Bicellum brasieri

Mature Bicellum brasieri. In image A, the boundary between the outer cell layer and the internal cell cluster can be clearly seen. In image E, the arrangement of the sausage-shaped elongated cells is clearly visible. (Image source: Strother, P. K. et al. (2021). ，採用 CC BY 4.0 授權）

Age

Proterozoic (Neoproterozoic)

1000 Ma

---

Taxonomy

Domain：Eukaryota

Genus：Bicellum

Species：Bicellum brasieri

---

Morphological description

The outermost layer of Bicellum brasieri consists of thicker-walled, sausage-shaped cells that form an enclosing layer. These sausage-shaped cells have diameters of about 1.5 to 2 μm and lengths approximately three to four times their diameter, with an average width-to-length ratio of 0.28. The peripheral cells are tightly packed together, often occurring in sets of four or more adjacent cells arranged parallel to one another, forming a rigid spherical outer shell. In cross-section, the spherical shell composed of sausage-shaped cells is only one cell thick. The cell walls of these outer cells appear darker in color, indicating that they are thicker.

Inside is a tightly compressed spherical cluster of cells. The cells that make up this cluster are closely packed with no spaces between them, indicating that these cells likely lacked rigid cell walls during life, or at least possessed very thin and somewhat plastic ones. Many of the contact lines between cells are straight, and the junctions where three cells meet form Y-shaped intersections of approximately 120°.

Naked cell clusters of Bicellum brasieri. The arrows indicate elongated cells forming within the cell cluster. (Image source: Strother, P. K. et al. (2021). ，採用 CC BY 4.0 授權）

In addition to typical specimens of Bicellum brasieri, fossils also preserve some naked cell clusters that lack the outer cell layer. These clusters are essentially indistinguishable in morphology from the internal cell clusters of Bicellum brasieri. After analysis, elongated cells were found within these naked clusters, and in some specimens these elongated cells appear to have migrated to the periphery of the cluster. However, compared with the outer cells of Bicellum brasieri, these cells do not possess thick cell walls. Researchers infer that these clusters likely represent the same organism, but with the outer cell layer still in the process of development.

Although the overall cell cluster is nearly perfectly spherical, the positions of the internal cells do not appear to be fixed. These cells could exchange positions, behaving somewhat like a liquid. The spherical shape may result from cohesive forces between cells that cause them to aggregate together.

---

Etymology

The genus name Bicellum derives from "Bi," meaning two, and "cell," referring to the presence of two different cell types.

The species name honors Martin Brasier, who devoted himself to studying the fossils of Bicellum brasieri until his death.

---

Biological description

Bicellum brasieri was discovered in strata near Loch Torridon in Scotland, United Kingdom. The sediments of this formation are interpreted as deposits from an ancient lake environment, suggesting that Bicellum brasieri lived in freshwater habitats.

Because the internal cell cluster lacks rigid cell walls, it is considered unlikely to belong to the plant lineage. Additionally, Y-shaped junctions are commonly associated with the cellular structures of animal tissues and are often used to infer phylogenetic relationships between ancient protists and animals. Based on these analyses, researchers consider Bicellum brasieri to be an early member of the Holozoa.

Schematic diagram of the developmental growth of Bicellum brasieri. (Image source: Strother, P. K. et al. (2021).，採用 CC BY 4.0 授權 ）

The cellular organization of Bicellum brasieri can be explained using the Differential Adhesion Hypothesis proposed by Malcolm Steinberg. This hypothesis suggests that when aggregates of cells behave like liquids, their overall structure is determined by the relative adhesion strengths between cells. In Bicellum brasieri, the adhesion among the internal cells is stronger than that between those cells and the newly formed elongated cells. As a result, the elongated cells are pushed to the outermost layer, while the remaining cells form a spherical cluster inside. This process may represent an early origin of tissue differentiation in multicellular organisms.

(Author: Bai Leng)

---

Reference

Strother, P. K., Brasier, M. D., Wacey, D., Timpe, L., Saunders, M., Wellman, C. H. (2021). A possible billion-year-old holozoan with differentiated multicellularity. Current Biology.

+57.2566, -5.3003

---