A microscopic bag-like sea creature dating to 540 million years ago is the earliest known ancestor of humans, research on the newly discovered species finds.

The ultra-tiny animal, named Saccorhytus coronarius, is now thought to be the first known “deuterostome” and is described in the journal Nature. Deuterostome refers to a broad biological group of animals that encompasses a number of sub-groups, including vertebrates, which are all animals with a backbone or spinal column.

“Humans are deuterostomes and so too are other vertebrates, as well as animals like sea-squirts and starfish,” co-author Simon Conway Morris of the University of Cambridge told Seeker. “What we want to know is what the common ancestor of all these animals looked like. In our opinion, Saccorhytus is the best candidate.”

To the naked eye, the creature resembles a black speck of dust. When magnified, however, “the preservation of detail in this (and the other) microscopic fossils is extraordinary,” he said.

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Conway Morris, lead author Jian Han and their international team of researchers analyzed the remains of Saccorhytus. They believe it lived in a shallow sea ecosystem with primitive mollusks, early representatives of arrow worms (nearly transparent worm-like marine predators) and other water-dwelling species.

There was not much to the animal, by the looks of it. The body was a rounded sack covered with a thin and flexible skin. A relatively large mouth was at the top, with small openings called “body cones” around the primary orifice.

“It could have slithered between the sediment grains, and the mouth seems to have been capable of considerable dilation and so, for its size, (it could) have swallowed relatively large prey with the excess water being discharged via the openings,” Conway Morris said.

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“We have not observed an anus,” he added, explaining that waste material could have left through the mouth.

The openings that are on the body were likely the precursors of gill slits that later evolved in fish and other marine life, the researchers believe. Gills were lost once the ancestors of humans and other related animals became terrestrial.

The discovery of Saccorhytus is just the latest major find from Conway Morris and his colleagues.

“Our team has notched up some important discoveries in the past, including the earliest fish and a remarkable variety of other early deuterostomes,” said co-author Degan Shu from Northwest University.” Saccorhytus now gives us remarkable insights into the very first stages of the evolution of a group that led to the fish, and ultimately, to us.”

Image: Artist’s reconstruction of Saccorhytus coronarius, based on the original fossil finds. The actual creature was probably no more than a millimeter in size. Credit: S Conway Morris/Jian Han