A recent study involving a fossil that died more than 525 million years ago may defy earlier theories on how brains evolved.
The study provides the first detailed description of a wormlike animal preserved in rocks in China's southern Yunnan province. Known as Cardiodictyon catenulum, the animal is barely half an inch long, and while its initial discovery was back in 1984, it hid an important secret- until now. That is, the presence of a nervous system comprising a brain.
The finding resolved a century-old debate about the origin and composition of the brain in arthropods - the group of animals (phylum) that includes insects, crustaceans, spiders, and other arachnids, as well as other lineages such as millipedes and centipedes.
Analyzing the delicately preserved nervous system led to new theories
Cardiodictyon was part of an extinct group of animals known as armored lobopodians, alive in the Cambrian period. This period lasted from roughly 541 million years ago - 485 million years ago.
According to a press release, lobopodians likely moved about the sea floor using "multiple pairs of soft, stubby legs that lacked the joints of their descendants, the euarthropods". Their closest relatives would be the velvet worms that primarily live in Australia, New Zealand, and South America.
The tiny size of the fossil made it difficult to x-ray the sample. Hence, a technique called "chromatic filtering" that employed high-resolution digitalized images to filter out light at different wavelengths was used.
The results were surprising. They revealed a segmented nervous system in the animal's trunk and a brain with three "confluent domains" in an unsegmented head.
"This anatomy was completely unexpected because the heads and brains of modern arthropods, and some of their fossilized ancestors, have for over a hundred years been considered as segmented," said Nicholas Strausfeld, a Regents Professor in the University of Arizona Department of Neuroscience who led the study.
Finding and tracing a common signature of brains
"Until very recently, the common understanding was 'brains don't fossilize,' so you would not expect to find a fossil with a preserved brain in the first place. This animal is so small you would not even dare to look at it in hopes of finding a brain," explained Frank Hirth, a reader of evolutionary neuroscience at King's College London who also led the study.
Upon combining their studies of the lobopodian fossils with analyses of gene expression patterns in their living descendants, the researchers identified a cerebral pattern that has been maintained from the Cambrian period 525 million years ago until the present.
"We have identified a common signature of all brains and how they formed. We realized that each brain domain and its corresponding features are specified by the same combination of genes, irrespective of the species we looked at. This identifies a common genetic ground plan for making a brain," said Hirth.
The results offer a message of continuity
According to Hirth and Strausfled, their findings could even be applied to creatures outside of arthropods.
They said that it could have "important implications when comparing the nervous system of arthropods with those of vertebrates, which show comparable architectures in which the forebrain and midbrain are genetically and developmentally distinct from the trunk’s spinal cord".
Strausfeld also added that their findings could symbolize a message of continuity, which is hopeful, considering the planet's dramatic change under climate shifts.
"At a time when major geological and climatic events were reshaping the planet, simple marine animals such as Cardiodictyon gave rise to the world's most diverse group of organisms – the euarthropods – that eventually spread to every emergent habitat on Earth, but which are now being threatened by our ephemeral species," concluded Strausfeld.
For more than a century, the origin and evolution of the arthropod head and brain have eluded a unifying rationale reconciling divergent morphologies and phylogenetic relationships. Here, clarification is provided by the fossilized nervous system of the lower Cambrian lobopodian Cardiodictyon catenulum, which reveals an unsegmented head and brain comprising three cephalic domains, distinct from the metameric ventral nervous system serving its appendicular trunk. Each domain aligns with one of three components of the foregut and with a pair of head appendages. Morphological correspondences with stem group arthropods and alignments of homologous gene expression patterns with those of extant panarthropods demonstrate that cephalic domains of C. catenulum predate the evolution of the euarthropod head yet correspond to neuromeres defining brains of living chelicerates and mandibulates.
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