In this video, we get a peek at how scientists have been working to pinpoint Earth’s first animal. It seems to be the sea sponge, just squeezing past the comb jelly. For a long time, sea sponges were actually presumed to be first, before comb jellies took the mantle. Now we’re back to sponges, thanks to some cool sleuthing by scientists at MIT.
It’s estimated that animals took over the Earth about 540 million years ago during what’s called the Cambrian explosion. But scientists have found some tantalizing molecules in rocks from 100 million years before that.
Since there so few pre-Cambrian fossils, it’s hard to know which animals came first. So, researchers at MIT have been examining the trace amounts of molecules found in old stones for clues. In particular, they’ve been looking at 24-isopropylcholesterol, or 24-ipc for short, because it’s in 640-million-year-old rocks, and modern sea sponges produce it.
To be certain 24-ipc indicated the presence of sea sponges, scientists had to be sure that these creatures are the only animal known to produce it. They analyzed genes and lipids through a range of living organisms including plants, fungi, and sponges. As it turns out, the SMT gene is responsible for production of 24-ipc, but it can only do so in an organism that has the correct number of SMT gene copies.
There are only two such organisms: algae and sea sponges. And the sea sponge developed this capability much earlier than algae. Sponges were making 24-ipc as long as, wait for it, 640 million years ago.