Microfossils found in China have revealed what could be our earliest known ancestor on the tree of life, researchers say. But don’t go looking for a resemblance. Saccorhytus, which looks a little like the “chestburster” from “Alien,” was a tiny, bag-like sea creature that lived 540 million years ago.
With a scientific name describing the shape of its body and wrinkled appearance, this millimeter-long creature wriggled around in the mud and lived between grains of sand on the seabed, according to a study published Monday in the journal Nature. Though the fossils were found on dry land, half a billion years ago, the creature’s location would have been a shallow sea.
Biologically, Saccorhytus belongs to a broad category of creatures called deuterostomes. Half a billion years ago, they began to rapidly evolve into diverse branches, including vertebrates such as humans as well as sea squirts, starfish, sea urchins and acorn worms.
Deuterostomes are the common ancestor of many species, creating a pathway of evolution that would lead to humans millions of years later, according to the study. But with such diversity stemming from one branch, researchers initially found it hard to imagine what the origins would have looked like. Saccorhytus provides that answer, but it wasn’t easy to discover.
In search of this answer, researchers from the University of Cambridge in England and Northwest University in China went through 3 tons of limestone just to find samples of tiny black specks in the rock. Under the microscope, a more detailed image of this creature emerged.
It had an elliptical body with bilateral symmetry, evidence of musculature and a thin yet flexible skin. But the most defining feature is the incredibly large mouth that could grow in size, capable of taking in food particles and larger prey. Although Saccorhytus lacked an anus, eight cone-like structures around the body appear to be primitive gills that could remove excess water. Unfortunately, anything larger would have come back out through the mouth, the researchers said.
For scientists, a finding like Saccorhytus brings up the mismatch between fossil evidence and the idea of the molecular clock.
“Molecular clocks suggest the origination of the main groups of animals significantly predated what the fossil record would indicate,” said Simon Conway Morris, fellow of St. John’s College at the University of Cambridge and one of the study authors. “One possibility is that the earliest animals were very small and, in normal circumstances of fossilization, very unlikely to survive. In this way, Saccorhytus might give us a glimpse of a long and cryptic history.”
Reconstructing the ‘tree of life’
Tom Harvey, a lecturer in geoscience at the University of Leicester, says tiny fossils such as Saccorhytus “provide a glimpse into a microscopic world that we rarely get to see.” His discovery of a complete loriciferan fossil was also published Monday in the journal Nature Ecology and Evolution. He did not participate in the Saccorhytus study.
Harvey’s discovery of the loriciferan fossil came as a complete surprise while he was studying fossilized crustaceans. Although other similar fossils had been found, they were lacking heads or other features that directly connected their identity, he said.
Although they were also about a millimeter in size and lived between sand particles on the sea bed, loriciferans are more related to arthropods like insects and shrimp than to Saccorhytus, and they are still alive today. Saccorhytus has no modern-day living counterpart.
Both were part of the Cambrian “explosion,” the sudden appearance of a wide variety of animal life in the fossil record half a billion years ago and an important turning point in the evolution of life on Earth, Harvey said.
“They help us to reconstruct the ‘tree of life,’ the series of branching events in animal evolution,” Harvey said. “But aside from that, they reveal that soon after the origin of animals in the late Precambrian, some groups of animals were already adapting to the ‘extreme’ lifestyle of living between grains of sand on the seabed, possibly as protection from being eaten by bigger animals.”
Modern-day loriciferans haven’t evolved much. They can still be found in their tiny ecosystem within beach sand or sea sediment. And loriciferans don’t belong to “our” part of the tree of life.
The researchers “do point out that Saccorhytus belongs to the same ‘branch’ of the tree of life as humans — so could be seen as one of our early human ancestors!” Harvey said. “Funny to think, but in half a billion years, a lot of evolutionary changes can take place.”
