The fossil of a tiny sea creature that died more than half a billion years ago could force science textbooks to rewrite how the brain evolved.
A study has been published science—led by Nicholas Straussfeld, a Regents Professor in the Department of Neuroscience at the University of Arizona, and Frank Harth Reader in Evolutionary Neuroscience at King’s College London —provided the first detailed description of the worm Cardiodictyon catenulum preserved in rocks in southern China. Yunnan province. Barely half an inch (less than 1.5 cm) long and initially discovered in 1984, the fossil had until now hid an important mystery: a finely preserved nervous system, including a the brain.
“As far as we know, this is the oldest fossil brain we know,” Straussfeld said.
Cardiodiction The armored belongs to an extinct group of animals known as lobopodians, which were abundant during the early period known as the Cambrian, when virtually all the major animal lineages appeared in a very short period of time, 540 million to 500 million years ago. Lobopodians probably walked on the sea floor using multiple pairs of soft, webbed feet that lacked the joints of their descendants, euarthropods—Greek for “true jointed feet.” The closest living relative of lobopodians today is the velvet worm, which lives mainly in Australia, New Zealand and South America.
A debate going back to the 1800s
its fossils Cardiodiction Express an animal with a segmented trunk containing a repeating system of nerve structures known as ganglia. This is in stark contrast to his head and brain, both of which show no evidence of division.
“This anatomy was completely unexpected because the head and brain of modern arthropods, and some of their fossil ancestors, have been thought to be segmented for over a hundred years,” Strausfeld said.
According to the authors, the discovery resolves a long and heated debate about the origin and structure of the head of arthropods, the world’s most species-rich group of animals. Arthropods include insects, crustaceans, spiders and other arachnids, plus some other genera such as millipedes and centipedes.
“Since the 1880s, biologists have noticed the distinctly segmented appearance of the trunk common to arthropods and essentially extrapolated it to the head,” Harth said. “Thus the field assumes 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 trunk nervous system evolved separately,” Strausfeld said.
The brain fossilizes
Cardiodiction It was part of the Chengjiang Fauna, a famous deposit of fossils discovered by paleontologist Xiangguang Hou in Yunnan Province. The soft, delicate bodies of lobopodians are well preserved fossil recordBut other than Cardiodiction None have been verified for their heads and brains, perhaps because lobopodians are generally small. The most prominent part Cardiodiction A series of triangular, saddle-shaped structures that define each segment and serve as attachment points for pairs of legs. They were found in much older rocks dating back to the Cambrian.
“This tells us that armored lobopodians were probably the earliest arthropods,” Straussfeld said, even predating trilobites, an iconic and diverse group of marine arthropods that went extinct about 250 million years ago.
“Until very recently, the general understanding was ‘brains don’t fossilize,'” Hirth said. “So you wouldn’t expect to find a fossil with a preserved brain in the first place. And, secondly, this animal is so small that you wouldn’t even dare look at it expecting to find a brain.”
However, work over the past 10 years, much of it by Straussfeld, has identified several cases of preserved brains of various fossil arthropods.
A simple genetic ground plan for building the brain
In their new study, the authors didn’t just identify the brain Cardiodiction But it compares with known fossil and living arthropods, including spiders and centipedes. By analyzing gene expression patterns in their living descendants along with detailed anatomical studies of lobopodian fossils, they concluded that a shared blueprint of brain organization has been maintained from the Cambrian to the present day.
“By comparing known gene expression patterns in living species,” Harth said, “we identified a common signature of all brains and how they are formed.”
inside CardiodictionThe three brain domains are each associated with a characteristic pair of head appendages and one of the three segments of the anterior digestive system.
“We realized that each brain domain and its associated properties are specified by the same combination of genes, regardless of the species we look at,” added Harth. “This suggests a common genetic ground plan for brain formation.”
Lessons for the evolution of the vertebrate brain
Harth and Strausfeld say the principles outlined in their study likely apply to other animals outside of arthropods and their close relatives. This has important implications when comparing nervous system Arthropods with vertebrates, which show a similar distinct architecture whereas the forebrain and midbrain are genetically and developmentally distinct from vertebrates, they say.
Strausfeld said their findings also provide a message of continuity at a time when the planet is changing dramatically under the effects of climate change.
“At a time when major geological and climatic events were reshaping the planet, common marine organisms such as Cardiodiction gave rise to the world’s most diverse group—the euarthropods—which eventually spread to every emerging habitat on Earth, but which are now threatened by our own transient species.”
The paper, “The Lower Cambrian Lobopodians Cardiodiction Resolves the Origin of the Arthropod Brain” is co-authored by Xiangguang Hou at the Yunnan Key Laboratory for Paleontology at Yunnan University in Kunming, China, and Marcel Sayre, who is at Lund University in Sweden and the Department of Biology. Macquarie University in Sydney. Science.
Nicholas J. Straussfeld et al., The Lower Cambrian lobopodian Cardiodiction resolves the origin of the urthropod brain, science (2022). DOI: 10.1126/science.abn6264. www.science.org/doi/10.1126/science.abn6264
Derek Easy Briggs et al., Keeping the Heads Together, science (2022). DOI: 10.1126/science.add7372
University of Arizona
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