Early insights into brain evolution

fossil brain

Fossil head of Cardiodictyon catenulum with brain remains (magenta). © Nicholas Strausfeld/ University of Arizona

Arthropods, such as insects, spiders, and millipedes, are the most diverse phylum in the animal kingdom. How their brain developed evolutionarily, however, has so far been controversial in science. A new study now brings clarity. In it, researchers describe the world's oldest known fossilized brain, preserved in the 525-million-year-old fossil of a worm-like sea creature — one of the earliest arthropods. The "modern" tripartite structure of the brain suggests that it evolved independently of the segmented nervous system of the rest of the body.

540 to 500 million years ago, almost all animal phyla that exist today arose during the Cambrian species explosion. Early in this phase, the so-called lobopods emerged, a group of armored sea creatures that probably propelled themselves along the sea floor using multiple pairs of soft, ungainly legs. Numerous fossils of these primeval arthropods, which are now extinct, have been preserved in fossil sites in southern China's Yunnan province, for example. Among them is a 1.5 centimeter long fossil of a lobopod named Cardiodictyon catenulum discovered in 1984. The special thing about this specimen: Its nervous system, including the brain, has been preserved in its fine structure.

Reconstruction of Cardiodictyon catenulum. © Nicholas Strausfeld/University of Arizona

Oldest fossil brain in the world

"To our knowledge, this is the oldest fossilized brain known to date," says Nicholas Strausfeld of the University of Arizona in Tucson. Together with his team, he has now examined the extraordinary fossil in detail. The researchers found that while the torso and the neuronal structures within it were segmented as expected, the head and brain showed no signs of segmentation. "This anatomy was completely unexpected, since the heads and brains of modern arthropods and some of their fossil ancestors have been thought to be segmented for over a hundred years," Strausfeld said.

Co-author Frank Hirth of King's College London explains: "Since the 1880s, biologists have noticed the distinct segmentation of the trunk typical of arthropods and essentially extrapolated this to the head. This led to the assumption that the head is an anterior extension of the segmented trunk.” As is typical for arthropods, the peripheral nervous system of Cardiodictyon consists of several nerve cell nodes, so-called ganglia, which are linked from segment to segment. Until now, it was assumed that the brain developed from a particularly large collection of interconnected ganglia. According to this, the head and brain of such an early representative as Cardiodictyon should show a clear segmentation. "But cardiodictyon shows that the early head was not segmented, nor was its brain, suggesting that the brain and trunk nervous system likely evolved separately," Strausfeld said.

Common genetic signature

To further test this thesis, Strausfeld and his colleagues compared the newly described primeval brain with the brains of other known fossils and living arthropods, including spiders and millipedes. In addition, they analyzed the gene expression patterns that are responsible for the blueprint of the brain in the representatives living today. "By comparing known gene expression patterns in living species, we have identified a common signature of all brains and their formation," reports Hirth.

Knowing this, the researchers at Cardiodictyon were able to identify three brain areas, each associated with a pair of head appendages and one of the three parts of the anterior digestive system. "We found that each area of ​​the brain and the corresponding traits are specified by the same combination of genes, regardless of the species studied," says Hirth. "This indicates a common genetic plan for the development of a brain." The authors assume that the basic principles they describe probably apply not only to arthropods, but also to other animal phyla, including vertebrates. Again, the forebrain and midbrain are genetically and developmentally separate from the spinal cord.

Source: Nicholas Strausfeld (University of Arizona, Tucson) et al., Science, doi: 10.1126/science.abn6264

Recent Articles

Related Stories

Stay on op - Ge the daily news in your inbox