Molecular fossils (or biomarkers) are key to unraveling the deep history of eukaryotes, especially in the absence of traditional fossils. In this regard, the sterane 24-isopropylcholestane has been proposed as a molecular fossil for sponges, and could represent the oldest evidence for animal life. The sterane is found in rocks ∼650-540 million y old, and its sterol precursor (24-isopropylcholesterol, or 24-ipc) is synthesized today by certain sea sponges. However, 24-ipc is also produced in trace amounts by distantly related pelagophyte algae, whereas only a few close relatives of sponges have been assayed for sterols. In this study, we analyzed the sterol and gene repertoires of four taxa (Salpingoeca rosetta, Capsaspora owczarzaki, Sphaeroforma arctica, and Creolimax fragrantissima), which collectively represent the major living animal outgroups. We discovered that all four taxa lack C 30 sterols, including 24-ipc. By building phylogenetic trees for key enzymes in 24-ipc biosynthesis, we identified a candidate gene (carbon-24/28 sterol methyltransferase, or SMT) responsible for 24-ipc production. Our results suggest that pelagophytes and sponges independently evolved C 30 sterol biosynthesis through clade-specific SMT duplications. Using a molecular clock approach, we demonstrate that the relevant sponge SMT duplication event overlapped with the appearance of 24-isopropylcholestanes in the Neoproterozoic, but that the algal SMT duplication event occurred later in the Phanerozoic. Subsequently, pelagophyte algae and their relatives are an unlikely alternative to sponges as a source of Neoproterozoic 24-isopropylcholestanes, consistent with growing evidence that sponges evolved long before the Cambrian explosion ∼542 million y ago.sponges | Porifera | sterols | steranes | Amorphea

Despite numerous attempts, classification of the Precambrian fossil Dickinsonia has eluded scientific consensus. This is largely because Dickinsonia and its relatives are structurally simple, lacking morphological synapomorphies to clarify their relationship to modern taxa. However, there is increasing precedence for using ontogeny to constrain enigmatic fossils, and growth of the type species Dickinsonia costata is well understood. This study formalizes the connection between ontogeny in Dickinsonia-which grows by the addition of metameric units onto one end of its primary axis-with terminal addition, defined as growth and patterning from a posterior, subtermial growth zone. We employ ancestral state reconstruction and stochastic character mapping to conclude that terminal addition is a synapomorphy of bilaterian animals. Thus, terminal addition allies Dickinsonia with the bilaterians, providing evidence that large stem- or crown-group bilaterians made up a significant proportion of the Precambrian biota. This study also illustrates the potential for combining developmental and phylogenetic data in constraining the placement of ancient problematic fossil taxa on the evolutionary tree.

The study of stem cells in cnidarians has a history spanning hundreds of years, but it has primarily focused on the hydrozoan genus Hydra. While Hydra has a number of self-renewing cell types that act much like stem cells--in particular the interstitial cell line--finding cellular homologues outside of the Hydrozoa has been complicated by the morphological simplicity of stem cells and inconclusive gene expression data. In non-hydrozoan cnidarians, an enigmatic cell type known as the amoebocyte might play a similar role to interstitial cells, but there is little evidence that I-cells and amoebocytes are homologous. Instead, self-renewal and transdifferentiation of epithelial cells was probably more important to ancestral cnidarian development than any undifferentiated cell lineage, and only later in evolution did one or more cell types come under the regulation of a "stem" cell line. Ultimately, this hypothesis and competing ones will need to be tested by expanding genetic and developmental studies on a variety of cnidarian model systems.