Here’s what you’ll learn when you read this story:

• The discovery of a collection of about 700 fossils known as the Jiangchuan Biota gives scientists a better understanding of the transition between the Precambrian supereon and the Phanerozoic, our current geologic eon.
• Because chordates and bilaterians were discovered in the Jiangchuan Biota, the Cambrian “explosion” of complex life might be a misrepresentation caused by missing evidence in the fossil record.
• Among the bilaterians found were deuterostomes, the ancestor of all vertebrates, including humans.

A very broad overview of the current scientific understanding of life’s evolution goes something like this: Around four billion or so years ago, the first microbes—anaerobic, single-celled creatures—appeared on the fiery ball of rock known as Earth. Next, about 3.5 billion more years of microbial development (along with the arrival of eukaryotes) essentially filled up the period known as the Precambrian. But after that, things got interesting, as complex life-forms proliferated rapidly during what’s known as the Cambrian Explosion.

But scientists still don’t know much about the transition between the Precambrian and the Phanerozoic, our current geologic eon. Now, however, a new fossil site in southwest China known as the Jiangchuan Biota helps bridge the gap between the Cambrian period and the Ediacaran period, which spans the 96 million years preceding the “explosion.” In a new study published in the journal Science, researchers from Yunnan University in China report that the 700 fossils at Jiangchuan are roughly 539 to 554 million years old. While some of the lifeforms were previously completely unknown, others were previously known to only exist in Cambrian rocks, so their presence among the Jiangchuan Biota seemingly pushes back the previously-accepted timeline of the origin of complex life.

Most of these fossilized animals are ancestors of modern invertebrates, including modern starfish, but a few among them count as ancient ancestors of humans—though you’d never know through resemblance alone. These worm-like creatures, known as bilaterians, feature bodies that, when bisected, could be divided in two mirror-image halves. This is known as “bilateral symmetry,” and it stands in stark contrast to animals like cnidarians (coral) or ctenophores (comb jellies). Interestingly, the presence of these two latter phyla in the fossil record means that animals we thought only lived in the Cambrian might’ve been around millions of years earlier.

“Our discovery closes a major gap in the earliest phases of animal diversification,” Oxford University’s Gaorong Li, lead author of the study (he was at Yunnan University at the time), said in a press statement. “For the first time, we demonstrate that many complex animals, normally only found in the Cambrian, were present in the Ediacaran period, meaning that they evolved much earlier than previously demonstrated by fossil evidence.”

Our distant ancestors, those wormy bilaterian critters known as deuterostomes, feature holdfasts that essentially secure the organism to the seabed as well as a feeding tube for grabbing grub. Eventually, this winning biological strategy gave rise to every vertebrate on Earth, and the Jiangchuan Biota now pushes their existence into the Ediacaran for the first time—but that wasn’t the only surprise discovery.

“The presence of…ambulacrarians [acorn worms] in the Ediacaran period is really exciting,” University of Oxford’s Frankie Dunn, a co-author of the study, said in a press statement. “The discovery of ambulacrarian fossils in the Jiangchuan Biota also means that the chordates—animals with a backbone—must also have existed at this time.”

One of the big questions is why this assemblage of new fossils was discovered only now, since similar fossils aren’t featured in other known Ediacaran sites. The researchers’ best guess is that the carbonaceous compressions found at the Jiangchuan site are rare among rocks of this period, so the fact that these fossils are so rare could be just an effect of their unusual preservation method, rather than an actual rarity of such creatures during the Ediacaran period in general.

If life’s evolution was a standard novel, the Cambrian Explosion would represent the rising action that eventually leads to the stunning climax of modern-day speciation, but if the Jiangchuan Biota tells us anything, that story’s build-up may need to be rewritten.