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525-million-year-old fossil contradicts textbook theory of brain evolution

ANI | Updated: Nov 27, 2022 01:02 IST

Arizona [US], November 27 (ANI): A rethink of how brains originated in science textbooks may be required in light of the fossils of a tiny sea creature that perished more than half a billion years ago.
In a study that was recently published in Science, Nicholas Strausfeld, a Regents Professor in the Department of Neuroscience at the University of Arizona, and Frank Hirth, a reader in evolutionary neuroscience at King's College London, describe Cardiodictyon catenulum, a worm-like creature that has been preserved in rocks in China's southern Yunnan province. The fossil, which is less than 1.5 centimetres in length and was first uncovered in 1984, has a delicately preserved neurological system, including a brain, that has been kept secret up until this point.
"To our knowledge, this is the oldest fossilized brain we know of, so far," Strausfeld said.
Cardiodictyon belongs to the extinct group of animals known as armoured lobopodians, which were widespread early in the Cambrian period, between 540 million and 500 million years ago, when nearly all major animal lineages originated over a very brief period of time. Numerous pairs of soft, stubby legs without the joints of their ancestors, the euarthropods (Greek for "true jointed foot"), helped lobopodians move about on the sea floor. The velvet worms, which are primarily found in Australia, New Zealand, and South America, are the lobopodians' closest living cousins at the present time.
A debate going back to the 1800s: Cardiodictyon fossils show an animal with a segmented trunk and repeating arrangements of neural cells called ganglia. This stands in sharp contrast to its head and brain, both of which show no signs of segmentation.
"This anatomy was completely unexpected because the heads and brains of modern arthropods, and some of their fossilized ancestors, have for over a hundred years been considered as segmented," Strausfeld said.
The discovery, according to the authors, settles a protracted and contentious argument on the origin and make-up of the head in arthropods, the animal kingdom group with the greatest diversity of species. Insects, crustaceans, spiders, and other arachnids are all considered arthropods, along with some other lineages like millipedes and centipedes.
"From the 1880s, biologists noted the clearly segmented appearance of the trunk typical for arthropods, and basically extrapolated that to the head," Hirth said. "That is how the field arrived at supposing the head is an anterior extension of a segmented trunk."
"But Cardiodictyon shows that the early head wasn't segmented, nor was its brain, which suggests the brain and the trunk nervous system likely evolved separately," Strausfeld said.
Brains do fossilized: Cardiodictyon was a member of the Chengjiang fauna, a well-known fossil site in the Yunnan Province that naturalist Xianguang Hou unearthed. Although lobopodians, with the exception of Cardiodictyon, have soft, fragile bodies, none have, maybe because they are typically small, been closely examined for their heads and brains in the fossil record.
Cardiodictyon's most noticeable features were a number of triangular, saddle-shaped structures that distinguished each segment and functioned as places of attachment for pairs of legs. Those have been discovered in even earlier rocks that date back to the Cambrian era.
"That tells us that armoured lobopodians might have been the earliest arthropods," Strausfeld said, predating even trilobites, an iconic and diverse group of marine arthropods that went extinct around 250 million years ago.
"Until very recently, the common understanding was 'brains don't fossilize,'" Hirth said. "So you would not expect to find a fossil with a preserved brain in the first place. And, second, this animal is so small you would not even dare to look at it in hopes of finding a brain."
However, work over the last 10 years, much of it done by Strausfeld has identified several cases of preserved brains in a variety of fossilized arthropods.
A common genetic ground plan for making a brain: The authors of the new study not only identified Cardiodictyon's brain but also compared it to the brains of known fossils and contemporary arthropods, such as spiders and centipedes. They come to the conclusion that a common blueprint of brain organisation has been preserved from the Cambrian to the present day by combining in-depth anatomical studies of the lobopodian fossils with analyses of gene expression patterns in their living descendants.
"By comparing known gene expression patterns in living species," Hirth said, "we identified a common signature of all brains and how they are formed."
In Cardiodictyon, three brain domains are each associated with a characteristic pair of head appendages and with one of the three parts of the anterior digestive system.
"We realized that each brain domain and its corresponding features are specified by the same combination genes, irrespective of the species we looked at," added Hirth. "This suggested a common genetic ground plan for making a brain."
Lessons for vertebrate brain evolution: According to Hirth and Strausfeld, besides arthropods and their close relatives, other organisms may also be affected by the theories presented in their study. When contrasting the neural systems of arthropods and vertebrates, which have a similar distinctive design in which the forebrain and midbrain are genetically and developmentally different from the spinal cord, they added, this has significant consequences.
At a time when the globe is drastically changing due to climate upheavals, Strausfeld said their findings also offer a message of continuity.
"At a time when major geological and climatic events were reshaping the planet, simple marine animals such as Cardiodictyon gave rise to the world's most diverse group of organisms -- the euarthropods -- that eventually spread to every emergent habitat on Earth, but which are now being threatened by our own ephemeral species." (ANI)