The Cambrian explosion is often told as an animal story: predators, filter feeders and a rapidly expanding food web. But what about the planktonic algae at the base of the food chain? Did they simply wait to be eaten?
A new discovery from the early Cambrian Kuanchuanpu Biota in Shaanxi, South China, offers a surprising answer. A team led by Prof. Zongjun Yin at the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, has described a new multicellular alga, Polyostiosphaera operculata gen. et sp. nov. The study is published as the cover article in Proceedings of the Royal Society B.
The fossils are tiny spheres, only 0.53—0.85 mm across. High-resolution X-ray microtomography reveals a three-part anatomy: an outer envelope of many small cells; an inner cluster of about 200—500 larger cells; and, in the best-preserved specimens, a non-cellular membrane between them. The envelope surface bears regular circular apertures. Some are sealed by plug-like opercula with concentric rings—like tiny hatches on a capsule.
Cell measurements show a clear bimodal size distribution, suggesting spatial and functional differentiation. The researchers interpret the small outer cells as somatic or vegetative cells, and the larger internal cells as reproductive cells or propagules. When propagules mature, the plugs may detach, allowing them to exit through pre-formed channels without catastrophic rupture of the parent. This makes staged or repeated release anatomically plausible, although the fossils do not directly prove multiple reproductive events, and the fate of released propagules remains unknown.
Using 27 phenotypic traits, the team compared Polyostiosphaera with Cambrian multicellular algae and modern Volvocaceae. It ranks among the most complex Cambrian algae examined and falls within the range of highly organized volvocine algae in preserved cell differentiation and reproductive architecture. It is not a direct ancestor of Volvox, but a useful functional benchmark.
Why evolve such an elaborate system? The study proposes a life-history trade-off. During the Cambrian radiation, animal consumers increased mortality risks, while environmental conditions fluctuated. Investment in a protective envelope and controlled release could protect fragile propagules and spread reproduction over time. Polyostiosphaera thus shows that early Cambrian primary producers were active participants in ecological change. Consumer–producer interactions may have shaped not only who ate whom, but also when and how producers reproduced.
This research was supported by the National Key Research and Development Program, the National Natural Science Foundation of China, and the Guizhou Provincial Science and Technology Fund.
Reference: Chen L, Yin Z, Liu W, Xiang K, Qin S, Yang X, Li G, Zhu M. 2026 Complex reproduction reflects life-history trade-offs in early Cambrian multicellular algae. Proc. R. Soc. B 293: 20261709. https://doi.org/10.1098/rspb.2026.1709.
The holotype of Polyostiosphaera operculata gen. et sp. nov. from the early Cambrian Kuanchuanpu Biota
The paratype of Polyostiosphaera operculata gen. et sp. nov. from the early Cambrian Kuanchuanpu Biota
Artistic reconstruction and hypothesized propagule-release process of Polyostiosphaera operculata gen. et sp. Nov
Comparison of complexity and traits in Cambrian and modern multicellular algae
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