Herpetologists don't know why a group of small tropical reptiles need green blood, but the search for an answer just took an unexpected turn.
New Guinea is home to multiple lizard species from the skink family with lime green blood. (Consequently, their tongues, muscles and bones are all various shades of green.)
Like humans, the reptiles have hemoglobin-rich red blood cells. Such cells do not last forever, and when they break down (in our bodies as well as the lizards'), the green-pigmented waste product biliverdin is made. Most vertebrates filter this stuff out of their circulatory systems. For them, excess biliverdin can harm cells, neurons and DNA.
Yet the lizards have a level of biliverdin in their veins that would kill a human. Moreover, the pigment is so densely-concentrated that it overrides the hemoglobin and makes their blood look green.
On May 16, 2018, a paper on the subject was published in the journal Science Advances. Its authors carried out a genetic survey of 51 different skinks in Australia, Asia and the islands between them. Six of the green-blooded New Guinea species were analyzed in the process. Turns out those biliverdin-loaded reptiles are not closely related to one other. In theory, they evolved green-bloodedness independently — and each of them is descended from red-blooded ancestors.
Such an unusual trait would not have evolved six times over if it didn't offer some kind of benefit. But scientists have yet to identify the advantage of having green blood. Predators who eat the lizards don't get sick afterward, and the skinks are no better camouflaged than their biliverdin-deficient cousins. It's possible that the special blood helped their ancestors kill parasites, but more research will be needed to confirm or refute this.