The tallest person who ever lived stood nearly nine feet tall, making him nearly double the size of the average human. What creates such remarkable extremes, and what's the absolute tallest a human being could possibly be?
In humans, gigantism is caused by the overproduction of growth hormones in the pituitary gland. This pea-sized structure creates the chemicals needed to stimulate growth of the body's tissues. If the production of this hormone reaches about 20% above the normal rate - which can either be caused by overproduction from the pituitary or unexpected production of the thyroid hormone - then the bones starting growing unusually long, with the individual's final height reaching somewhere above seven feet tall.
A key point in the development of the bones is the fusion of the epiphyseal plate at the ends of the long bones, such as those in the arms and legs. If the overproduction of growth hormone begins or continues after epiphyseal fusion, then the resulting condition is known as acromegaly, which is most immediately recognizable by the enlargement of certain features like the hands, feet, nose, lips, and ears. Generally speaking, the treatment for runaway growth is the irradiation of the pituitary gland, which prevents further overproduction of the growth hormones.
It's important to recognize that we're not simply talking about people who are tall, or even very tall. People can reach heights well in excess of seven feet without any pituitary imbalance involved, with this extreme height simply being the result of genetics and nutrition. And, while extreme height often brings with it serious medical conditions - as we will discuss in a moment - people as tall as 7'6" or more have been able to become world-class athletes.
What we really want to look at are the absolute upper limits of human height, as well as some of the people who vie for the title of the tallest person who ever lived. So then, an obvious question arises...
The quick answer is that we actually don't know. There are twelve verified cases in medical history of people being taller than 8 feet. Unsurprisingly, eleven of these were men, although the Chinese woman Zeng Jinlian stood over 8'1" at the time of her death. Of all these cases, only one was definitely taller than 8'3".
Robert Pershing Wadlow was 8'11" when he died at the age of 22, and he grew continuously throughout his life - had he lived a little longer, it's possible he would have hit nine feet and kept on growing. Indeed, while his death was certainly related to his height - one of his leg brace irritated his ankle, which led to an infected blister - it wasn't strictly speaking the direct cause of his death. There's no medical reason to think that 8'11" represents an upper limit on human height.
Still, while nine feet does appear just within the realm of possibility, anything much more than that is probably impossible. It all goes back to something called the square-cube law. First demonstrated by Galileo, the law simply states that if an object increases in size, its surface area will increase by the square of the multiplier, whereas its volume will increase by the cube of the multiplier. That means that if you took a six foot tall, 200 pound man and doubled his height to 12 feet tall, his mass would have to increase eightfold to 1600 pounds, which is way more than human bones could support.
In his classic 1928 essay "On Being the Right Size", evolutionary biologist and geneticist J.B.S. Haldane lays out the basic problem with all this, using a famous literary example:
Let us...consider a giant man sixty feet high-about the height of Giant Pope and Giant Pagan in the illustrated Pilgrim's Progress of my childhood. These monsters were not only ten times as high as Christian, but ten times as wide and ten times as thick, so that their total weight was a thousand times his, or about eighty to ninety tons. Unfortunately the cross sections of their bones were only a hundred times those of Christian, so that every square inch of giant bone had to support ten times the weight borne by a square inch of human bone. As the human thigh-bone breaks under about ten times the human weight, Pope and Pagan would have broken their thighs every time they took a step. This was doubtless why they were sitting down in the picture I remember.
The human frame is very specifically adapted to be roughly between five and six feet tall, give or take a few inches in either direction. (Indeed, one statistical analysis finds that 96% of all Ameirican men are between 5'4" and 6'4".) You don't need to be anywhere close to sixty feet tall to begin placing unbearable structural pressures on your bones, and it's probably somewhere not much taller than Robert Wadlow's 8'11" stature where the strain becomes impossible.
This is borne out by pretty much every case of extreme height - all twelve people who were known to be taller than eight feet dealt with serious height-related medical issues, generally experiencing severe curvature of the spine, being unable to walk without the help of braces, and dealing with a severely reduced life expectancy.
I've been careful here to say that only twelve people in medical history were taller than eight feet. Of course, if you're willing to just say to hell with "documentation" or "facts" or "the slightest shred of plausibility", then there are plenty more fascinating cases to consider! For instance, there's John Middleton, who supposedly lived from 1578 to 1623 and is chiefly known through English folklore and oral history. He supposedly was 9'3", and he was so tall that he slept with his feet handing out the window.
