The fossils of tiny marine creatures suggests complex skeletons evolved about 550 million years ago, which is much earlier than was previously believed.

The study is the first to demonstrate that some of the earliest complex species on Earth lived millions of years earlier than thought, the University of Edinburgh reported. Before these findings, the oldest known evidence of complex animals dated to around 541 million years ago, but scientists had always wondered if this type of life existed even earlier on.

Complex animals called bilaterians are believed to have evolved before the Cambrian Period, but may have lived as much as 550 million years ago during the late Ediacaran Period. This means complex life would have existed before the Cambrian explosion, which is a time that saw a wealth of new animal group developments.

The analysis of these ancient fossils, dubbed Namacalathus hermanastes, could rewrite what we know about our evolutionary history. The animal possessed rigid calcium carbonate skeletons that had a complex structure similar to what is seen in modern deep sea-dwelling animals.

"This fossil has been known for a long time, and was assumed to have been a primitive animal, such as a sponge or coral. This study suggests that it was, in fact, more advanced. We have suspected that these complex animals were present in the Ediacaran, but this study provides the first proof," said professor Rachel Wood, of the University of Edinburgh's School of GeoSciences, who led the study.

The study, published in the journal Proceedings of the Royal Society B, and was funded by the Natural Environment Research Council.