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It's somewhat misleading to compare evolutionarily distant species' morphology without accounting for their respective evolution. In short, it's inaccurate to label Mammals  more or less complicated than other vertebrates based upon the number of cones they possess, just as it is misleading to develop evolutionary relationships for all species based upon singular criteria (ex. humans are more closely related to ants than to cats because cats have tails while ants and humans do not). Mammals and the "general pool" of vertebrates have distinct evolutionary histories that may better account for differences in eye anatomy than a complex/primitive definition (group A has more cones than group B because they are more/less complex).                                                                                                                                                                                                      Very good review of the evolution of color vision: http://www.optometrists.asn.au/ceo/backissues/vol87/no4/3268              Mammals diverged from other vertebrates approximately 300 million years ago, so roughly half of their history as multi-cellular life forms comprises independent evolution. Over subsequent millennia, Mammalia has evolved into its ~4,600 species. Paleeontological evidence suggests that early mammals were small and nocturnal. This basic nocturnality has had a profound impact on the nature of mammalian vision. Unlike other groups of vertebrate, the retinas of the vast majority of contemporary mammals are dominated by rod photoreceptors. In addition to a dramatic alteration in the mix of rods and cones, there have been conspicuous losses of potential color vision mechanisms. For example, the colored oil droplets that characterize the color vision machinery of many birds and reptiles are missing from mammalian retinas and two of the vertebrate cone opsin gene families are not represented in placental mammals (SWS2 and RH2) having apparently been lost in the evolution of this group. This reduction in the number of potential cone photopigment types and the loss of an important source of selective spectral filtering has yielded greatly simplified color vision in mammals.