There's also Trijntje Keever, a Dutchwoman who supposedly lived from 1616 to 1633. If the reports are true - and that's about as big an "if" as you can imagine - then she stood 8'4" tall, making her easily the tallest woman in history and the only woman to surpass eight feet. Perhaps even more amazingly, she supposedly was already over 6'7" by the age of nine, and she allegedly wore about size 20 shoes.
Middleton and Keever, assuming for the moment that they existed, were notable only for their extreme height, which was enough to bring them into contact with kings and queens and other powerful figures curious to see them. But the Roman general Maximinus Thrax, who supposedly stood 8'6", did them one better by actually becoming emperor. His superhuman stature was almost certainly apocryphal, as our main source for his height comes from a known fictitious source in the decidedly fantastical Historia Augusta. That said, his appearance on ancient coins does suggest he showed symptoms of acromegaly, so he was probably still of some unusual height.
But perhaps my favorite story is one that's more or less true: that of Feodor Machnow, who lived from about 1878 to 1912 in what is now Belarus. Modern researchers generally agree that his maximum height was "only" 7'10", but during his touring days his promoters liked to claim he was 9'3" making him easily the tallest person who ever lived. Why such a massive discrepancy? The reason is hopelessly mundane - he liked wearing a very tall, very furry Cossack hat along with very tall boots, which together added at least an extra foot to his frame.
Things start getting seriously weird when you look at stories of mythical giants. There have been several claims over the years of giants' skeletons being discovered ranging anywhere from 10 to 25 feet tall, but none of these claims hold up to even the slightest scrutiny. Most of the recent claims are just meant as practical jokes or as a test of Photoshop skills, such as the fake photos a few years back that purported to show the discovery of giant skeletons in Saudi Arabia.
That said, some of these claims do have a larger purpose. Certain strands of creationism are interested in proving the existence of giant races, known as the Nephilim, that are mentioned in the Old Testament - this is also a plot point in Eegah, the movie that taught that we should always watch out for snakes. There isn't that much to say about this from any sort of scientific perspective, but I do want to take a look at one of the very few times such claims made their way into actual scientific discussion, however briefly.
Back in 1890, a French anthropologist named Georges Vacher de Lapouge claimed to have discovered the femur of an ancient French giant that must have been 11'6" tall. He published one paper about it in the French scientific journal La Natura, and then...nothing. Lapouge, a staunch eugenicist and Aryan supremacist, had apparently already moved onto other work. (It's possible that his theories on race led him to fabricate these results, although that's just supposition.)
The Giant of Castelnau remains one of those weird footnotes in pseudoscience, something that was very briefly able to get some minor scientific credibility through being published in a journal, but thereafter it entirely disappeared from the historical record, and there's no record of what happened to these bones, assuming they ever existed, even as fabrications. About the only tangible record this incident left behind is an article in the August 1890 issue of The Boston Journal of Chemistry and Pharmacy. Here's an excerpt:
At Castelnau near the above town is a prehistoric cemetery dating from the ages of polished stone and bronze. A large number of human bones were found including about forty skulls, one of which formerly belonged to an individual about eighteen years old who judging from the size of skull must have boen over six feet in height. But the most remarkable "finds" of M Lapouge were three pieces of bone which must have belonged to some pre-historic of extraordinary size. The first piece on the left of the engraving is a part of femur or thigh bone and the one on right a part of a tibia or shin bone. In middle is represented a humerus or bone of upper arm from the same ancient cemetery of normal size. At the bottom is a small fragment which may be either a of a femur or a humerus; if the latter, it must also have formerly made up of the skeleton of the giant as can be by comparison with the normal above it of terror to the savage men of those times and was doubtless treated with all the honor which in these modern days is bestowed upon a successful prize fighter.
All the honor of a successful prize-fighter? I never realized this, but 1890s medical journals were kind of awesome. But that little curio, doubtless based on some incredibly shaky - if not outright fraudulent - science, represents the closest that impossibly tall giants have ever come to mainstream credibility in the entire history of science. Which is just as well - some basic understanding of physical relationships tells us there's a very, very good reason why humans are better off staying at just the height they are.
For more on giant cryptids, check out our earlier post "Where did all the world's giants go?".