According to an unusual claim circulating in the dark corners of social media, some people may be aliens and not even know it. The evidence, they claim, is in their blood. More specifically, people with the rhesus-negative blood type may be descended from extraterrestrials. So what’s going on here?

The “aliens” did it?

A TikTok video currently doing the rounds shows a clip from The History Channel’s 2009 series Ancient Aliens, which explored how extraterrestrials may have been visiting our planet for millions of years. In the clip, a range of “experts” explain how these celestial visitors could have meddled with our genetic heritage, either through interbreeding or deliberate genetic engineering, which left a small portion of the world with a relatively rare blood type – rhesus (Rh) negative blood. 

This blood type only appears in about 15 percent of the entire global population and has been a source of interest among scientists and medical professionals since it was first identified in the late 1930s. The reason for this, save for its rarity, is that we do not currently know where it comes from. This apparent gap in our knowledge has allowed some theorists to insert their own ideas for how the blood type came to be, e.g., through external intervention.

It is a wonderful example of the “Aliens of the Gaps” argument – a variation of the “God of the Gaps” argument used by creationists where a phenomenon is credited to a deity if there is no agreed explanation for it.

Although this claim lacks any actual evidence, it is presented as an intriguing possibility through a typical rhetorical sleight of hand. This involves meshing the views of scientists and other experts with non-experts in a way that makes them look like they are all contributing to the same conversation. 

In this instance, a professor of Anthropology provides a brief overview of the science behind rhesus-negative blood but is then sidelined by authors of alien-related ideas who add their own spin to the discussion without challenge. The effect makes it look like the viewer is watching a discussion of equally valid perspectives. 

What is Rh-negative blood?

Karl Landsteiner and Alexander Wiener played the most significant role in identifying the Rhesus blood type. In the early 20^th century, Landsteiner had also made important contributions to our understanding of blood types as he identified the ABO system. But then in 1937, he and Wiener turned their attention to a perplexing issue related to blood transfusions. They were trying to figure out why some people experienced adverse reactions when receiving blood from certain donors. Through their experiments, the two scientists identified a new antigen on the surface of red blood cells which they named the Rhesus factor (or Rh factor) after the Rhesus macaques used in the experiments.

In essence, the discovery meant that blood types can be characterized as either Rh-positive or Rh-negative, a factor that doubled the number of known blood types from four (A, B, AB, and O) to eight.

For most people, whether they are Rh-negative or positive will not be a concern unless they require a blood transfusion. There does not appear to be any other significant differences resulting from the blood type, though some have hypothesized that it may affect resistance to certain parasites, especially the one that causes toxoplasmosis. Some have also claimed that Rh-negative people have higher IQs, are more sensitive to the environment, and may have lower body temperature and blood pressure, among other things, but this is far from universally accepted. 

The Rh blood group can play an important role in fatal blood diseases, especially erythroblastosis fetalis, which can develop in fetuses and newborns if an Rh-negative mother gives birth to an Rh-positive child. In this case, the mother’s antibodies will attack the child’s blood. It is possible this seemingly weird phenomenon is what has caused the alien explanation to appear so compelling as, on the surface at least, it looks like the mother is rejecting the baby, as if they were from two different species. In reality, the baby has different DNA from its mother, which leads to complications during pregnancy.  

So if not aliens, where does Rh-negative blood come from?

At the moment, there is no clear consensus on why some people are Rh-positive and others are not. As mentioned above, Rh-negative blood is quite rare – around 15 percent of Caucasians are negative, while only 8 percent of Black people and only 1 percent of Asians are. But before we try to squish aliens into this explanatory gap, there are some simpler, albeit less sensational, explanations worth considering first, and they all have to do with genes, which are prone to mutating naturally.

A good example here is eye color. Originally, human eyes were brown, but around 6,000 to 10,000 years ago a genetic mutation slipped in which turned off the ability to produce enough melanin to sustain this color in some people. This led to the emergence of other colors such as blue, green, and hazel eyes. The eye color is not the result of a particular pigment but rather the relative absence of higher concentrations of melanin to make them brown. 

The fact that blue eyes have survived down successive generations has led some researchers to posit that blue eyes carried some evolutionary benefit, though they are not sure what exactly. A similar phenomenon may be going on with our blood. 

In 2012, a study published in the journal Human Genetics tried to find out whether there was an advantage to being Rh-negative that would have kept it present in our genetic heritage, despite it sometimes leading to difficult pregnancies. They did not find any clear benefit, leading them to conclude that, if one did exist, then it occurred in our evolutionary past and has no significance today, or maybe the blood type occurs due to random change. 

Returning to the parasite hypothesis, although we do not know for certain whether having Rh-negative blood does affect our defense against toxoplasmosis, we do know that similar variations in blood can have comparable benefits. For instance, carriers of sickle cell anemia (SCA), a genetic mutation that affects red blood cells, are known to be resistant to malaria. 

It is important to note that only the carriers of the disease benefit from added protection; unfortunately, those with SCA do not have greater resistance. But the idea here is that the benefits of being a carrier of SCA, and the protection it offers, outweigh the costs of having children who develop SCA, especially in areas where malaria is common.

It is likely that something similar is going on with Rh-negative blood, though we are not yet sure what. It may be less intriguing than believing that aliens somehow gave some people different blood, but it makes more sense. 

This article is part of our Inconceivable series debunking unscientific stories on the Internet.

All “explainer” articles are confirmed by fact checkers to be correct at time of publishing. Text, images, and links may be edited, removed, or added to at a later date to keep information current.