Around 540 million years ago, a wrinkled, bag-like sea creature measuring just one millimeter (0.04 inches) in size nestled between grains of sand on the ocean floor in what is now central China. It was oval in shape and symmetrical, and its most striking feature was its large mouth, which it used to engulf particles of food, or even other miniscule creatures. Meet Saccorhytus coronaries, the species that scientists in a new study believe is the earliest known step on the evolutionary path that led to fish and—eventually—to humans.

An international team of scientists uncovered the microfossils of the speck-size creature they are calling Saccorhytus coronaries in Shaanxi Province, central China. According to their findings, published this week in the journal Nature, Saccorhytus lived 540 million years ago, during the early Cambrian period, when a shallow ocean would have covered the region where it was found. At that advanced age, it is believed to be the oldest known member of a category of animals known as “deuterostomes,” which are common ancestors to a wide range of other species, from starfish, sea urchins and acorn worms to the large group classified as vertebrates (backboned animals).

In other words, according to a statement about the study from the University of Cambridge, the creature “may be the earliest known step on an evolutionary path that eventually led to the emergence of humans.”

After first isolating the 45 individual microfossils from the surrounding rock, and then studying them both under an electron microscope and using a CT scan, the researchers were able to construct a picture of what the creature must have looked like. Its body was oval in shape and symmetrical, a characteristic passed down to many of its descendants, including humans. Around one millimeter (0.04 inches) in size, it was covered with a thin, relatively flexible skin. According to the researchers, this suggests the creature had muscles beneath that skin, and that it may have moved by contracting those muscles and wriggling around.

Microfossil found in China. (Credit: Jian Han)
Microfossil found in China. (Credit: Jian Han)

The scientists dubbed the tiny creature Saccorhytus coronaries in honor of its bag-like, wrinkled appearance. (“Saccus” means “sac” in Latin, while “rhytis” means “wrinkle” in Greek.) The creature’s mouth, its most striking feature, was quite large relative to the size of its body. The researchers concluded it probably ate by engulfing food particles, or maybe even other miniscule creatures, in its mouth.

Saccorhytus also had small conical structures on its body, which the researchers believe may have allowed any water the creature swallowed to escape, functioning like a primitive form of gills. Interestingly, the scientists found no evidence that the creature had an anus, which means it would have had to consume food and excrete waste from a single orifice

Previously, the oldest deuterostomes scientists had identified were between 510 and 520 million years old. By that time, the group had already begun the process of diversification into echinoderms (marine invertebrates such as sea urchins and starfish), hemichordates (acorn worms) and vertebrates. Because of the vast difference between such species, scientists were hard-pressed to determine what a common ancestor could have looked like.

With the discovery of Saccorhytus coronaries, they may have found their answer. “To the naked eye, the fossils we studied look like tiny black grains, but under the microscope the level of detail is jaw-dropping,” Simon Conway Morris, a professor of evolutionary palaeobiology at the University of Cambridge, said in the university statement. “All deuterostomes had a common ancestor, and we think that is what we are looking at here.”