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For billions of years, life remained stagnant—stuck in a mode of single-celled simplicity.

But the latter part of this long stretch of time, a period known as the “boring billion” for its presumably low levels of oxygen, may not have been so boring after all.

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A team of researchers, including Andrew Knoll of Harvard University and Shixing Zhu of the China Geological Survey in Tianjin, has discovered fossil evidence of what appear to be multi-celled eukaryotes—organisms in which differentiated cells contain a nucleus with genetic material—in the Yanshan region of China’s Hebei province. The fossils were preserved in mudstone and are dated to 1.56 billion years ago. The scientists published their findings in the journal Nature Communications.

Previously, scientists believed that insufficient oxygen would have stifled organismic growth during the “boring billion.” They also hadn’t found any eukaryotic fossils of similar size that were more than 635 million years old—about 100 million years before the Cambrian Explosion, the rapid proliferation of complex plant and animal life.

But this new report is true, it could shatter the 635 million-year mark by another one billion.

The team says that the variety of fossils is an indication of complexity—a third of the 167 samples are in one of four different shapes, and exhibit a fine, regular cell structure. But while the team is using these details to claim that the leap from single-celled to multi-celled organisms happened earlier than once thought, some scientists are skeptical. They argue that these specimens are merely “colonies” of single-celled bacteria.

Here’s Paul Rincon, reporting for BBC News:

Prof Knoll told BBC News: “It looks like the leap from single cells to simple multi-cellularity is easy—in relative terms. It was done many times (over the course of evolution) and this really cements the case that it was done early in the history of eukaryotes.”

The study is very much up for debate—and that matters not just for the history of life but also for the early evolution of Earth’s chemistry. If Earth was somehow more suitable for multi-cellular life 1.56 billion years ago, we may have to rewrite our planet’s origin story.