Researchers trace evolution of central nervous system
Although to the untrained eye, worms and people may appear to have very little, if nothing, in common, researchers from at the European Molecular Biology Laboratory (EMBL) have now confirmed that our brains evolved from the same ancestor. The results of the study, partly funded by the EU, are published in the current issue of Cell. The paper suggests that this common ancestor is none other than a lowly marine worm, called Platynereis dumerilii, the nervous system of which has remained unchanged for eons. Scientists have known for some time that vertebrates, insects and worms evolved from the same ancestor called Urbilateria. But their central nervous systems are different and were thought to have evolved only after their lineages had split during evolution. While vertebrates have a central nervous system in the shape of a spinal cord running along their backs, insects and annelid worms like the earthworm have a rope-ladder-like chain of nerve cell clusters on their belly side. Other invertebrates on the other hand have their nerve cells distributed diffusely over their body. So did the Urbilateria have a central nervous system to begin with and, if so, how might it have looked? Also, how did it give rise to the diverse range of nervous systems seen in animals today? These were the questions the researchers at EMBL sought to answer in their study. To do so, they investigated the molecular architecture of the trunk nervous system in the annelid Platynereis dumerilii. 'Platynereis can be considered a living fossil,' says Detlev Arendt, who led the study. 'It still lives in the same environment as the last common ancestors used to and has preserved many ancestral features, including a prototype invertebrate CNS [central nervous system].' Using in vivo time-lapse imaging, the researchers explored how the Platynereis dumerilii's neuroectoderm - the region in embryos that develops into the brain, spinal cord and nervous tissue of the peripheral nervous system - initially forms. They also tracked, using neural differentiation markers, the timing and spatial extent of early neurogenesis - the process by which neurons are created in the brain. This allowed the researchers compare the molecular fingerprint of Platynereis nerve cells with what is known about vertebrates. It revealed some surprising similarities. 'Our findings were overwhelming,' says Alexandru Denes, one of the researchers involved in the study. 'The molecular anatomy of the developing CNS turned out to be virtually the same in vertebrates and Platynereis. Corresponding regions give rise to neuron types with similar molecular fingerprints, and these neurons also go on to form the same neural structures in the annelid worm and vertebrate.' The new findings support a theory first proposed in 1875 by zoologist Anton Dohrn, which states that vertebrate and annelid nervous systems are of common descent, and that vertebrates turned themselves upside down during the course of evolution. 'This explains perfectly why we find the same centralised CNS on the backside of vertebrates and the bellyside of Platynereis,' explains Dr Arendt says. 'How the inversion occurred and how other invertebrates have modified the ancestral CNS throughout evolution are the next exciting questions for evolutionary biologists.'
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