The oldest known relative of all vertebrates on Earth swam in the oceans 518 million years ago, according to a new study.
Researchers in China analyzed the fossils of the Yunnanozoic, an extinct soft-bodied organism that lived in the Cambrian period of our planet’s history.
Fossils found in the Yunnan province of China show that this creature belongs to the Earth. the oldest known “stem vertebrate” is a vertebrate that is extinct but very closely related to modern vertebrates.
The Yunnanozoa were very simple fish-like organisms that lived underwater, but they had “basket” skeletons similar to modern vertebrates.
They are also thought to have been deuterostome, meaning that their anus formed in front of their mouth during embryonic development.
An artistic reconstruction of the underwater inhabitants of the Yunnanozoic shows “basket” skeletons similar to modern vertebrates.
Vertebrates, including fish, amphibians, reptiles, birds, mammals, and humans, share unique features such as a backbone and skull, while invertebrates are animals without backbones.
The new study was conducted by experts from the Nanjing Institute of Geology and Paleontology and Nanjing University in China’s Jiangsu province.
“The Yunnanozoi are Cambrian animals with a taxonomic position that has long been debated,” their article says.
“Our phylogenetic analysis confirms that the Yunnanozoans are stem vertebrates.”
Scientists have long puzzled over a gap in the fossil record that could explain the evolution of invertebrates into vertebrates.
The evolutionary process that led invertebrates to develop backbones, and what these earliest vertebrates looked like, has remained a mystery for centuries.
As scientists have studied vertebrate evolution, the focus has been on the pharyngeal arches, paired structures that grow on either side of the future head and neck of the developing embryo and fuse in the middle.
Mammalian embryos have five pairs of these pharyngeal arches. As the mammalian embryo grows, the arches of the pharynx form parts of the face and neck, such as muscles, bones, and connective tissue.
The authors say the pharyngeal arches are a “key innovation” that likely contributed to the evolution of vertebrate jaws and skulls.
The pharyngeal arch is thought to have evolved from a “bar” of non-articulated cartilage in vertebrate ancestors such as the chordate amphioxes, small “fish-like” organisms, and close invertebrate relatives of vertebrates.
In an effort to better understand the role of the pharyngeal arch in ancient vertebrates, the research team studied soft-bodied Yunnanzoan fossils found in Yunnan, China.
But whether such anatomy really existed among ancient ancestors is not known for certain.
In an effort to better understand the role of the pharyngeal arch in ancient vertebrates, the research team examined the fossils of 127 Yunnanozoic specimens.
The samples have well-preserved carbonaceous residues, allowing the team to conduct detailed analysis using microscopy, spectrometry and other methods.
The results confirmed that the Yunnanozoans have cellular cartilages in their pharynx, a feature that is thought to be characteristic of vertebrates, suggesting that they are stem vertebrates.
During their study, the team also noticed that all seven pharyngeal arches in the Yunnanzoic fossils are similar to each other.
All arches have bamboo segments and threads. Neighboring arches are all connected by horizontal rods both above and below, forming a basket.
The basket-shaped pharyngeal skeleton is a feature found today in modern jawless fish such as lampreys and hagfish.
The pharyngeal arches are paired structures that grow on either side of the future head and neck of the developing embryo and merge in the middle. Shown here is a rat embryo with pharyngeal arches.
“Two types of pharyngeal skeletons – basket-shaped and isolated – are found in Cambrian and living vertebrates,” said study author Tian Qingyi.
“This means that the shape of the pharyngeal skeletons has a more complex early evolutionary history than previously thought.”
The researchers describe the Yunnanozoans as “controversial” because their classification has been debated for about three decades.
But new anatomical observations confirm the evolutionary position of the Yunnozoic at the very bottom of the vertebrate life tree.
New study published in the journal The science.
