On September 28 2021, the following post made a series of claims about humans having “stripes” that are visible but aren’t actually stripes, and that they also have stripes that do exist but which are not visible. The post’s claim was via a series of screenshots from Tumblr; the gallery of screenshots was shared to Imgur on September 30 2021:
Google Trends data indicated a spike in searches for “humans have stripes” beginning on September 30 2021. Related queries included “do humans have stripes under uv,” “can cats see human stripes,” “humans have invisible stripes,” human stripes uv,” and “Blaschko lines.”
First and foremost, “humans have stripes that we normally cannot see” was not a new claim. Posts on Reddit dating back more than a decade (and intermittently after that) covered the issue:
September and October 2021 posts claiming that “humans have stripes” typically involved screenshots of a November 15 2020 Tumblr post by u/keuhkopussirota. That post was framed in terms of aliens discussing human stripes (and possibly confusing stripes with stretch marks), but later “notes” on the original post made the claim that “cats can see them”:
At least one of the Reddit posts linked above cited a 2012 Gizmodo item, “Humans have stripes! You just can’t see them.” It described human stripes as “Blaschko Lines,” and concluded:
While Blaschko Lines sound like a cheat – “We have stripes! But they’re invisible.” – but there are some dramatic examples of them. Humans with chimerism can show them dramatically. Some people (and animals), happen to have been made with two different sets of DNA. Sometimes different fertilized cells will get mixed up with each other and build one human between them. In rare cases, they can send out different waves of epidermal cells, one alternating with another. Often the skin color of the two types is indistinguishable, or subtle enough that it can only show up under black light. Sometimes, however, the two different sets of DNA code for skin types that are dramatically different, which leaves people with literal stripes on their skin in the pattern of Blaschko Lines.
An undated post (published in 2012 or earlier) on a site attributed to “Department of Genetics, Stanford School of Medicine” addressed a television reference to a human with stripes, and explained:
In the episode, what the investigator discovered was something called Blaschko’s lines. These swirling patterns are found on the backs of many chimeras. Often you need UV light to see them. These patterns arise from the fact that chimeras start out with two cells, each with different DNA. Remember, DNA is a set of instructions for creating and running an individual. Because a chimera starts out with two cells with different DNA, the chimera ends up with some cells that have one set of instructions and others that have a different set. The skin of a chimera is made up of two sets of cells, each with different DNA. One of the instructions DNA has is how dark to make the skin.
The Blaschko’s lines result from the fact that some of a chimera’s skin cells say make darker skin and some say to make lighter skin. When there is a big difference between the two DNA’s instructions on how dark to make the skin, then you get obvious Blaschko’s lines. If the differences are more subtle, then you might need something extra like UV light to see the pattern. A German dermatologist named Alfred Blaschko first noted these lines more than a century ago. He noticed that in some skin diseases, the pattern was linear on the arms and legs, S-shaped on the stomach and V shaped on the back. The patterns are just a consequence of how the skin develops. If it developed differently, you would end up with a different pattern.
So there you have it. These Blaschko’s lines are there for the rest of us but invisible because all of our cells have the same instructions for how dark to make our skin. They only become apparent with certain skin conditions or if the two populations of cells have different DNA.
In November 2012, biologist Joe Hanson published a post (“Humans have stripes and you just can’t see them”) about Blaschko’s lines on Tumblr, referencing the above-linked Gizmodo article:
Another of the Reddit posts linked to a 2015 Mental Floss article about the claim (“Our Skin Is Covered With Invisible Stripes”), which explained, in part:
… today we know what they are: cellular relics of our development from a single cell to a fully formed human. Each one of us started out as a single cell, and then a little glob of cells. As the cells divided, they differentiated. Some became muscles, others bones, still others organs. And some became skin. As those skin cells continued dividing, they expanded and stretched to cover a quickly growing body. One cell line pushed and swirled through another like steamed milk poured into an espresso to make a latte.
Most people will never see their own stripes. As Dr. Blaschko noted, there are dozens of skin conditions that follow these lines, but most of them affect patches of skin or a single body part, not the entire body. Lined and whorled nevoid hypermelanosis can create beautiful patterns.
