The Cambrian explosion, characterized by the rapid increase of animal body plans and organismal complexity, laid the foundation for animal evolution. However, whether non-metazoan eukaryotes, such as algae, experienced a similar increase in complexity is still inadequately explored. As the initial stage of the Cambrian explosion, the Kuanchuanpu Biota preserves cellular-level structures of microfossils, providing a crucial taphonomic window for investigating the complexity evolution of non-metazoan eukaryotes in the early Cambrian.
Here, we report a spherical fossil (520–798 μm in diameter) from the Kuanchuanpu Biota with well-preserved internal structures. Using high-resolution X-ray microtomography, we reconstructed its three-dimensional organization, revealing a three-layered structure: an outer envelope composed of cells (~24 μm) with evenly distributed openings, some retaining plug-like structures; a middle membranous layer; and an inner body comprising larger cells (~40 μm). Statistical analyses indicate significant size differences between the cells composing the envelope and those in the inner body, suggesting cellular differentiation into somatic and germ cells. Additionally, reconstruction of the reproductive mechanism indicates that the detachment of plug-like structures likely coincided with reproductive cell maturation and the rupture of the membranous layer, which may have facilitated germ cell release through the envelope openings.
Based on the available evidence, we propose that these multicellular fossils are most likely eukaryotic green algae. Moreover, by comparing these fossils in cell differentiation and reproductive processes with extant Volvox, we suggest that these Cambrian algae may have been as organizational complexity as living green algae. This provides important evidence for understanding the complexity of non-metazoan eukaryotes.
 

cell differentiation,Kuanchuanpu Biota,Cambrian explosion