In an effort to determine how, why, and when multicellularity arose from single-celled ancestors, Xiao and his collaborators looked at phosphorite rocks from the Doushantuo Formation in central Guizhou Province of South China, recovering three-dimensionally preserved multicellular fossils that showed signs of cell-to-cell adhesion, differentiation, and programmed cell death — qualities of complex multicellular eukaryotes such as animals and plants.
The discovery sheds light on how and when solo cells began to cooperate with other cells to make a single, cohesive life form.
The complex multicellularity evident in the fossils is inconsistent with the simpler forms such as bacteria and single-celled life typically expected 600 million years ago.
Actually, we might find multicellulars from earlier eras still. Don’t the comb jellies, far more than merely multicellular, date back to that era? And that pre-Cambrian animal with muscles (560 mya)? Let’s see what else turns up.
Here’s the abstract:
Phosphorites of the Ediacaran Doushantuo Formation (~600 million years old) yield spheroidal microfossils with a palintomic cell cleavage pattern1, 2. These fossils have been variously interpreted as sulphur-oxidizing bacteria3, unicellular protists4, mesomycetozoean-like holozoans5, green algae akin to Volvox6, 7, and blastula embryos of early metazoans1, 2, 8, 9, 10 or bilaterian animals11, 12. However, their complete life cycle is unknown and it is uncertain whether they had a cellularly differentiated ontogenetic stage, making it difficult to test their various phylogenetic interpretations. Here we describe new spheroidal fossils from black phosphorites of the Doushantuo Formation that have been overlooked in previous studies. These fossils represent later developmental stages of previously published blastula-like fossils, and they show evidence for cell differentiation, germ–soma separation, and programmed cell death. Their complex multicellularity is inconsistent with a phylogenetic affinity with bacteria, unicellular protists, or mesomycetozoean-like holozoans. Available evidence also indicates that the Doushantuo fossils are unlikely crown-group animals or volvocine green algae. We conclude that an affinity with cellularly differentiated multicellular eukaryotes, including stem-group animals or algae, is likely but more data are needed to constrain further the exact phylogenetic affinity of the Doushantuo fossils. (paywall)