Researchers working in northern China have unearthed 1.56-billion-year-old fossils that are just a few dozen centimeters long. They think the fossils belong to surprisingly complex organisms called multicellular eukaryotes. And if that’s the case, large multicellular life began a lot earlier than we thought; eukaryotes visible to the naked eye didn’t become common in the fossil record until just 635 million years ago. The findings are published in Nature Communications this week.
Most living things we see on a daily basis – from plants to animals – are made of eukaryotic cells. That means DNA is contained within a membrane-enclosed nucleus, unlike with bacteria (which are unicellular prokaryotes). Eukaryotes can be single-celled (like most protists) or multi-celled. Fossils of macroscopic eukaryotes rarely date back to before the Ediacaran Period, which spanned 635 million to 541 million years ago, and those that do date back that far remain controversial.
A team led by Maoyan Zhu from the Chinese Academy of Sciences discovered 167 of these macroscopic fossils – preserved as carbon-rich compressions – in mudstones from the lower Mesoproterozoic Gaoyuzhuang Formation in China’s Yanshan area.
They categorized 53 of the fossils into four distinct shapes. About half of them have a linear shape, and the others were oblong or shaped liked a wedge or a tongue (pictured to the right). They’re up to 30 centimeters (12 inches) long and nearly 8 centimeters (3 inches) wide. The team also found organic fragments of 10-micrometer-long cells that were packed closely together and arranged in a thick sheet.
Taken together, the researchers think the fossils are multicellular eukaryotes – and not just mats of microbes. These multi-celled organisms may have lived in the shelf areas of prehistoric oceans, and based on comparisons with living organisms, these eukaryotes may have been photosynthetic. Their findings suggest that multicellular eukaryotes with relatively large dimensions lived in the oceans at least a billion years before the Cambrian Explosion, during a time period known as the "boring billion."
Image in the text: Multicellular fossil from 1.56-billion-year-old rocks in China. Maoyan Zhu