Editor's note: Oops—this story was an April Fool's Day prank!

The pointy ears were the first indication that Star Trek Vulcan Mr. Spock was not of this Earth. As conceived by Trek creator Gene Roddenberry, Spock was the measured voice of reason on the Enterprise, balancing the actions of an impassioned, drop-kicking Captain James T. Kirk. But the ears and demeanor weren't the only giveaways. Occasionally, viewers of the original 1966-69 television series caught sight of Spock’s unusual body chemistry. His blood was green.

The reason for that is, as Spock would say, fascinating.

In a recent video uploaded by the Mütter Museum of medical oddities in Philadelphia, museum director Robert Hicks discusses how Roddenberry drew inspiration from Siamese twins Chang and Eng Bunker when developing the character of Spock. Born in 1811 in Siam (now Thailand), the brothers developed an international reputation as sideshow performers and fathered 21 children. A roughly 4-inch band of flesh connected them at the torso.

According to Hicks, Roddenberry’s longtime interest in pathology once led him to examine the autopsy report of the Bunkers. The 1875 document made note of the twins having green blood, a curious fact that he adopted for Spock.

The Mütter Museum retains a number of specimens related to the twins, including their conjoined liver. In examining the collection, Hicks discovered a blood sample bottle that was dried out but still held a green tint. Though rare, the presence of sulfur in hemoglobin (called sulfhemoglobinemia) can cause blood to appear green. A 2007 report in the Lancet medical journal detailed a case of a man who had the strange hue because of the migraine medication he was taking.

It’s not known why the Bunkers had green blood. Their health had been compromised, and a variety of factors could have affected a number of their bodily functions. As an achievable special effect, Roddenberry found it to be an excellent way to further characterize Spock as something only half-human.

Hicks says that wasn’t the only way Roddenberry was inspired by Chang and Eng. In the autopsy, which relates details of their final moments, Chang was the first to die due to a blood clot. Eng soon followed. Addressing one of their 21 children, his final words were said to be, “Live long and prosper, my dear son.”

[h/t Mutter Museum]

This article was originally published on www.mentalfloss.com as The Surprising Reason Star Trek's Mr. Spock Has Green Blood.