Say some government, or the nba, wanted to push human physiology to the limit and create a population of maximum height. First of all, would picking the (non-tumorous) tallest 1000 or so men and women and secluding them be enough? Or would you need to go in and remove the short individuals of each subsequent generation? Secondly, how tall could people get? Would the average height be 6-2 or 6-3, slightly taller than the Scandanavians, or reach 7 plus? What exactly is the limiting factor here?
Regression to the mean. Even if you have 2 people in the 99th percentile of height for their sex, chances are their children will not be as tall as them, despite their genes.
You’d eventually wind up with a population who never stop growing, which has quite a few negative side effects. http://en.wikipedia.org/wiki/Acromegaly
As those side effects don’t really kick in until middle age, there’d be no evolutionary preference for those who simply grew tall but then stopped.
Folks whp never stop growing generally do not make it to/past eight feet in height.
For selective breeding to work, you’ve got to do it every generation.
However, their children will very likely be significantly taller than the general population. Regression to the mean doesn’t mean that there’s no correlation between the heights of parents and offspring.
Supposedly there was a tribe in ancient Polynesia that tried to do this, at least for their warrior class. There’s a stone at Taputapuatea marae on the island of Raiatea that’s about 8 feet tall, and lots of smaller blocks 1 foot tall. The legend is that if a warrior wannabe was shorter than the big stone, they’d chop his head off over one of the small stones. But apparently it counted if their hat was higher than the stone.
Or that’s what I heard, anyway. I searched Google and Wikipedia (link) and found pictures of the stone, but no mention of the selective breeding program.
There’s no telling how big humans could get. All you have to do is do what animal breeders do. Select a foundation stock. Breed them together. Select the offspring that express the traits you want and cull the ones that don’t. Continue as desired.
Note that there are some problems. If your foundation stock is small and your culling severe, you’ll quickly have a small gene pool in your breed. And this inbreeding means you’re likely to fix undesirable traits as well as desirable traits.
And there are physiological limits. All the selection in the world isn’t going to get you green hair unless the trait is present in your foundation stock. There are lots of genes that contribute to height, and it’s likely that you won’t have all of them in your foundation stock. We’ve been breeding dogs for small size for hundreds of years, but there are no mouse-sized chihuahuas. We’ve been breeding dogs for large size for hundreds of years, but there are no horse sized mastiffs.
And your breed is going to suffer from secondary effects. Large size means more stress on the heart and other organs. Wild-type humans are selected for many different traits, your breed will ignore that in favor of a single measure of fitness.
This appears to be the key factor. People around 8 feet in height have so many problems, as their circulatory systems can’t keep up. You don’t get much past it. It might be possible to compensate, but historical examples suggest we’d need to massively increase in size and need to eat a lot just to survive. The largest carnivores (and we’d be looking at this or omnivores*) on land were the Spinosaurus under the Dinosaurs, and the Arctodus Bear. Spinosaurus probably benefited from scavenging or fishing much more than hunting. Arctodus was big enough to kill and eat a Buffalo.
Both were heavily dependant on food, however. On a human body-plan, we can probably not match Arctodus at all, because so much fuel goes to upright stance and the brain. But that puts a pretty reasonable far-upper limit.
*Omnivores haven’t really gotten that big. We are among the largest, probably because omnivores tend to breed for cleverness and versatility than size and strength. Omnivores have much too different digestion to plausibly breed in, in a plausible amount of time. It may not even be practical to breed in at all at this point. Thus, large carnivores are the most probably comparison.
The largest omnivore in the world is the brown bear. That’s pretty darn big.
The problem is that brown bears aren’t bipedal. Humans still haven’t fully adapted to a bipedal lifestyle. Giant bipeds are certainly possible (see Tyrannosaurus rex), but you can’t scale a human up to T. rex size. We’re just not built for it.
The giant ground sloth was probably omnivorous, and more bipedal than a grizzly, and somewhat larger. Of course they were remarkably stupid.
I wouldn’t be surprised if some of the Andrewsarchus type omnivores were even larger.
Come to think of it, hippos are mildly omnivorous. I remember seeing one of those nature shows where a hippo ate a drowned wildebeast. Of course plenty of other herbivores will sometimes eat carrion or opportunistically chomp small animals.
As a 5’ 4" man, I can assure you that there is already selective pressure towards increased height in humans, with no outside interference necessary.
So, just wait around a few hundred years, and as long as there’s enough food, we’ll see what the limit is.
Cite that Eremotherium or Megalotherium ate a significant amount of meat? Their teeth certainly don’t seem to indicate it. Or did you have another kind of ground sloth in mind?
I’ve been able to google up lots of references to the idea that ground sloths might have been omnivorous, but the only papers I can find are about analysis of sloth dung to find out what plants they were eating. Modern sloths are pretty much folivores, but seem to occasionally eat carrion and small animals.
We’d have to assume that a shortened lifespan, ala large breed dogs, is acceptable for purposes of the experiment. Big dogs have a bazillion major problems and people still like them - if it’s possible to take a wolf and make a Chihuahua and an Irish Wolfhound out of it, it seems that a large size variation is possible given sufficient time. (Not to say that there isn’t a ceiling, of course; rather that we should assume less sensitivity on the part of our researchers with regards to health problems. If a dog that is only expected to live seven or eight years is an acceptable specimen, than so is a human who only lives 35. And you should see how much my 65 pound dog has to eat. Gonna run me out of house and home.)
Also, people like Shaq and Yao Ming have their fair share of aches and pains I’m sure, but aren’t exactly hobbled by their height. Maybe most of the problems come from acromegaly, not just the height.
Most participants in the thread are probably already familiar with it, but the square-cube law is a limiting factor in how tall us humans can be. At some point, our bones would have to evolve to be able to support our weight.
http://www.jstor.org/pss/50663
It’s bit unclear just what their tooth and jaw structure indicates about diet. They weren’t adapted to tough, fibrous material and had only moderate chewing capability. The jaws generally suggest a diet more similar to that of bears or pigs rather than to extant sloths.
They were certainly descended from primarily herbivorous ancestors and retained the same general dentition, but then pandas retain their ancestral carnivore dentition, so that may not tell us a lot. As already noted, extant slots consume carrion, eggs and so forth when they can get it. It’s not a huge leap to posit that the ground sloths, which were much faster moving and living in an environment with much more animal food also ate much more meat, if only because t was more available.
The general idea seems to be that these were browsers that supplemented their diet with easy meat such as carrion, stolen kills, neonates and eggs. IOW they probably occupied a niche similar to pigs or baboons. Some authors have suggested they were specialised predators/scavengers more akin to bears.