Posted on 03/12/2007 6:25:00 PM PDT by blam
Source: University of Utah
Date: March 12, 2007
These Legs Were Made For Fighting: Human Ancestors Had Short Legs For Combat, Not Just Climbing
Science Daily — Ape-like human ancestors known as australopiths maintained short legs for 2 million years because a squat physique and stance helped the males fight over access to females, a University of Utah study concludes.
This drawing of a male gorilla skeleton illustrates their very short legs. Male gorillas fight to gain access to reproductively mature females. Relatively short legs increase the stability and strength of great apes, and should therefore increase fighting performance. A new University of Utah study suggests human ancestors known as australopiths had short legs for the same reason, not just for climbing trees. (Credit: From Alfred Brehm, "Brehms Tierleben" ("Brehm's Life of Animals"), small edition, 1927)"The old argument was that they retained short legs to help them climb trees that still were an important part of their habitat," says David Carrier, a professor of biology. "My argument is that they retained short legs because short legs helped them fight."
The study analyzed leg lengths and indicators of aggression in nine primate species, including human aborigines. It is in the March issue of the journal Evolution.
Creatures in the genus Australopithecus -- immediate predecessors of the human genus Homo -- had heights of about 3 feet 9 inches for females and 4 feet 6 inches for males. They lived from 4 million to 2 million years ago.
"For that entire period, they had relatively short legs -- longer than chimps' legs but shorter than the legs of humans that came later," Carrier says.
"So the question is, why did australopiths retain short legs for 2 million years? Among experts on primates, the climbing hypothesis is the explanation. Mechanically, it makes sense. If you are walking on a branch high above the ground, stability is important because if you fall and you're big, you are going to die. Short legs would lower your center of mass and make you more stable."
Yet Carrier says his research suggests short legs helped australopiths fight because "with short legs, your center of mass is closer to the ground. It's going to make you more stable so that you can't be knocked off your feet as easily. And with short legs, you have greater leverage as you grapple with your opponent."
While Carrier says his aggression hypothesis does not rule out the possibility that short legs aided climbing, but "evidence is poor because the apes that have the shortest legs for their body size spend the least time in trees -- male gorillas and orangutans."
He also notes that short legs must have made it harder for australopiths "to bridge gaps between possible sites of support when climbing and traveling through the canopy."
Nevertheless, he writes, "The two hypotheses for the evolution of relatively short legs in larger primates, specialization for climbing and specialization for aggression, are not mutually exclusive. Indeed, selection for climbing performance may result in the evolution of a body configuration that improves fighting performance and vice versa."
Great Apes' Short Legs Provide Evidence for Australopith Aggression All modern great apes -- humans, chimps, orangutans, gorillas and bonobos -- engage in at least some aggression as males compete for females, Carrier says.
Carrier set out to find how aggression related to leg length. He compared Australian aborigines with eight primate species: gorillas, chimpanzees, bonobos, orangutans, black gibbons, siamang gibbons, olive baboons and dwarf guenon monkeys. Carrier used data on aborigines because they are a relatively natural population.
For the aborigines and each primate species, Carrier used the scientific literature to obtain typical hindlimb lengths and data on two physical features that previously have been shown to correlate with male-male competition and aggressiveness in primates:
The weight difference between males and females in a species. Earlier studies found males fight more in species with larger male-female body size ratios. The male-female difference in the length of canine teeth, which are next to the incisors and are used for biting during fights. Carrier used male-female body size ratios and canine tooth size ratios as numerical indicators for aggressiveness because field studies of primates have used varying criteria to rate aggression. He says it would be like having a different set of judges for each competitor in subjective Olympic events like diving or ice dancing.
The study found that hindlimb length correlated inversely with both indicators of aggressiveness: Primate species with greater male-female differences in body weight and length of the canine teeth had shorter legs, and thus display more male-male combat.
There was no correlation between arm length and the indicators of aggression. Carrier says arms are used for fighting, but "for other things as well: climbing, handling food, grooming. Thus, arm length is not related to aggression in any simple way."
Verifying the Findings
Carrier conducted various statistical analyses to verify his findings. First, he corrected for each species' limb lengths relative to their body size. Primates with larger body sizes tend to have shorter legs, humans excepted. Without taking that into account, the correlation between body size and aggression indicators might be false.
Another analysis corrected for the fact different primate species are related. For example, if three closely related species all have short legs, it might be due to the relationship -- an ancestor with short legs -- and not aggression.
Even with the corrections, short legs still correlated significantly with the two indicators of aggressiveness.
The study also found that females in each primate species except humans have relatively longer legs than males. "If it is mainly the males that need to be adapted for fighting, then you'd expect them to have shorter legs for their body size," Carrier says.
He notes there are exceptions to that rule. Bonobos have shorter legs than chimps, yet they are less aggressive. Carrier says the correlation between short legs and aggression may be imperfect because legs are used for many other purposes than fighting.
