A 525-million-year-old fossil contradicts the textbook explanation of brain evolution
Fossils of a tiny sea creature that died more than half a billion years ago may cause a science textbook to rewrite how brains evolved.
A study published in Science— led by Nicholas Strausfeld, Regents Professor in the Department of Neurology at the University of Arizona, and Frank Hirt, Reader in Evolutionary Neurology at King’s College London — provides the first detailed description of Cardiodictyon catenulum, a worm-like animal preserved in rocks in the southern part of China Yunnan Province. Just half an inch (less than 1.5 centimeters) long and originally discovered in 1984, the fossil held an important secret until now: a delicately preserved nervous system, including brain.
“As far as we know, this is the oldest fossilized brain that we know of so far,” Strausfeld said.
Cardiodiction belong to an extinct group of animals known as armored lobopods that were abundant early in a period known as the Cambrian, when almost all major animal lineages appeared in an extremely short time between 540 million and 500 million years ago. Lobopods probably moved along the sea floor using multiple pairs of soft, thick legs that lacked the joints of their descendants, the euarthropods, Greek for “true jointed foot.” Today’s closest living relatives of lobopods are the velvet worms, which live mainly in Australia, New Zealand and South America.
A debate dating back to the 1800s
Fossils of Cardiodiction reveal an animal with a segmented trunk in which there are repeating arrangements of nerve structures known as ganglia. This contrasts sharply with his head and brain, both of which lack evidence of segmentation.
“This anatomy was completely unexpected because the heads and brains of modern arthropods and some of their fossilized ancestors had been considered segmented for over a hundred years,” Strausfeld said.
According to the authors, the find resolves a long and heated debate about the origin and composition of the head in arthropods, the most species-rich group in the animal kingdom. Arthropods include insects, crustaceans, spiders and other arachnids, plus some other lineages such as millipedes and centipedes.
“Beginning in 1880, biologists noticed the distinctly segmented appearance of the body typical of arthropods and basically extrapolated that to the head,” Hirt said. “This is how the field arrived at the suggestion that the head is an anterior extension of a segmented trunk.”
“But Cardiodiction shows that the early head was not segmented, nor was its brain, suggesting that the brain and the body’s nervous system probably evolved separately,” Strausfeld said.
Cardiodiction is part of the Chengjiang Fauna, a famous fossil site in Yunnan Province discovered by paleontologist Xianguang Hou. The soft, delicate bodies of the lobopodia are well preserved in fossilsbut besides Cardiodiction none has been carefully examined for its head and brain, probably because the lobopodia are usually small. The most prominent parts of Cardiodiction were a series of triangular, saddle-shaped structures that defined each segment and served as attachment points for pairs of legs. They were found in even older rocks dating back to the onset of the Cambrian.
“This tells us that armored lobopods may have been the earliest arthropods,” Strausfeld said, predating even trilobites, an iconic and diverse group of marine arthropods that went extinct about 250 million years ago.
“Until very recently, the common understanding was ‘brains don’t fossilize,'” Hirth said. “So you wouldn’t expect to find a fossil with a brain preserved in the first place. And secondly, this animal is so small that you wouldn’t even dare to look at it in the hope of finding a brain.”
However, work over the past 10 years, much of it by Strausfeld, has identified several instances of preserved brains in various fossil arthropods.
A common genetic blueprint for creating a brain
In their new study, the authors not only identified the brain of Cardiodiction but also compared it with those of known fossils and of living arthropods, including spiders and centipedes. Combining detailed anatomical studies of fossil lobopods with analyzes of gene expression patterns in their living descendants, they conclude that a shared blueprint of brain organization has been maintained from the Cambrian to the present.
“By comparing known patterns of gene expression in living species,” Hirt said, “we identified a common signature of all brains and how they form.”
in Cardiodictioneach of three brain domains is 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 characteristics were determined by the same combination of genes, regardless of the species we were looking at,” Hirth added. “This suggests a common genetic blueprint for brain creation.”
Lessons in the evolution of the vertebrate brain
Hurt and Strausfeld say the principles described in their study likely apply to other creatures beyond arthropods and their direct relatives. This has important implications when comparing nervous system of arthropods with those of vertebrates, which show a similarly distinct architecture in which the forebrain and midbrain are genetically and developmentally distinct from the spinal cord, they said.
Strausfeld said their findings also offer a message of continuity at a time when the planet is changing dramatically under the influence of climate change.
“At a time when major geological and climatic events were changing the planet, simple marine animals like Cardiodiction gave rise to the world’s most diverse group of organisms—the arthropods—which eventually spread to every emergent habitat on Earth, but are now threatened by our own ephemeral species.
The report “Lower Cambrian Lobopodium Cardiodiction Resolves the Origin of Euarthropod Brains’ is co-authored by Xianguang Hou of the Yunnan Key Laboratory of Paleontology at Yunnan University in Kunming, China, and Marcel Sayre, who holds appointments at Lund University in Lund, Sweden, and in the Department of Biological Sciences in Macquarie University in Sydney.
Nicholas J. Strausfeld et al, The Lower Cambrian lobopodian Cardiodictyon resolves the origin of the euarthropod brain, Science (2022). DOI: 10.1126/science.abn6264. www.science.org/doi/10.1126/science.abn6264
Derek EG Briggs et al., Assembling the Heads, Science (2022). DOI: 10.1126/science.add7372
University of Arizona
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