In 1969, two biologists wrote about three lizards from New Guinea whose insides were green. The color ranged from a deep blue-green to a vivid lime hue, and it was everywhere. The lizards’ bodies, when dissected, revealed green bones, muscles, and blood. Their mouths, when opened, were green. Their eggs, when held up to a light, looked green inside their shells.

Christopher Austin was just 3 years old when the two scientists, Allan Greer and Gary Raizes, wrote about the lizards, and he was 22 when he finally learned about them. But he quickly became enthralled. Green! Why green? “It’s very, very striking,” he says.

Animal blood comes in a rainbow of hues because of the varying chemistry of the molecules it uses to carry oxygen. Humans use hemoglobin, whose iron content imparts a crimson color to our red blood cells. Octopuses, lobsters, and horseshoe crabs use hemocyanin, which has copper instead of iron, and is blue instead of red—that’s why these creatures bleed blue. Other related molecules are responsible for the violet blood of some marine worms, and the green blood of leeches. But the green-blooded lizards use good old hemoglobin. Their red blood cells are, well, red. Their green has a stranger origin.

Red blood cells only live for four months or so, and when they die, our bodies recycle the iron within them. That process inadvertently creates a green pigment called biliverdin, which is then converted into a yellow one called bilirubin. Both of these are toxic, and our livers quickly filter them out of our blood. When you get a bruise, the unsightly green and yellow colors come from the biliverdin and bilirubin unleashed by dying red blood cells. When infants get jaundice, that’s the result of bilirubin building up before their newborn livers kick into action.

Greer and Raizes guessed that the New Guinean lizards have green innards because of biliverdin, and Austin confirmed their hunch much later. The lizards’ blood contains so much of the green pigment that it completely overshadows the normal red of their hemoglobin.

A tube containing some of the lizards’ blood. (Christopher Austin)

They should be dead. Biliverdin can damage DNA, kill cells, and destroy neurons. And yet, the lizards have the highest levels of biliverdin ever seen in an animal. Their blood contains up to 20 times more of it than the highest concentration ever recorded in a human—an amount that proved to be fatal. And yet, not only are the lizards still alive, they’re not even jaundiced. How do they tolerate the chemical? Why did they evolve such high levels of biliverdin in the first place? And why, as Austin’s colleague Zachary Rodriguez has just discovered, did they do so on several occasions?

“It’s an unusual physiological trait that’s only found in New Guinean lizards, so it probably evolved once,” says Austin. “That’s the obvious expectation,” and it’s why all the green-blooded species are classified as Prasinohaema, from the Greek for “green blood.” It’s also completely wrong. By comparing the genes of the five green-blooded species to those of other Australasian lizards, Rodriguez built a family tree that shows their evolutionary relationships. And to his huge surprise, the tree suggests that the green-blooded species evolved from red-blooded ancestors on four separate occasions. They all independently showed up to life’s party with green in their veins.

The team can’t rule out the possibility that the lizards evolved green blood once and then reverted to red on several occasions, or that there’s an even more complicated history of color-switching. But whatever the case, the pattern is more intricate than he suspected. At the very least, it means the lizards need to be reclassified.

“Even if the trait only evolved once, the fact that it has been retained across several species indicates that it confers a tremendous advantage,” says Adriana Briscoe, an evolutionary biologist from the University of California at Irvine. It’s hard to think what that might be, especially because the five species are so different. Some live at sea level, others at 8,000 feet. Some lay eggs, others give birth to live young. Some are green on the outside, others are brown or black.

There’s just one thing that the green-blooded species share and their red-blooded relatives don’t: a type of malaria.

Malaria is caused by microscopic Plasmodium parasites. Five species of these infect humans, but there are hundreds that infect lizards. And one of them infects all the green-blooded New Guinean lizards and none of the red-blooded ones, according to Susan Perkins, a colleague of Austin’s from the American Museum of Natural History. It’s called Plasmodium minuoviride, from the Latin for “to draw green blood.”

Other studies have shown that biliverdin can block the growth of Plasmodium parasites and kill infected red blood cells. Perhaps that’s why the lizards have so much of it. “It could be an anti-parasite strategy,” says Perkins.

The green-blooded lizards still get malaria, though. Perhaps their ancestors were heavily infected with many types of Plasmodium, and so repeatedly evolved high levels of biliverdin to control the parasites. Perhaps one parasite—P. minuoviride—evolved its own protective countermeasures, and still manages to infect the lizards that other types of malaria can’t touch. “The naïve view is that if green blood evolved to prevent malaria, there would be no malaria in green-blooded lizards,” says Austin. “But you might also expect to find higher levels of malaria in them.”

“Figuring out how the lizards tolerate biliverdin is potentially of great biomedical interest,” says Briscoe. It might lead to new ways of treating jaundice, or perhaps even malaria. “For a scientist, it’s like discovering green gold.”