Humans "are a special case" and are not less aggressive because they have longer legs, Carrier says. There is a physical tradeoff between aggression and economical walking and running. Short, squat australopiths were strong and able to stand their ground when shoved, but their short legs made them ill-suited for distance running. Slender, long-legged humans excel at running. Yet, they also excel at fighting. In a 2004 study, Carrier made a case that australopiths evolved into lithe, long-legged early humans only when they learned to make weapons and fight with them.
Now he argues that even though australopiths walked upright on the ground, the reason they retained short legs for 2 million years was not so much that they spent time in trees, but "the same thing that selected for short legs in the other great apes: male-male aggression and competition over access to reproductively active females."
In other words, shorter legs increased the odds of victory when males fought over access to females -- access that meant passing their genetic traits to offspring.
Yet, "we don't really know how aggressive australopiths were," Carrier says. "If they were more aggressive than modern humans, they were exceptionally nasty animals."
Why Should We Care that Australopiths Were Short and Nasty?
"Given the aggressive behavior of modern humans and apes, we should not be surprised to find fossil evidence of aggressive behavior in the ancestors of modern humans," Carrier says. "This is important because we have a real problem with violence in modern society. Part of the problem is that we don't recognize we are relatively violent animals. Many people argue we are not violent. But we are violent. If we want to prevent future violence we have to understand why we are violent."
"To some extent, our evolutionary past may help us to understand the circumstances in which humans behave violently," he adds. "There are a number of independent lines of evidence suggesting that much of human violence is related to male-male competition, and this study is consistent with that."
Nevertheless, male-male competition doesn't fully explain human violence, Carrier says, noting other factors such as hunting, competing with other species, defending territory and other resources, and feeding and protecting offspring.
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| Yo! Dobie! Yeah you - you're lucky I'm on this leash!!! |
The very first line is a lie. It should read: Ape like monkey ancestors .... Not ape like "human" ancestors. There is absolutely no proof that this animal was a direct ancestor of humans. My tag line says it all.
I think you must mean creationism, not ID. (See, even you guys have trouble maintaining the distinction, unless you're admitting there is none.)
None of the differences separating man and apes have been cited by IDers as indicative of "intelligent design". All the things that have been -- the blood clotting cascade, various other molecular machines or processes, the genetic code in general, etc -- are common to and substantially (or absolutely) identical in humans and anthropoid apes.
Many IDers admit the possibility if not probability of substantial common ancestry, often up to or beyond the range of diversity separating man and apes. A few even have no objection to universal common descent.
So THAT's why Wild Bill is built like that!
Those of you who don't know my stepdad, I gotta tell you . . . he's taller than me (so over 5'10"), and his legs are noticeably shorter than mine. I mean, WAY shorter, like you wonder what happened to his legs, and then you realize he had to have been born that way since he's almost six feet.`
Sorry to deflate your bubble of incisive analysis, but that drawing is the skeleton of a modern gorilla. The feet just look small because you're looking at them head on.
Howard Dean...
Short legs and nasty...
Nice of you to hijack this thread. How attractive were YOUR remote ancestors, hmmm?
Wow. That's an impressive concentration of ignorance packed into a couple short sentences: Effectively an announcement, that; "Now I'm going to debunk an entire subject I thoroughly misunderstand and know virtually nothing about"!
Where to start? First apes evolved from monkeys (or monkey-like ancestors) not monkeys from apes, so you have that completely backwards.
Second monkeys and apes are vastly more distinct from each other by nearly every objective measure -- gross anatomy, genetic distance, geologic time separating first appearances in the fossil record; excepting, of course, only the uniquely extreme development of the neocortex in humans -- than humans are from apes. Therefore if you object to the notion that humans and apes are related by evolution, you logically ought also to object to the notion that monkeys and apes are so related. In fact the fossil evidence of early apes is actually worse, relatively speaking, than that of early humans.
"How did he do such increcible fighting...with such tiny feet?" (with apologies to Blazing Saddles)
No, they're taking skeletons of early humans, comparing them to apes and proving that we're related.
Um, no "they" aren't doing that. Was there a point to this contrived, bald and gratuitous assertion; or did you just suddenly feel the need to go public with this bit of delusional paranoia?
Or how that belt slowly creeps up to the chest area as one grows older?
I thought we had short legs for loving. Well, half of us, at least.
*Yet, "we don't really know*,
Exactly, they really don't know. This is a lot of conjecture.
Well then, if that's the way you want to look at it -- lumping monkeys and apes into one category, even though to biologists (including pre-Darwinian ones) they are entirely separate orders, as distinct if not more so than cats and dogs -- then that's fine.
The problem is that doing so, and then preceding to airily dismiss the entire field of evolutionary biology, is exactly like saying; "Electrons and protons and photons are all the same to me, but now I'm going to tell you why modern particle physics is all a load of crap."
But can we trust a person who can't recognize a gorilla, and can't be troubled to read a caption, to have legitimately formed and reasonable doubts?
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