Scientists relieved as microscopic creature with no anus is NOT the earliest human ancestor
It was certainly heartbreaking when a 2017 study concluded that an anusless microscopic creature resembling an angry Minion was the earliest human ancestor.
However, new research has found that the spiky, wrinkly sac called Saccorhytus — which would be right at home in “Despicable Me” — isn’t actually related to humans.
Saccorhytus had pores around its mouth that were first interpreted as gills – a primitive feature of the Deuterostomia animal group from which we sprang.
However, analysis of 500-million-year-old fossils from China has shown that these pores are, in fact, the basis of spines that have broken off during their conservation process.
The research team, led by scientists at the Nanjing Institute of Geology and Paleontology, has instead placed Saccorhytus in a different evolutionary group, relating it to arthropods such as spiders, crabs and insects.
Saccorhytus had pores around its mouth that were first interpreted as gills — a primitive feature of the Deuterostomia group of animals from which humans evolved. Pictured is an artist’s reconstruction of Saccorhytus coronarius
The research team, led by scientists at the Nanjing Institute of Geology and Paleontology, instead placed them in a different evolutionary group and associated them with arthropods such as spiders, crabs and insects. Shown is an artist’s reconstruction of a side (left) and dorsal (right) view of Saccorhytus coronarius
Saccorhytus is said to look like an angry Minion from ‘Despicable Me’ (stock image)
All animals that are bilaterally symmetrical – having a left and a right side – are descended from one of two different groups; protostomes and deuterostomes.
With protosomes, the mouth forms in front of the anus during embryonic development, with deuterostomes this happens the other way around.
Insects, crabs and clams are all part of the evolutionary lineage from the protosome, while vertebrates like humans came from deuterostomes.
Until the new findings, published today in NatureSaccorhytus was thought to be the earliest representative of the Deuterostomia group.
Fossils of the creature, about a millimeter in diameter, were first discovered in rocks in China’s Shaanxi province in 2012, and were described in 2017.
These fossils appeared to have filter-feeding organs, known as pharyngeal openings, around their mouths, leading to their identification as deuterostomes.
Since they were dated at 535 million years old, it was concluded that they were the earliest animal within that group.
The scientists have since hypothesized that the Saccorhytus belongs to a group within Protosomia known as ecdysoszoan, which contains arthropods and nematodes. Shown is the phylogenetic position of Saccorhytus coronarius (shown in red)
More recently, however, paleobiologists have dug for additional specimens of Saccorhytus and recovered hundreds of specimens that were better preserved.
“Some fossils are so perfectly preserved that they look almost alive,” said Yunhuan Liu, a professor of paleobiology at Chang’an University, Xi’an, China.
“Saccorhytus was a curious beast, with a mouth but no anus, and rings of complex spines around its mouth.”
hundreds of X-rays were taken of a new fossil using a particle accelerator at the Swiss Light Source in Switzerland to construct a detailed 3D digital model.
This showed that the creature had spines around its mouth created by a decay-resistant cuticle layer extending through the pores.
“We think this would have helped Saccorhytus capture and process its prey,” suggests Huaqiao Zhang of the Nanjing Institute of Geology and Paleontology.
Crucially, they weren’t gills, scrapping any evidence to suggest they were deuterostomes like humans.
The Saccorhytus microfossils studied in the analysis were found in central China’s Shaanxi province (shown on map)
WHAT ARE DEUTEROSTOMES?
Deuterostome is one of the most important groups of the animal kingdom.
Most deuterostomes are either chordates or echinoderms.
Most early deuterostome groups are from about 510 to 520 million years ago, when they already started to diversify into not only the vertebrates, but also the sea pipes, echinoderms, animals such as starfish and sea urchins, and hemichordates.
This level of diversity has made it extremely difficult to figure out what a previous, common ancestor might look like.
The scientists have since hypothesized that the Saccorhytus belongs to a group within Protosomia known as ecdysoszoan, which contains arthropods and nematodes.
Professor Philip Donoghue, from the University of Bristol, said: ‘We have considered many alternative groups that Saccorhytus could be related to, including the corals, anemones and jellyfish which also have a mouth but no anus.
“To solve the problem, our computational analysis compared the anatomy of Saccorhytus with all other living groups of animals, concluding a relationship with the arthropods and their relatives, the group that includes insects, crabs and roundworms.”
Saccorhytus’ lack of an anus means that all the waste material in his body would have come back through the mouth.
But it’s also made for an intriguing slice of evolutionary history, suggesting that the trait disappeared within its group at some point.
When it was believed to be part of the Deuterostomia group, the disappearing anus contributed to our understanding of how the modern vertebrate body came to be.
But now this event should be considered part of the history of ecdysozoa, which have a continuous gut extending from mouth to anus.
Shuhai Xiao of Virgina Tech, USA, said: “Saccorhytus’ membership of the group indicates that it has declined in evolutionary terms, inheriting the anus that its ancestors would have inherited.”
Ecdysozoa traditionally have worm-like bodies, so the sac-like Saccorhytus implies that the ancestor of the ecdysozoa may not be worm-like.
Xiao said, “We still don’t know the exact position of Saccorhytus in the tree of life, but it may reflect the ancestral condition from which all members of this diverse group evolved.”
Hundreds of X-rays were taken of a new fossil using a particle accelerator at the Swiss Light Source in Switzerland to construct a detailed 3D digital model. This showed that the creature had spines around its mouth created by a decay-resistant cuticle layer extending through the pores. Shown are microscopic images of Saccorhytus coronarius
Further research may include how the Saccorhytus lived, for example floating in the sea or among grains of sand.
Researchers also want to determine exactly what they used their mouth spears for, such as deterring predators or stabilizing their prey.
The earliest human ancestor is also still sought after, as the Saccorhytus is not considered part of the Deuterostomia.
“The second oldest deuterostome fossil is almost 20 million years younger,” Xiao says.
Protostomes have a fossil record dating back to 550 million years ago, but deuterostomes are estimated to be only 515 million years old.
Xiao added: “This means there is a big hole in the fossil record of deuterostomes, or our side of the animal tree.
“So we’re going to continue to dig and look for the real first deuterostome fossils.”
Oldest species with a spine dating back more than 500 million years
The oldest known relative of all vertebrates on Earth swam in the oceans 518 million years ago, a new study reveals.
Researchers in China have analyzed fossils of yunnanozoa, an extinct gentle organism that lived during the Cambrian period of our planet’s history.
The fossils, found in Yunnan province, China, show that the creature is from Earth oldest known ‘stem vertebrate’ – a vertebrate that is extinct, but very closely related to living vertebrates.
Yunnanozoa were very simple fish-like organisms that lived underwater, but they had “basket-like” skeletons similar to today’s vertebrates.
They are also thought to have been deuterostomes — meaning their anus formed in front of their mouths during embryonic development.
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