Science books wrong? 525-million-year-old fossil defies common explanation for brain evolution

Fossils of a tiny sea creature with a delicately preserved nervous system resolve a century-old debate about how the brain evolved in arthropods, the most species-rich group in the animal kingdom, according to a new study.

Fossils of a tiny sea creature that died more than half a billion years ago may prompt 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 half an inch (less than 1.5 cm) long and initially discovered in 1984, the fossil still conceals a crucial secret: a delicately preserved nervous system, including the brain. Published in the journal Sciences On November 24, the research was led by Nicholas Strausfeld, Regent Professor in the Department of Neuroscience at the University of Arizona, and Frank Hirth, Reader of Evolutionary Neuroscience at King’s College London.

“As far as we know, this is the oldest fossilized brain that we know of so far,” said Straussfeld.

the heart It belongs to an extinct group of animals known as the lobotomy, which was plentiful early on during a period known as the Cambrian, when almost all major animal lineages appeared during a very short period of time between 540 million and 500 million years ago. Lobopodians likely moved on the seafloor using multiple pairs of soft, stubby legs that lack the joints of their descendants, euarthropods — a Greek word for “true articulated foot.” Today’s closest living relatives of the lobotomy 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. Credit: Nicholas Straussfeld/University of Arizona

A debate dating back to the nineteenth century

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

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

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

“Since the 1880s, biologists have noted the distinctly segmented appearance of the trunk typical of arthropods, extrapolating that mainly to the head,” Heath said. “This is how the field came to the assumption 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 its brain, which indicates that the brain and trunk nervous system likely developed separately,” Strausfeld said.

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

Brains do not ossify

the heart It was part of the Zhengjiang Fauna, a famous fossil deposit in Yunnan Province discovered by paleontologist Xianguang Hu. The soft, delicate bodies of lobotomy have been well preserved in the fossil record, but otherwise the heart None have been examined for their head and brain, perhaps because lobules are generally small. Highlight parts the heart They were a series of triangular, saddle-shaped structures that defined each segment and acted as attachment points for pairs of legs. Those are found in older rocks dating back to the advent of the Cambrian period.

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

“Until very recently, the common understanding was that brains don’t fossilize,” 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 numerous cases of brains preserved in a variety of fossilized arthropods.

Common genetic floor plan to make a brain

In their new study, the authors not only identified 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 the fossil lobopods with analyzes of gene expression patterns in their living descendants, they concluded that a common blueprint for brain organization was preserved from the Cambrian period until 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 head appendages and with one of the three segments of the anterior digestive tract.

Heath added, “We realized that every area of ​​the brain and its corresponding features are determined by the same combination genes, regardless of which species we looked at.” “This suggested a common genetic floor plan for making a brain.”

Lessons for vertebrate brain development

Hirth and Straussfeld say the principles described in their study may apply to other organisms outside of arthropods and their immediate relatives. This has important implications, they said, when comparing the nervous system of arthropods with that of vertebrates, which show a similarly distinctive architecture in which the forebrain and midbrain are genetically and developmentally different from the spinal cord.

Their findings also provide a communication message at a time when the planet is changing dramatically under the influence of climate shifts, Strausfeld said.

“At a time when major geological and climatic events were reshaping the planet, simple marine animals such as… 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 Decides the Origin of Orthropod Brains” By Nicholas J. Straussfeld, Xiangwang Hu, Marcel E. Sayre and Frank Hirth, Nov. 24, 2022, Available Here. Sciences.
DOI: 10.1126/science.abn6264

The paper was co-authored by Xianguang Hou at the Yunnan Key Laboratory of Paleontology at Yunnan University in Kunming, China, and Marcel Sayer, who has appointments 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.