Six hundred million years ago, in what is now China, a small, sponge-like animal laid eggs that started to divide into embryos.
Then disaster struck, and the tiny embryos died. But they were preserved in the shales and limestone deposits of Guizhou Province in southwest China.
Geologists who found them have used advanced X-ray and other imaging techniques to see what the embryos looked like, and report on Thursday that they have caught these fossilized embryos literally in the act of dividing.
"The fact that you can catch cells in process of division or in the process of dividing is astonishing because we are talking about little blobs of jelly," said Whitey Hagadorn of Amherst College in Massachusetts, who led the study.
Writing in the journal Science, Hagadorn and colleagues said the embryos consisted of anywhere between a few cells to nearly 1,000 cells, and appear to have been dividing in a way similar to the way modern embryos divide.
"We're learning something about how the very earliest multicellular animals formed embryos and how the embryos developed," said biologist Rudolf Raff of Indiana University, who worked on the study.
"This gives us an enormous and entirely surprising look at half-a-billion-year-old embryos in the act of cleaving. What a window on the past. We've had no prior idea what they might have done."
The researchers have spotted what appear to be specialized structures inside the cells, such as bubble-like vesicles that cells use to transport, store or metabolize molecules.
"We see things that look like organelles," Hagadorn added in a telephone interview. These include spindle bundles, which in a modern cell helps it pull apart into two new cells.
The international team of researchers used X-ray computed tomography, along with scanning electron microscopy and other high-tech equipment to examine the embryos in three dimensions.
The samples came from the Doushantuo Formation in China, a 635-million-year-old to 551-million-year-old fossil bed. The team chose about 160 embryos to look at.
They are not easy to find, Hagadorn said. "It's like looking for a needle in a haystack. "You go out and take a sample ... take it back to the lab, dissolve it and you sieve through the particles under a microscope with a paintbrush."
Most look like modern-day blastocysts — little balls of cells that are formed soon after conception. These early embryos divide, but not in perfect timing.
"Imagine eight cells dividing to form 16. They don't all do that instantaneously at exact same millisecond," Hagadron said. This is how the researchers knew the embryos were still dividing — because one might have 17 cells, while another might have 30 cells.
Hagadorn said no one knows what kind of animals would have grown from these embryos, but guessed it might be a sponge-like creature.