A new study claims to have found the answer to the question of how the brain develops in arthropods, the most species-diverse group in the animal kingdom. This question had been debated for more than a century, thanks to the remains of a tiny sea creature whose nervous system has been carefully preserved.
In light of the fossils of a tiny sea creature that disappeared more than half a billion years ago, it may be necessary to rethink how the brain originated in science textbooks.
A new study offers the first comprehensive description of Cardiodictyon catenulum, a worm-like creature found preserved in rocks in southern China's Yunnan province. The fossil, which is less than 1,5 centimeters in length and was first discovered in 1984, has a delicately preserved neurological system, including the brain, that has been kept secret until now.
The study's principal researchers are Nicholas Strausfeld, Regents Professor of Neuroscience at the University of Arizona, and Frank Hirth, a reader in evolutionary neuroscience at King's College London. The study was published November 24 in the journal Science.
According to Strausfeld, “this is the oldest fossilized brain we know to date.”
Cardiodictyon belongs to the extinct group of animals known as armored lobopodians, common 540 million to 500 million years ago, during the early Cambrian period, when nearly all major animal lineages emerged in a very short time. The multiple pairs of soft, stubby legs lacking the joints of their ancestors, the euarthropods (Greek for "true jointed feet"), likely explain how lobopods walked on the seafloor. Found primarily in Australia, New Zealand, and South America, velvet worms are today's closest living cousins to lobopodians.
A dispute dating back to the 1800s
Cardiodictyon fossils show an animal with a segmented body and repetitive strings of nerve cells called ganglia. This contrasts sharply with the skull and brain, both of which show no signs of segmentation.
"This anatomy was completely unexpected," Strausfeld said, as the heads and brains of some of the modern arthropods and their fossilized relatives were thought to be segmented for more than a century.
According to the authors, this discovery resolves a long and controversial debate about the origin and structure of the head in arthropods, the most species-diverse group of animals. Insects, crustaceans, spiders, and other arthropods are considered arthropods, along with some other lineages such as centipedes and centipedes.
According to Hirth, biologists began observing the distinctively segmented body of arthropods in the 1880s and reflected this mainly on the head. The theory that the head is the anterior extension of a segmented body was thus developed.
But according to Strausfeld, Cardiodictyon shows that neither the early head brain nor its segments are like this, meaning that the brain and trunk nervous system most likely developed independently.
Brains Can Be Boiled
Cardiodictyon was a member of the Chengjiang fauna, a well-known fossil site in Yunnan Province unearthed by naturalist Xianguang Hou. Although lobopodians with the exception of the cardiodictyon had soft and fragile bodies, their heads and brains were not closely studied in the fossil record, perhaps because they were typically small. The most notable features of the cardiodictyon are a series of triangular, saddle-shaped structures that separate each segment and serve as attachment sites for pairs of legs. These have been discovered in even older rocks dating back to the Cambrian period.
According to Strausfeld, this suggests that armored lobopodians may have been the first arthropods to even predate the famous and diverse group of marine arthropods known as trilobites, which went extinct about 250 million years ago.
According to Hirth, until recently it was generally accepted that “brains do not fossilise”. “Therefore, you wouldn't expect to find a fossil with a preserved brain to begin with. Also, you wouldn't even dare to look at this creature in hopes of discovering a brain because of how small it is.
The discovery of a large number of intact brain cases in various fossilized arthropods over the past decade, most of them conducted by Strausfeld, is a culmination of this research.
A single genetic blueprint for the development of the brain
The authors of the new study not only described Cardiodictyon's brain, but also compared it with the brains of known fossils and contemporary arthropods such as spiders and centipedes. Combining in-depth anatomical studies of lobopodian fossils with analysis of gene expression patterns in their living descendants, the researchers concluded that a common blueprint of brain organization has been preserved from the Cambrian to the present.
"We've identified a common feature of all brains and how they develop," said Hirth. "By comparing known patterns of gene expression in living species."
Each of the three parts of the anterior digestive tract and one of three different pairs of head appendages is connected to one of the three brain areas in the Cardiodictyon.
“We found that regardless of the species we studied, each brain area and accompanying traits are determined by the same set of genes,” Hirth said. This pointed to a "common genetic framework for the development of a brain."
According to Hirth and Strausfeld, besides arthropods and their close relatives, other organisms may be affected by the theories presented in their study. They added that this had important implications when they compared the nervous systems of arthropods and vertebrates with a similar design, where the forebrain and midbrain were genetically and developmentally different from the spinal cord.
Strausfeld said his findings also offer a message of continuity, at a time when the earth is changing drastically due to climate change.
Simple marine animals like Cardiodictyon gave rise to euarthropods, the most diverse group of organisms on earth at a time when important geological and climatic processes were transforming the earth. However, these euarthropods are now threatened by our own transient species.
Source: “The Lower Cambrian Lobopodian Cardiodictyon Resolves the Origin of Euarthropod Brains” by Nicholas J. Strausfeld, Xianguang Hou, Marcel E. Sayre and Frank Hirth, 24 November 2022, Science.
Günceleme: 28/11/2022 13:13