Our very primitive beginnings look to have been very primitive indeed. It turns out the earliest known ancestor of humans was a sea creature a millimeter in size that likely lacked an anus. In a study published in Nature on Monday, scientists named Saccorhytus coronaries as a 540 million-year-old member—at that age the oldest one—of a category of animals called deuterostomes. The BBC explains deuterostomes gave rise to vertebrates. "All deuterostomes had a common ancestor, and we think that is what we are looking at here," researcher Simon Conway Morris tells the BBC. As for what they're looking at, "the bag-like body bears a prominent mouth and associated folds," the researchers write.
Live Science reports the deuterostome groups scientists have previously discovered were at most 520 million years old, and had begun evolving into vertebrates, echinoderms (ie, starfish), and hemichordates (acorn worms). That left scientists stumped as to what a common ancestor would have looked like. Now they say they know. "We had to process enormous volumes of limestone—about [3 tons]—to get to the fossils, but a steady stream of new finds allowed us to" answer that question, says co-author Jian Han of the find, made in China. Two other details, from a Cambridge press release: The minuscule creature is thought to have lived between grains of sand on the sea bed, and it featured small conical structures that the water it ingested may have exited through "and so were perhaps the evolutionary precursor of the gills we now see in fish." (Scientists have found Earth's oldest civilization.)