The fossil of a tiny sea creature that died more than 5 billion years ago could force a science textbook to rewrite how the brain evolved.
A study published in the journal Science led by Nicholas Straussfeld, a professor in the Department of Neuroscience at the University of Arizona, and Frank Hess, a reader in evolutionary neuroscience at King’s College London, describes in detail for the first time Cardiodictyon chain, a worm-like animal preserved in rocks in Yunnan Province, southern China. Measuring just half an inch (less than 1.5 centimeters) long, the fossil was first discovered in 1984, and until now harbored an important secret: a well-preserved nervous system, including the brain.
“As far as we know, this is the oldest brain fossil we know of to date,” Straussfeld said.
electrocardiogram Belonging to an extinct group of animals known as armored phyllopods, they abounded during the early Cambrian period, when almost all major animal lineages arose in the extremely short period of time between 540 and 500 million years ago. Lobopodians likely moved across the ocean floor using multiple pairs of soft, stubby legs that lacked the joints of their descendants, euarthropods—Greek for “truly articulated feet.” The closest relatives of phyllopods today are the velvet worms that live mainly in Australia, New Zealand and South America.
A debate dating back to the 1800s
the fossil electrocardiogram have revealed an animal with a segmented trunk with repeating arrangements of neural structures called ganglia. This is in stark contrast to its head and brain, both of which lack any evidence of segmentation.
“This anatomy was completely unexpected, since the heads and brains of modern arthropods, as well as 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-standing and intense debate about the origin and composition of the head of arthropods, the most species-rich group in the world’s animal kingdom. Arthropods include insects, crustaceans, spiders and other arachnids, as well as other lineages such as millipedes and centipedes.
“Since the 1880s, biologists have noticed the distinctly segmented appearance of the torso typical of arthropods and essentially extrapolated it to the head,” Hirth said. “Assuming the head is an anterior extension of the segmented torso, that’s what the field has concluded.”
“but electrocardiogram shows that the early head was not segmented, and neither was its brain, suggesting that the brain and trunk nervous systems may have evolved separately,” Strausfeld said.
Brains do become fossils
electrocardiogram It is part of Chengjiang fauna, a famous fossil deposit in Yunnan Province discovered by paleontologist Hou Xianguang. The soft, delicate bodies of lobopodians are well preserved in the fossil record, but apart from electrocardiogram No one has examined their heads and brains closely, probably because leaf-pods are usually small.most prominent part electrocardiogram are a series of triangular saddle-shaped structures that define each section and serve as attachment points for the paired legs. Those are found in even older rocks dating back to the Cambrian period.
“This tells us that armored phyllopods were probably the earliest arthropods,” Strausfeld said, even before trilobites, an iconic and diverse group of marine arthropods that went extinct about 250 million years ago.
“Until recently, the common understanding was that ‘the brain doesn’t petrify,'” Hirth said. “So first you wouldn’t expect to find a well-preserved fossil brain. Second, the animal is so small that you wouldn’t even look at it to find a brain.”
However, work over the past 10 years, much of it by Strausfeld, has uncovered several cases of well-preserved brains in various arthropod fossils.
A Common Genetic Basis Plan for Making the Brain
In their new study, the authors not only identified electrocardiogram But it was also compared to known fossils and those of living arthropods, including spiders and centipedes. Combining detailed anatomical studies of fossilized phyllopods with analyzes of gene expression patterns in their extant descendants, they concluded that a shared blueprint of brain organization persisted from the Cambrian period to the present day.
“By comparing known gene expression patterns in extant species,” Hirth said, “we identified features common to all brains and how they were formed.”
exist electrocardiogramThree brain regions were each associated with a pair of characteristic head appendages and one of the three parts of the predigestive system.
“We realized that each brain region and its corresponding traits were specified by the same combination of genes, regardless of the species we looked at,” Hirth added. “This suggests a common genetic basis plan for making the brain.”
Lessons from vertebrate brain evolution
Hirth and Strausfeld say the principles described in their study may apply to organisms other than arthropods and their immediate family members. This, they say, has important implications when comparing the nervous systems of arthropods to those of vertebrates, which display a similarly unique structure in which the forebrain and midbrain are genetically and developmentally distinct from the spinal cord.
Their findings also add to the message that Earth is changing dramatically under the effects of climate change, Strausfeld said.
“At a time when major geological and climatic events are reshaping the Earth, simple marine animals such as electrocardiogram This gave rise to the world’s most diverse group of organisms – the euarthropods – which eventually spread to every emerging habitat on Earth, but are now threatened by our own ephemeral species. “
Funding for this work was provided by the National Science Foundation, University of Arizona Regents Fund and the UK Biotechnology and Biological Sciences Research Council.