Multicelled fossils from the 1 Ga old Torridonian of Scotland

Beinn Alligin and Loch Torridon, Northwest Highlands of Scotland. The hills are formed by Torridonian terrestrial sediments (credit: Stefan Krause, Wikimedia Commons)

Palaeobiologists interested in the origin of animals have generally focussed on sedimentary rocks from southern China: specifically those of the 635 to 550 Ma Doushantuo Formation. Phosphorus-rich nodules in those marine sediments have yielded tiny spheroids whose structure suggests that they are fossil embryos of some unspecified eukaryote. The Doushantuo Formation lies on top of rocks associated with the Marinoan episode of global glaciation during the Neoproterozoic; a feature which suggested that the evolutionary leap from single- to multi-celled eukaryotes was associated with environmental changes associated with Snowball Earth events. In a forthcoming issue of Current Biology that view will be challenged and the origin of multicellular life pushed back to around 1 billion years ago (Strother, P.K. et al. 2021. A possible billion-year-old holozoan with differentiated multicellularity. Current Biology, v. 31, p. 1-8; DOI: 10.1016/j.cub.2021.03.051). Spherical fossils of that age have been teased out of phosphatic nodules deposited in lacustrine sediments from the lower part of the Mesoproterozoic Torridonian Group of the Northwest Highlands of Scotland.

The internal structure of the fossils has been preserved in exquisite detail. Not only are cells packed together in their interiors, but some reveal an outer layer of larger sausage-shaped cells. So, cell differentiation had taken place in the original organisms, whereas such features are not visible in the Doushantuo ‘embryos’. A few of the central cells show dark, organic spots that may be remains of theirnucleii. Whatever these multicellular spheres may have developed into, the morphology of the Torridonian fossils is consistent with a transition from single-celled holozoans to the dominant metazoans of the Phanerozoic; i.e. the stem of later animals. The younger, Chinese fossils that are reputed to be embryos cannot be distinguished from multicellular algae (see: Excitement over early animals dampened, January 2012)

Photomicrograph of Bicellum brazieiri: scale bar = 10μm; arrows point to dark spots that may be cell nuclei (credit: Charles Wellman, Sheffield University)

Interestingly, the Torridonian Group is exclusively terrestrial in origin, being dominated by sediments deposited in the alluvial plains of huge braided streams that eventually buried a rugged landscape eroded from Archaean high-grade metamorphic rocks. Thus the environment would have been continually in contact with the atmosphere and thus oxygen that is vital for eukaryote life forms. The age of the fossils also rings a bell: a molecular clock based on the genomics of all groups of animals alive today hints at around 900-1000 Ma for the emergence of the basic body plan. Because its host rocks are about that age, could Bicellum brazier be the Common Ancestor of all modern animals? That would be a nice tribute to the second author, Martin Brazier (deceased) of Oxford University, who sought signs of the most ancient life for much of his career.

See also:Billion-year-old fossil reveals missing link in the evolution of animals (Press release, Sheffield University; 29 April 2021)

Excitement over early animals dampened

Alga (Volvox sp.)
Volvox cyst. Image via Wikipedia

The Neoproterozoic lagerstätte in the Doushantuo Formation in the south of China was until recently thought to be a source of astonishing information about Earth’s earliest animals (See Ancestral animal? in EPN August 2004) that preceded the appearance of those with hard parts at the start of the Phanerozoic.  It contains well-preserved fossils that resemble embryos, algae, acritarchs, and small bilaterians. Dated at between 580 to 600 Ma(See Age range of early fossil treasure trove  in EPN February 2005), the Doushantuo directly overlies cap carbonates representing the emergence of Earth’s climate from a Snowball epoch represented by a tillite beneath the carbonate sequence. A detailed examination using synchrotron X-ray tomography of the putative animal embryos does show clear signs of cell doubling or palintomy (Huldtgren, T. et al. 2011. Fossilized nucluei and germination structures identify Ediacaran ‘animal embryos’ as encysting protists. Science. V. 334, p. 1696-1699) but also internal cell features most likely to be nuclei, but which have no counterparts in animal embryos. The organisms which the fossils most resemble are indeed eukaryotes, but of a kind separate from animals known as Holozoa. Yet there are striking resemblances with eukaryotes more distant from animals, such as the modern Volvox, a type of alga (Butterfield, N.J. 2011. Terminal developments in Ediacaran embryology. Science. V. 334, p. 1655-1656), that developed from an ancestor further back in time than the separation of metazoan animals from holozoans.