What the researchers found instead suggests that the ancestors of all of the lizards had red blood, and that green blood then evolved independently four times, in separate lineages. But evolutionarily, the trait isn't so unusual. But in a new Science Advances study, a team of biologists reveal that they've uncovered the evolutionary history of the skinks' green blood, which may offer some clues into how we can use it too.
Green blood is one of the most unusual characteristics in the animal kingdom, but it's the hallmark of a group of lizards in New Guinea. The lizards have it in such great abundance-20 times more than the highest concentration recorded in a human, according to The Atlantic-that such a level could kill other animals. The excess of green bile pigment essentially eclipses the normal ruddy hue of their red blood cells.
Prasinohaema virens, a green-blooded skinkCHRISTOPHER AUSTINTo find out how the trait arose in lizards, Austin and his colleagues examined the genomes of 51 species of skinks, including six with green blood.
"The green-blooded skinks of New Guinea are fascinating to me as a parasitologist because a similar liver product, bilirubin, is known to be toxic to human malaria parasites".
Austin tells NPR that the biliverdin likely didn't give the animals an advantage in deterring predators, as animals are able to eat the skinks without untoward GI troubles (he himself ate them, saying it was like "bad sushi").
Biliverdin appears whenever hemoglobin, the particle that transports oxygen, breaks down to produce bilirubin, another bile pigment. "They have other cool traits such as giving birth to live young and [having] adhesive toe pads", Rodriguez added.
So Austin, Perkins, and their colleague Zachary Rodriguez made a decision to create a kind of lizard family tree by studying the DNA of 51 Australasian skink species, including six species that have green blood. "The problem is that there's green-blooded lizards that aren't green, and there's red-blooded lizards that are green", he explains. We only have low levels of biliverdin in our bodies, and we can spot it at the surface whenever we get hurt and develop bruises. The pigment might have caused them to have green blood and green bones, but they became immune to developing adverse reactions to it.
If the green blood is protective against malaria, it's not perfectly so, because they've found a malarial parasite living inside a green-blooded lizard, says Perkins. This suggests that green blood might have been an adaptive reaction, but researchers can not tell why the process had occurred. Several fish, frog and insect species also are green-blooded.