Abstract

The fossil record is a (backwards) time machine that provides us with the occasional snapshot of former life on Earth. Phosphatic nodules from the Torridonian Sequence in in the NW Scottish Highlands preserve lake bottom sediments from 994 ± 48 Ma and some layers include 3-dimensionally preserved cells and cell clusters. The simple morphologies of unicellular organisms throughout much of the Precambrian record have permitted the recognition of only a handful of protist-level clades. The Torridonian, however, has preserved more complex morphology in the form of various cell clusters, some of which contain 2 clearly distinct cell types. A process of elimination, based on cell-cell morphology led us to propose the Ichthyosporea as the closest morphological analog to this new form, Bicellum Brasieri. Nick Butterfield pointed out years ago that individual Precambrian deposits might be conducive to the preservation of life-cycle varieties, and, indeed, we found several populations of what we infer to be the intermediate phases in the Bicellum life cycle. These include the initiation of multicellularity through the formation of a (multinucleate) syncytium, followed by cellularization into isodiametric cells forming a parenchymatous stereoblast. Some stereoblasts preserve an admixture of a small number of elongate cells within the largely isodiametric cell mass. Others show a single enclosing layer of elongate cells, that we infer to have migrated to the periphery of the cell mass. In the inferred mature, cyst, form, the elongated cells have thickened walls and they are devoid of cell contents.

Even though the dynamics of this reconstruction are speculative, fossil themselves do provide the basis for an independent morphogenic model from which to explore the early evolution of morphogenesis eukaryotes. For example, Malcolm Steinberg’s Differential Adhesion Hypothesis (DAH) provides a possible causal mechanism for the movement of elongate cells to the cell mass periphery. Similar dynamics have been shown in both adhesion experiments with living cell masses and computer simulations. It is interesting to speculate that mechanistic aspects of development in early eukaryotes may have incorporated a combination of D’Arcy Thompsonian ‘physical forces’ (or Kauffman’s ‘order for free’) in conjunction with the chemical environmental signalling that eventually led to genetic control of cell development and differentiation in the evolving holozoan lineage. The extent to which such elements will become incorporated into new models of hypothetical protistan ancestors to the Metazoa remains to be seen, but Bicellum does demonstrate that simple cell-sorting in protists was taking place in lake bottoms almost a billion years ago.