Going back further, a 1976 article in a medical journal discussed Blaschko’s lines, with an abstract stating:
Blaschko’s lines are the pattern assumed by many different naevoid and acquired skin diseases on the human skin and mucosae. They were described and drawn by Blaschko 75 years ago. These lines are to be distinguished from other linear patterns such as Voight’s lines, Langer’s lines, and the lines of innervation of the spinal nerves. They do not follow any known nervous, vascular or lymphatic structures in the skin. The epidermis and its appendageal structures, the melanocytes, the vascular system, and the fatty hypoderm, all, separately or in combination, may be involved in the morphological manifestations which follow Blaschko’s lines. Many of the naevoid skin conditions are lifelong (e.g. linear sebaceous naevus, unilateral naevoid telangiectasia); many of the acquired skin diseases (e.g. lichen striatus, linear psoriasis) are of relatively short duration (e.g. 1-2 years). The cause of the distribution pattern is unknown. It is possibly a form of human ‘mosaicism’ where certain specific cells or groups of cells react differently from other cells due to chromosomal abnormalities. The embryological explanation of Blaschko’s lines is not at all clear. Other markers in addition to the skin findings are needed to determine the time and the nature of the change responsible for these lines. The main purpose of this article is to introduce the concept of Blaschko’s lines into the medical, paramedical, and general biological fields of science. In this way, it is hoped that some inter-reaction can occur between those who regularly see Blaschko’s lines and those who regularly see and study other chromosomal and embryological abnormalities.
Posts claiming that humans have invisible stripes began appearing with regularity in 2012 on social media, but none of the linked sources claimed that “cats can see them.” Something of a cycle of interest seemed to encompass the claim, where it would begin spreading and lead people to look up the claim “humans have stripes” or “humans have invisible stripes.”
A 2014 study we were able to track down seems to have generated much of that claim. The study, which was published in Proceedings of the Royal Society B, concluded that many mammals — particularly nocturnal or partially nocturnal animals — can see a far wider spectrum than originally thought:
A previous study has shown that the reindeer, whose lens transmits 26.5% of UVA and which does not have a visual pigment with λmax below 400 nm, nevertheless responds electrophysiologically to 372 nm light . It therefore seems likely that species with similar or more UV lens transmission, such as cattle, pig, ferret, dog, okapi and cat, for example, will also be sensitive at these short wavelengths (table 1).
The realization that many mammals have some UV sensitivity may be important for understanding aspects of their behaviour as they could be responding to visual signals undetectable to humans. It may also have implications for the lighting conditions of captive and domestic species. On the one hand, some UV may be required for normal behaviour, while on the other, excessive UV exposure might put species with UV-transparent ocular media at increased risk of retinal damage….
That paper included several examples of animals, only a few of which seemed to ignite the imaginations of headline writers everywhere:
….[H]edgehogs, dogs, cats, ferrets and okapis had lenses transmitting significant amounts of UVA (315–400 nm), suggesting that they will be UV-sensitive even without a specific UV visual pigment.
However, it is not clear how that paper’s discussion points made the leap to cats being able to specifically see Blaschko’s lines.
Curious Google users were also likely to land on the extremely concise Wikipedia entry for Blaschko’s lines. The minimal text on Wikipedia might have been widely misread, leading to the secondary claim:
Blaschko’s lines, also called the lines of Blaschko, named after German dermatologist Alfred Blaschko, are lines of normal cell development in the skin. These lines are invisible under normal conditions. They become apparent when some diseases of the skin or mucosa manifest themselves according to these patterns. They follow a “V” shape over the back, “S” shaped whirls over the chest and sides, and wavy shapes on the head.
The lines are believed to trace the migration of embryonic cells. The stripes are a type of genetic mosaicism. They do not correspond to nervous, muscular, or lymphatic systems. The lines can be observed in other animals such as cats and dogs.
Alfred Blaschko is credited with the first demonstration of these lines in 1901.
To recap, since 2012 or earlier, waves of social media users searched for “humans have stripes,” “humans have invisible stripes,” or “Blaschko’s lines,” likely landing on Wikipedia’s short entry. Prior to the 2020 Tumblr post, “humans have stripes” was its own curiosity. “And cats can see them” seemed to enter the discussion between then and the September/October 2021 viral Facebook post, but we were unable to validate that addition to our satisfaction.