The evolution of forebrain contralaterality as a response to eye development: the path of least resistance.
The origins of vertebrate forebrain contralaterality remain to be adequately explained. A new and novel idea outlined previously by this author as the Inversion Hypothesis (2009) proposed that the vertebrate forebrain developed its ‘wrong-sidedness’ as a response to the inversion of visual images in an ancestral vertebrate possessing a single frontal eye. Here we develop further the likely mechanisms such processes would entail. Using the single frontal eye of ancestral protochordates as a theoretical starting point for the evolution of bilateral vision in early vertebrates, a simple evolutionary pathway involving incremental improvements in retinal design shows how forebrain contralaterality could evolve due to the requirements set by geometric optical physics alone. This untested proposition, while challenging in many respects, provides a concise and credible framework based on established biological data, from which testable corollaries can now be drawn.
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