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Photoreceptor evolution in vertebrates. The common ancestor of tetrapods and amniotes are tetrachromats. Asteroid impact at 65 million years ago (mya) led to the Cretaceous-Paleogene extinction event when most mammals became di-chromates to adjust to underground habitats. ‘Re-evolution’ occurred around 35 mya when primates became trichromats (Bowmaker, 2008; Jacobs, 2009).
