Rethinking evolution

The detailed images of embryos more than 500 million years old have been revealed by an international team of scientists, led by the University of Bristol.

Writing in Nature, Dr Phil Donoghue and colleagues reveal the various developmental stages of fossilised embryos, from the first splitting of cells to pre-hatching, using synchrotron-radiation X-ray tomographic microscopy (SRXTM).

In one instance this has exposed the internal anatomy of the mouth and anus of a close relative of the living penis worm. Another case has revealed a unique pattern for making embryonic worm segments, not seen in any animals living today.

The University of Bristol’s Phil Donoghue said: ‘Because of their tiny size and precarious preservation, embryos are the rarest of all fossils.

‘They are just gelatinous balls of cells that rot away within hours. But these fossils are the most precious of all because they contain information about the evolutionary changes that have occurred in embryos over the past 500 million years.’