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300 Million Year Old Fossil Fish Likely Had Color Vision
westlake writes Nature is reporting the discovery of mineralized rods and cones in a 300-million-year-old fossil fish found in Kansas. The soft tissues of the eye and brain decay rapidly after death, within 64 days and 11 days, respectively, and are almost never preserved in the fossil record — making this the first discovery of fossil rods and cones in general and the first evidence for color vision in a fossilized vertebrate eye.
... I'm hard pressed to believe that there is an advantage for colorblindness that would have been selected for in the earliest mammals.
There didn't have to be an advantage for partial colorblindness (they were never totally colorblind), there just doesn't have to be any penalty for the trait to be lost. Same with the inability of some mammals to synthesize vitamin C, no particular advantage to losing it, but with a vitamin C rich diet there was no penalty either and so it could get lost over time. Color vision only works in bright light. Mammals spent a lot of their early evolutionary history as nocturnal creatures, and so could lose this trait without penalty. In fact it appears there were multiple function S cone loss events in the mammalian line, not just one (genomics gives us powerful insights into this today). The article does point out though that "the fact that these gene mutations have spread throughout the populations allows the possibility that the loss of S cones may in some way enhance visual fitness". It is entirely possible that processing of images in dim light could be better optimized through evolution with the loss of the unneeded bright-light color vision baggage.
Starships were meant to fly, Hands up and touch the sky - Nicky Minaj
Most mammals can see color. They (except some primates) are colorblind in the sense that they can't tell the difference between red and green, but they can tell the difference between red and blue. Because of the similarites in the proteins expressed, it is believed that human ancestors inherited a mutated gene for red that had a peak receptivity at green together with the original red gene from another parent. That's why most people now have both red and green cones.