On 12 January 2007 this blog drew attention to an unexpected turn in a controversy over the origin of animals: the reclassification of the “oldest known animal fossils” as fossilised giant bacteria.

Now opponents of the bacterial interpretation have responded with two papers in Nature upholding the earlier view that the fossils are animal embryos. Leiming Yinand colleagues argue that some of the fossils have an elaborate casing, indicating that they are animal embryos in an “egg cyst” stage.

Furthermore, Shuhai Xiao and colleagues argue that the fossils without an outer casing – that look most like bacteria – once had an egg casing, but this was removed during the processes of decay and fossilisation.

Jake Bailey and colleagues, who suggested the bacterial interpretation of the fossils, have responded that modern day giant sulphur bacteria are surrounded by a multilayered structure and a mucus-filled sheath. If this was fossilized, they claim, it would produce the structures identified by the other researchers as an egg casing. “We are therefore not convinced that the bacterial hypothesis has been falsified,” they conclude.

If these fossils are of animal embryos, not bacteria, then the abruptness of the Cambrian explosion would appear to be somewhat diminished. Darwin saw the abrupt appearance of animals in the Cambrian explosion as one of the most serious objections that could be lodged against his theory.