies growing on surfaces" which they say are known to "coordinate their behavior" often via "repulsive chemotaxis." In other words, cells detect the presence of neighbors and tend to disperse in ways that can generate patterns similar to those seen in these fossils. Sometimes, bacteria may coordinate their activities in more sophisticated ways, which wouldn't leave fossil traces.
A commentary on the article is titled "Origins of multicellularity", but most people would expect more from a multicellular organism than a little coordination among cells. We expect differentiation, for example, where different cells specialize for different functions. Trichoplax is thought to have four different cell types, including cells with flagella that move these simple multicellular animals along, and cells that excrete digestive enzymes. In larger animals, a few cells specialize in reproduction, a potential source of conflict, especially if there are genetic differences among cells. I don't think we can tell from these fossils whether there was any such specialization.
A consistent size and shape is another criterion for true multicellularity, met by Volvox, for example. The fossils don't look any more consistent in size and shape than one would expect from bacterial colonies. On the other hand, Trichoplax individuals seems to vary somewhat, but I wouldn't argue against calling them multicellular organisms.
Interestingly, the authors found traces of the chemical sterane, which is typically found in eukaryotes. But apparently there's some possibility that it could have diffuses into the fossil rock from younger materials.
I've been reading about multicellularity recently and was particularly impressed by a 1998 paper by Boraas et al., in Evolutionary Ecology, and titled "Phagotrophy by a flagellate selects for colonial prey: a possible origin of multicellularity". (I would link to the paper, but the publisher Springer has this stupid system that sends you to their main web page.) Boraas et al. exposed unicellular algae to predators and evolved multicellular clumps in only 10-20 generations. The first clumps contained hundreds of cells, but eventually they evolved an 8-cell phenotype, too big for the predators to eat, but with better access to nutrients for each cell than if they were in a larger clump. Surprisingly, nobody seems to have done further evolution experiments with this system.