Science books wrong?  The 525-million-year-old fossil defies the popular explanation of brain evolution

Science books wrong? The 525-million-year-old fossil defies the popular explanation of brain evolution

An artist’s impression of a 525-million-year-old line of cards on the shallow coastal sea floor, emerging from a small stromatolite shelter built by photosynthetic bacteria. Credits: Nicholas Straussfeld/University of Arizona

Fossils of a small sea creature with a delicately preserved nervous system resolve an age-old debate about how brains evolved in arthropods, the most species-rich group in the animal kingdom, a new study reveals.

Fossils of a tiny sea creature that died more than half a billion years ago could lead to a rewriting of the science textbook on how the brain evolved.

A new study provides the first detailed description of catenulum of the heartA worm-like animal preserved in rocks in Yunnan Province, southern China. Measuring less than 1.5 cm long and first discovered in 1984, the fossil still hides an important secret: a carefully preserved nervous system, including the brain. Published in the journal Sciences On November 24, the study was led by Nicholas Strausfeld, Regent Professor in the Department of Neuroscience at the University of Arizona, and Frank Hirth, Lecturer in Developmental Neuroscience at King’s College London.

“As far as we know, this is the oldest fossilized brain known to date,” Strausfeld said.

the heart It belonged to an extinct group of animals known as lobopods, which proliferated early on during a period known as the Cambrian when almost all major animal lineages appeared in a very short period of time between 540 million and 500 million years ago. Lobopodians likely moved on the seafloor using several pairs of soft, stubby legs that lacked the joints of their descendants, euarthropods—Greek for “true articulated foot.” The closest living relatives of the lobectomy today are the velvet worms that live mainly in Australia, New Zealand and South America.

The fossilized Cardiodictyon series was discovered in 1984 among a variety of extinct creatures known as Chengjian fauna in Yunnan, China. In this photo, the animal’s head is on the right. Credits: Nicholas Straussfeld/University of Arizona

A debate dating back to the nineteenth century

fossils the heart It reveals an animal with a segmented trunk that regularly contains nervous structures known as ganglia. This is in stark contrast to his head and brain, both of which lack any evidence of segmentation.

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“This anatomy was completely unexpected because the heads and brains of modern arthropods and some of their fossilized ancestors had been considered fragmentary for over a hundred years,” Strausfeld said.

According to the authors, this discovery resolves a long and heated debate about the origin and formation of cephalopods in arthropods, the most species-rich group in the animal kingdom. Arthropods include insects, crustaceans, spiders, and other spiders, as well as some other genera such as centipedes and centipedes.

“Since the 1880s, biologists have noted the distinctly segmented appearance of the trunk typical of arthropods, extrapolating this particularly to the head,” Heath said. “This is how the field hypothesized that the head is an anterior extension of a segmented torso.”

“But the heart “It shows that the early head was not segmented, and neither was the brain, which indicates that the brain and the nervous trunk developed separately,” Strausfeld said.

Petrified head and ghost of the heart chain

Fossil head of the Cardiodictyon series (front right). Purple deposits indicate fossilized brain structures. Credit: Nicholas Straussfeld

Brains do not freeze

the heart It was part of the Zhengjiang Fauna, a famous fossil deposit in Yunnan Province that was discovered by paleontologist Xianguang Hu. The soft, delicate, lobed bodies are well preserved in the fossil record, but they are different the heart None of them have been examined on their heads and brains, perhaps because the lobes are generally small. Post highlight the heart They were a series of triangular, saddle-shaped structures that defined each segment and acted as attachment points for pairs of legs. They are found in ancient rocks dating back to the arrival of the Cambrian period.

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“This tells us that the armored lobules were probably the oldest arthropods,” Strausfeld said, even predating the trilobites, an iconic and diverse group of marine arthropods that went extinct about 250 million years ago.

“Until recently, the general belief was that brains don’t freeze,” Heath 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 in hopes of finding a brain.”

However, work over the past 10 years, much of which Strausfeld has done, has identified several cases of brains preserved in a variety of fossilized arthropods.

A common genetic map of brain functioning

In their new study, the authors didn’t just identify a brain the heart But we also compared it to known fossils and living arthropods, including spiders and centipedes. By combining detailed anatomical studies of fossil lobotomy with analyzes of gene expression patterns in their living descendants, they concluded that a common blueprint for brain organization had survived from the Cambrian period to today.

“By comparing known gene expression patterns in living species, we identified a common signature for all brains and how they are formed,” Heath said.

in the heartThree cerebral domains are associated with a distinct pair of major appendages and with one of the three segments of the anterior GI tract.

“We’ve realized that every part of the brain and its associated characteristics are determined by the same combination genes, no matter what species we’re looking at,” Heath added. “This suggests a common genetic plan for making a brain.”

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Lessons for vertebrate brain development

Hirth and Straussfeld say the principles described in their study could apply to organisms other than arthropods and their relatives. This has important implications, they said, when comparing the nervous system of arthropods with that of vertebrates, which exhibit similar characteristic architecture in which the forebrain and midbrain differ genetically and developmentally from the spinal cord.

Their findings also provide a communication message at a time when the planet is undergoing dramatic change under the influence of climate change, Straussfeld said.

“At a time when great geological and climatic events were reshaping the planet, simple marine organisms like… the heart It gave rise to the world’s most diverse group of organisms – the euarthropods – which eventually spread to every nascent habitat on Earth, but are now threatened by ephemeral species. “

Reference: “Lobopodian Lower Cambrian the heart Outlines the Origin of Osteoblasts” by Nicholas J. Strausfeld, Xiangwang Hu, Marcel E. Sayre, and Frank Hirth, 24 Nov. 2022, Available here. Sciences.
DOI: 10.1126/science.abn6264

This paper was co-authored by Xianguang Hou of the Yunnan Key Laboratory of Paleontology at Yunnan University in Kunming, China, and Marcel Sayer, who hold positions at Lund University in Lund, Sweden and in the Department of Biological Sciences at Macquarie University. in Sydney.

This work was funded by the National Science Foundation, the University of Arizona Regents Fund, and the UK Biotechnology and Biological Sciences Research Council.

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