Relatives of the humble sea sponge have filtered Earth's waters for hundreds of millions of years or more, long before the first plants took to land. Their simplicity has led scientists to suggest sponges were the earliest animals to arise on our planet. But exactly when that happened remains under debate.

Now, a study published in the journal Nature suggests that mesh-like structures in an ancient reef may be 890-million-year-old sponges. If confirmed, the fossil sponges, found in the "Little Dal" limestones in northwest Canada, would predate the earliest undisputed fossils of any animal by more than 300 million years.  

However, most claims of extremely old fossilized life kick up controversy. The creatures that flourished in ancient seas may