Signs point to sponges

Just a few weeks ago there was a nature paper based on 'chemical fossils': Fossil steroids record the appearance of Demospongiae during the Cryogenian period.

MIT, where some of the researchers are based, had a press release: Signs point to sponges as earliest animal life 'Chemical fossils' provide evidence for first multicelled creatures.

The Cryogenian is that period in the Precambrian era when there was an extensive series of severe ice ages - often referred to as 'snowball earth'.

Soft-bodied animals such as sponges are very rarely preserved as fossils, so finding evidence of their early appearance required some clever detective work. The key turned out to be an examination of unusual chemicals: steroids of a particular type produced abundantly by sponges but virtually never by simpler organisms.

Studying an unusually well preserved long sequence of strata found in Oman, the research team was able to extract these "chemical fossils" from a large number of samples spanning a range of tens of millions of years -- before, during and after the Ediacarian period. This provided clear evidence that sponges must have evolved long before the great variety of multicellular organisms that proliferated at the dawn of that period.

At that time in geological history, the Earth was just coming out of the last of its "snowball Earth" phases, when the entire planet was shrouded in ice. Since the new findings show that complex life seems to have begun tens of millions of years before that, that means these organisms were able to survive through that extreme episode of glaciation, something that many scientists had thought was impossible. This provides new evidence that the freezing was not absolute, but instead left some open patches of water.

"There's plenty of evidence in these rocks that there were places on Earth where life was flourishing" during this snowball episode, known as the Cryogenian, Summons says. "There must have been some refugia. Life certainly didn't shut down."