Usually, when you're looking for creatures from a billion and a half years ago, you do it through the lens of the microscope.
But when Maoyan Zhu cracked open sheets of hardened mud chipped from an ancient outcropping near Beijing, it took little more than a glance to see he'd discovered something special.
Inside the rocks were more than 100 ribbon-like fossils, some of them nearly a foot long and all clearly visible to the naked eye. They can only have come from large, many-celled organisms, Zhu and his colleagues reported in the journal Nature Tuesday — making these ancient seaweed-like creatures some of the earliest multi-cellular life scientists have ever found.
The find challenges the popular conception of what is sometimes called "the boring billion," writes Zhu, a paleontologist at the Chinese Academy of Sciences. Traditionally, scientists have believed that complex cells (known as eukaryotes) spent the first 10 million centuries after their initial evolution just chilling. The environment didn't change much, and neither did the creatures that inhabited it. They may have occasionally banded together into very simple multi-cellular organisms, but such instances were rare and fleeting. Truly complex life wouldn't really get going until the "Cambrian explosion" about 600 million years ago, when eukaryotes began to evolve into sponges and slime molds, then fish, fungi, plants, insects, dinosaurs, birds, squirrels, bats, badgers and eventually us.
The new find shows that the "boring billion" actually had quite a bit going on, said Harvard paleontologist Andrew Knoll, a co-author on the report.
"Things were larger and more diverse and arguably more widespread than we thought," Knoll said.
Most of the fossils, which were found in a 1.56 billion-year-old layer of rock, were long, thin and blade-like, resembling the algal seaweeds still found in oceans around the world. This and other similarities suggest to Knoll that they were probably photosynthetic — a find that could help researchers understand how eukaryotes adopted the ability to turn sunlight into energy.
"A number of molecular clocks indicate that the time table should be that eukaryotes swallowed cyanobacteria (incredibly old, simple, single-celled organisms capable of photosynthesis) and started photosynthesis very early and this would be consistent with that," Knoll said.
The discovery also adds to scientists' understanding of how difficult it was for complex multi-cellular life to evolve. Until the last five years or so, it was believed that eukaryotes lived a life of microscopic monotony during their "boring billion." But recent fossil discoveries show evidence of very simple multi-cellular organisms pretty soon after the first eukaryotes emerged.
Though they were big enough to be seen without a microscope, these creatures weren't too different from their single-celled ancestors. Their component cells weren't different from one another — unlike our cells, which come in hundreds of different forms, each serving a specialized purpose. They were basically "sheets of cells" that had all chosen to work together and get along, Knoll said.
"There's not a lot of genetic makeover required to make something like that."
On the other hand, the fact that it took roughly a billion years to make the jump from simple multi-cellularity to, well, everything that's happened since, suggests that complexity is the trait that may be harder to gain. After all, the genetic differences between humans and simple seaweeds are far greater than the difference between those seaweeds and single-celled algae.
"It's clearly a bigger challenge to get from simple to complex than it is to get from single-celled to multi-cellular," Knoll said.
There are also other reasons why it may have been hard for life to diversify during the "boring billion": not enough oxygen and essential minerals in the oceans, too many other toxins, a long period of tectonic stability that kept the environment from changing.
The fossils discovered in China show it was possible for some eukaryotes to become large and complex despite those constraints. Indeed, the long stable period may have been essential for eukaryotes' development, some researchers say.
As Martin Brasier, the late University of Oxford palaeobiologist, once told the New Scientist: “I argue that the boring billion was the anvil on which the eukaryote cell was forged."