| By Jonathan Fildes Science and technology reporter, BBC News |
The exact moment when a 550-million-year-old cell began to divide has been captured in an exquisite 3D image. 
In picturesThe picture is one of a series taken by researchers examining ancient fossil embryos from Guizhou Province, China.
The specimens, described in the journal Science, are the oldest known examples of fossil embryos, and shed light on the early evolution of complex life.
Scientists used an advanced X-ray technique to peer inside the balls of cells to reveal the structures inside.
"We have been able to tease apart every structure, geological or biological," said Professor Phil Donoghue of the University of Bristol in the UK and one of the team which worked on the 162 pristine specimens.
Digital probe
The tiny fossils are part of South China's Doushantuo Formation, a limestone bed deposited between 635 and 551 million years ago that contains layers composed almost entirely of fossil embryos.
The team behind the research believes the fossils are the developing offspring of extremely primitive sponge-like creatures.
To resolve the delicate internal structures, the scientists used a technique known as microfocus x-ray computed tomography (microCT). The method allowed the team to construct 3D images of the tiny fossils. 
It is amazing that such delicate biological structures can be preserved in such an ancient deposit 
Virginia Tech
Computer software was then used to analyse individual cells.
"We digitally extracted each cell from the embryos and then looked inside the cells," said Shuhai Xiao of Virginia Tech University in the US.
Inside, the team found kidney-shaped structures that it believes could be nuclei or other subcellular components.
"It is amazing that such delicate biological structures can be preserved in such an ancient deposit," said Professor Xiao.
In some four-celled embryos, each cell had two of the kidney-shaped structures, suggesting they were caught in the process of splitting prior to cell division.
Explosion of life
Although the bed is packed full of the tiny fossils, the team has been unable to find any adult specimens.
Previous research has suggested that the embryos were the product of complex animals, the ancestors of modern organisms.
If true, this would suggest that complex multi-cellular life got started much earlier than previously thought, prior to the "Cambrian Explosion" 542 million years ago.
At this time, fossils record a dramatic change in animal diversity with many of today's modern groups suddenly making an appearance.
Some researchers believe that the Cambrian Explosion marked the emergence of modern animal life. Although complex animals had started evolving before 542 million years ago, their development accelerated at this point.
Others maintain that complex animals lived long before this event and that the period just marks a time of exceptional fossil preservation.
The Doushantuo Formation is important because it gives a window into the time leading up to the Cambrian and the new analysis goes some way towards resolving the dispute.
Unique insights
Using the microCT technique to analyse late-stage embryos, with up to 1,000 cells, the team was able to gain insights into the creature that produced them.
This work provides a constraint on when advanced groups evolved,
University of Bristol
Although the cells show some modern traits, they crucially lack others.
"Even in these late-stage embryos, there is no evidence of the formation of a tissue layer," said Dr Donoghue.
"You would expect to see that in modern embryos, even those of sponges."
The team believes the cells probably came from extremely simple creatures.
"They would have developed into sponge-like creatures, but more primitive," said Dr Donoghue.
If right, this means that the Cambrian Explosion theory for the origin of complex animal life would still stand.
"This work provides a constraint on when advanced groups evolved," Dr Donoghue said.
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