What was at the beginning of the development that led to the living beings to which we belong? New research results on a mysterious Cambrian sea creature are now shedding light on the early evolutionary history of vertebrates: researchers have uncovered characteristics in approximately 518 million-year-old fossils from Yunnanozoon that make these creatures appear as the oldest known representatives of the group from which the vertebrates evolved.
A small shoot formed, which became stronger and stronger and finally branched out profusely: Life on our planet grew like a tree, they say. First, multicellular organisms emerged from microbes, then the first animals appeared on the stage of evolution, which subsequently produced ever more complex forms. This is how the basic lines of development came about, to which today’s animals can still be assigned. The vertebrates, which include fish, amphibians, reptiles, birds and mammals, are characterized by features such as a spine and a skull. It is assumed that the vertebrates once emerged from invertebrate ancestors, in which the bases of these body characteristics have gradually formed.
Controversy over Cambrian beings
Exactly how this happened and what the first representatives of this evolutionary history might have looked like remains an exciting scientific question. An important aspect is which known fossil beings could be the oldest that can be clearly linked to the evolution of vertebrates. In this regard, there has been a controversy among paleontologists for about 30 years about strange sea creatures whose fossils have been found in Yunnan province of China.
These beings, called Yunnanozoa, have been dated to be around 518 million years old. They had worm-like characteristics, but structures also emerged that some scientists interpreted as archetypes of the so-called gill arches. They are anatomical features that make up the muscles, bones and connective tissue of parts of the face and neck in vertebrates. However, how exactly the structures of the Yunnanozoa are to be assigned has so far remained controversial.
The researchers led by Qinqyi Tian from Nanjing University are now presenting the results of newly discovered Yunnanozoan fossils. As they explain, the 127 specimens in total show amazingly well-preserved carbonaceous residues, which have allowed them to carry out previously impossible structural studies and detailed geochemical analyzes of the former body structures of the Yunnanozoans. Various modern analysis methods were used, including X-ray microtomography and scanning and transmission electron microscopy.
Basal position in pedigree actuated
The researchers were able to confirm that Yunnanozoa had cellular cartilage structures in their pharynx that formed a kind of pharyngeal skeleton. Specifically, they found that there were seven arcs shaped by an extracellular matrix of so-called microfibrils. All of these units had special sub-segments and appendages. The adjacent arches were connected by dorsal and ventral horizontal rods, forming a kind of basket. According to the scientists, this combination of tissues and structures is considered a characteristic feature of vertebrates: A basket-like pharyngeal skeleton can still be found today in jawless fish such as lampreys and hagfish, which are considered primordial, Tian and his colleagues explain.
The scientists conclude that the new anatomical discoveries support the evolutionary classification of the Yunnanozoa in the very basal part of the vertebrate tree of life. With their age of about 518 million years, they now represent the oldest known beings that can be clearly assigned to the phylogenetic group from which the vertebrates – and ultimately us too – emerged.
Source: Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, Article: Science, doi: 10.1126/science.abm2708