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symmakhos
Friday, October 13th, 2006, 01:25 PM
Telegraph.co.uk 13/10/2006 (http://www.telegraph.co.uk/news/main.jhtml?xml=/news/2006/10/13/wfossil13.xml)

By Roger Highfield, Science Editor

The discovery of a group of half-billion-year-old fossilised embryos in China has given a glimpse of the very first animals to evolve on Earth, overturning the accepted picture of how life evolved.

Dr James Hagadorn of Amherst College, Massachusetts, who led a team of 15 scientists from five countries, reports in the journal Science that the 160 fossilised embryos, each of which consists of up to 1,000 cells and dates back as far as 580 million years, form the root stock of all of today's animals and were a critical part of the so-called Cambrian Explosion, in which animals became bigger, more diverse and ecologically dominant.
The fossils are thought to be of relatively simple creatures, the most primitive animals that were the ancestors of sponges.

The findings are controversial as a competing Chinese-American group of scientists has used embryos from the same deposit, from the Doushantuo Formation of south central China, to argue for the existence at that time of living groups of sea urchins and cnidarians (jellyfish and their kin) and that animals emerged much earlier.

But using a method called high-energy X-ray tomography to study the embryos with unprecedented precision, Dr Hagadorn's group managed to carry out a "virtual dissection" of the embryos, which are around half a millimetre across, to challenge this conclusion, revealing how the embryos show some features of animals but are distinct from anything alive today.
"We've completely characterised the embryos, and we see no features of any living animal group," said Dr Philip Donoghue of the University of Bristol, one of the team. "In fact, we argue that these are embryos of a grade of animal evolution preceding all living groups. There is going to be a huge scrap over this."

"The really big deal is that we see no evidence of more complex animals, indicating that at 580 million years ago, they had not yet evolved."
The team, which also includes Neil Gostling and Maria Pawlowska of the University of Bristol, also reports the first direct evidence that the primitive animals were capable of asynchronous cell division during development, where an embryo would have an odd number of cells, which does not fit with the expected pattern of cell division and allows the formation of unique shapes.

The scientists found evidence of cell differentiation - where embryonic cells are developing a new identity -and cells about to divide. They also believe they have identified specialised structures inside the cells, called organelles, that the cells might have used to transport, or store molecules.

Slight aberrations during the fossilisation of dead embryonic cells even reveal what appear to be dividing nuclei, the compartments at the heart of cells that contain the genetic instructions.

"We're learning something about how the very earliest multi-cellular animals formed embryos," said Indiana University biologist Rudolf Raff. "This gives us an enormous and entirely surprising look at half-a-billion-year-old embryos in the act of cleaving. We've had no prior idea what they might have done."

Prof Simon Conway Morris of Cambridge University, an expert on the Cambrian Explosion, said: "We have incredible structures within the cells. Claims for a much older date seem now to have been abandoned."

Fossilised embryos are very rare. The Doushantuo Formation has proved a boon to palaeontologists and evolutionary developmental biologists interested in the evolution of animal species.