Fossils tens of centimetres in length which resemble multicellular eukaryotes and date back to 1.56 billion years ago are described in a paper published in Nature Communications this week. Eukaryotes of similar size do not start to become common in the fossil record until 635 million years ago, so this discovery could represent new evidence for the early evolution of eukaryotic organisms large enough to be visible with the naked eye.
Maoyan Zhu and colleagues found 167 fossils in the mudstones of the Gaoyuzhuang Formation in North China and assigned 53 fossils to four distinct shapes: nearly half of them have a linear shape; the others are wedge-shaped, oblong, or tongue-shaped. The fossils, which are preserved as carbon-rich compressions, measure up to 30 centimetres in length and eight centimetres in width. The authors also found fragments of closely packed, ten micrometre-long cells, which they interpreted as further evidence for multicellularity in these fossils.
Based on detailed analyses of the fossil specimens, they interpret them as eukaryotes rather than fragments of microbial mats, and speculate that they may represent photosynthesizing organisms living in shelf areas of ancient oceans. However, the exact affinity of the specimens remains uncertain. Further research will help to shed light into these ancient marine ecosystems.