Fossilized Finds Prompt Rethinking of Natural History
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LONDON — The discovery of a group of half-billion-year-old fossilized embryos in China has given a glimpse of the very first animals to evolve on Earth, overturning the accepted picture of how life evolved.
James Hagadorn of Amherst College, Massachusetts, who led a team of 15 scientists from five countries, reports in the journal Science that the 160 fossilized 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 being debated 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, Mr. Hagadorn’s group managed to carry out a “virtual dissection” of the embryos, which are around half a millimeter across, to challenge this conclusion, showing how the embryos show some features of animals but are distinct from anything alive today.
“We’ve completely characterized the embryos, and we see no features of any living animal group,” Philip Donoghue of the University of Bristol, one of the team, said. “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.
