This tiny 'wrinkled bag' might be humans' earliest known ancestor

A tiny marine creature that lived about 540 million years ago in the seabed mud was found in China and may be the earliest-known animal in the evolutionary path that led to humans.

First Look

Evolutionarily speaking, humans haven't been around very long – only about 200,000 years. But while humans might be able to find common ground with their close evolutionary ancestors, like primates and other vertebrates, it may be harder to see the connection with this earliest known ancestor, Saccorhytus coronarius, a tiny deuterostome that lived around 540 million years ago.

The odd-looking creature was only four millimeters long, living between sand particles in a shallow ocean during the Cambrian Period. But despite its diminutive size and primitive digestive system, scientists suggest that the creature may be one of the earliest common ancestors of a range of species, including the evolutionary line of vertebrates that would eventually lead to Homo sapiens.

Of course, it's a little difficult to see the family resemblance. But there are a few features that have become popular in many kinds of animals since Saccorhytus – bilateral body symmetry, for instance, is a trait that both humans and the deuterostome share. The baglike animal also bears some similarities to modern fish, featuring conical structures that allowed water to escape after being swallowed, a possible precursor to gills.

"Our team has notched up some important discoveries in the past, including the earliest fish and a remarkable variety of other early deuterostomes," Degan Shu, one of the researchers involved with the study, said in a statement from the University of Cambridge. "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."

A team of researchers discovered more than 40 tiny fossils of the deuterostome buried in limestone from Shaanxi, China. To the naked eye, the remains of the creatures look like little more than black specks of sand embedded in the rock. But with the help of an electron microscope and CT scans, scientists were able to observe telltale signs of a tiny lifeform: facelike features attached to general body structure.

"We had to process enormous volumes of limestone – about three tons – to get to the fossils, but a steady stream of new finds allowed us to tackle some key questions: was this a very early echinoderm, or something even more primitive?" asked Jian Han, one of the researchers involved in the study. "The latter now seems to be the correct answer."

Saccorhytus is clearly not as sophisticated as many of its evolutionary offspring. For instance, its proportionally large mouth and lack of an exit point at the other end of the digestive system lead the researchers to conclude that the creature engulfed food particles or other small creatures whole before expelling waste through the same orifice, according to a study published in the journal Nature. 

Most known early deuterostome groups are much younger than Saccorhytus, first appearing about 510 to 520 million years ago, after they had already begun to diversify into other groups such as vertebrates, animals like sea urchins and starfish, and even types of sea worms.

"We think that as an early deuterostome this may represent the primitive beginnings of a very diverse range of species, including ourselves ... under the microscope the level of detail is jaw-dropping," Simon Conway Morris, study co-author, said in the statement. "All deuterostomes had a common ancestor, and we think that is what we are looking at here."

For scientists, that means that while Saccorhytus may be an ugly creature – its name means "Wrinkled Sack," after all – it represents a significant step forward in the development of a range of complex and majestic creatures found across the entire planet.

"And is not beauty in the eye of the beholder?" Dr. Conway Morris asked.

This report includes material from Reuters.