Vertebrates are a subphylum of animals that arose 525 million years ago. They are characterized as chordates with backbones and spinal columns. The development of vertebrates poses some deep evolutionary mysteries due to the speed with which the complex vertebrate brain evolved. In terms of evolutionary time, it seems to develop extremely rapidly, without any invertebrate taxa displaying vertebrate-like brain structure.
Invertebrates that share the closest common ancestry with vertebrates, the sea squirt and lancelet, do not have the characteristic signalling centers possessed by all vertebrates. This has led to the belief that some evolutionary pressure caused the vertebrate brain to arise for the first time within vertebrates, without any precursor elsewhere in the animal kingdom.
|All vertebrate brains share several homologous features derived from common ancestry|
However, Pani et al., surprisingly found evidence for characteristic brain signaling centers in the acorn worm (Saccoglossus kowalevskii), which is a hemichordate. Although hemichordates are more distantly related to vertebrates than both sea squirts and lancelets, sharing a common ancestor over 500 million years ago, this species has the necessary signaling centers to form the ‘scaffold’ of the vertebrate brain.
This finding provides the first evidence that the last common ancestor of hemichordates and vertebrates had the genes necessary to construct a very complex vertebrate-like nervous system. It also provides us with an understanding of the type of signalling centres present during the development of the modern vertebrate brain.
Future research will need to be done in order to understand why modern sea squirts and lancelets do not possess these structures, why the acorn worm still possess them, and what pressures led the development of the vertebrate brain so quickly.
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Pani, A.M. et al. 2012. Ancient deuterostome origins of vertebrate brain signaling centres. Nature 483, 289-294.