1.56-billion-year-old fossils add drama to Earth’s ‘boring billion’

A form of multicellular life visible to the naked eye may have emerged nearly a billion years earlier than scientists once thought.

At 1.56 billion years old, fossils discovered in north China represent the best evidence yet for the early existence of large eukaryotes, paleobiologist Maoyan Zhu of the Chinese Academy of Sciences in Nanjing and colleagues report May 17 in Nature Communications.

Eukaryotes, which have cells containing nuclei and other membrane-wrapped machinery, include everything from plants to people. The new find could mark when single-celled eukaryotes became multicellular organisms capable of drawing energy from the sun, says geobiologist Shuhai Xiao of Virginia Tech in Blacksburg, who was not involved in the research. “That’s why it’s important,” he says. “The fossils represent one of the major transitions in evolution.”
Remains of early life on Earth are scarce and sometimes hard to interpret (SN: 2/8/14, p. 16). Some scientists point to evidence of life from as early as 3.8 billion years ago; rocks in Greenland, for example, contain traces of carbon that could be cellular remnants. Harder evidence for microbes comes from what appear to be fossils of actual cells speckling 3.4-billion-year-old sandstone in Australia.

Signs of multicellular life crop up later in the fossil record, roughly 1.2 billion years ago, Zhu says. But until now, clear evidence for large-scale multicellular organisms, like the ones Zhu’s team reports, date back just 635 million years. The team’s new find — consisting of 167 fossils, some up to 30 centimeters long and 8 centimeters wide (roughly the size of a man’s footprint) — places these life-forms much further back in time, when Earth was hot and oxygen was scarce.

The fossils are compressions: cells squashed flat into ribbons — like a garden hose crushed beneath a car’s tire, or like kelp. In fact, the organisms may have looked “very similar to some algae living in the shallow sea today,” Zhu says.

After stretching ropy bodies along the shore, the ancient sea life was preserved in layers of mudstone. Inside the stone, the researchers found closely packed cells and organic carbon, evidence of cellular remains. Zhu can’t explain why such a large time gap exists between his find and similar large-scale fossils, but he suspects the problem may be with preservation of such old material.

Scientists once considered the time period when these organisms lived to be the “boring billion,” though that view seems to be shifting (SN: 11/14/15, p. 18). People thought that “there was very little evolutionary change, and not much going on geochemically,” Xiao says. The new work “tells us a different story.”