What prompted primitive man to become bipedal?

[gib65]

What prompted primitive man to become bipedal? I mean, what were the environmental pressures that made it necessary to stop swinging from trees and start walking upright? Note I'm not asking what advantage it gave us (freeing our hands to use tools), I'm asking what prompted it.

[CDarwin]

Humans probably had knuckle-walking ancestors, not ones that would "swing through the trees." The latter notion is known as the gibbon theory of the evolution of bipedality and it was once very popular, but the pretty indisputable realization that chimpanzees and humans form a clade distinct from gorillas has brought it into doubt.

 

Why? Because if the common ancestor of all the African apes was a brachiator, then knuckle-walking would have had to independently evolve twice in both gorillas and chimpanzees. Not impossible, but unlikely. There's also been a recent study done of the forearms of early hominids that has reported some of the remnants of the wrist-locking mechanism seen in modern chimps and gorillas essential in keeping that joint stable for knuckle-walking, which if its right, would pretty much lock up the case. I'll find it in the morning.

 

That's important, because if our prebipedal ancestors were knuckle-walkers, that changes the game somewhat. Bipedality is a more efficient means of striding long distances on the ground than knuckle-walking.

 

Now that doesn't answer your question, but it's a good preface for any answer. You should also consider the fact that the apes radiated explosively during the Miocene, and that's when bipedality would have evolved. When you have forests crawling with apes trying out every conceivable ecological niche, then you can imagine there is going to be a great amount of diversity in the way these apes will get around. Some species might just naturally be more inclined to stand and walk upright for short distances (like modern chimpanzees do) than others. If a group of one of those species found itself isolated in some environment where striding might be at a premium, like perhaps the forest's edge, then who knows?

[ecoli]

there's a theory that sort of coincides with the geological record, from what I've heard. Increased desertification created patches of jungles with stretches of plains or desert (I forget which). This would favor bipedals rather quickly, esp. runners who could avoid predators, namely the big cats.

[CDarwin]

there's a theory that sort of coincides with the geological record, from what I've heard. Increased desertification created patches of jungles with stretches of plains or desert (I forget which). This would favor bipedals rather quickly, esp. runners who could avoid predators, namely the big cats.

 

Either way, that would open up more forest edge, and that's the sort of environment that early hominids tend to be found in. It's not a bad place to be if you're mobile. You can range out onto into open woodland or savanna to find what food might be there and always have a treeline to run back to in case one of your big cats or a large bird or other predator shows up.

Archaeologyinfo.com describes the environment for Australopithecus afarensisthus:

 

The paleoecology of the site seems to indicate a forest margin or savanna-woodland environment, a much different environment from the savanna-grassland environment initially assumed by many researchers.

 

 

That study of early hominid wrist joints is here, from Nature.

[bombus]

Swimming.

 

Without a doubt IMHO.

 

Savanahh living primates are more likely to run on all fours compared to forest dwellers (c/f baboons v orangutans)

 

C/f penguins, seals, sea lions, otters, cetaceans, all have a fully 'upright' stance (seals, cetaceans etc lie flat, but the body design is similar)

[CDarwin]

Swimming.

 

Without a doubt IMHO.

 

Savanahh living primates are more likely to run on all fours compared to forest dwellers (c/f baboons v orangutans)

 

C/f penguins, seals, sea lions, otters, cetaceans, all have a fully 'upright' stance (seals, cetaceans etc lie flat, but the body design is similar)

 

Early humans didn't live on the savanna, they lived on the forest edge, and they didn't evolve from baboons who are highly specialized to move on all fours on the ground as efficiently as they do and evolved from ancestors who also moved on all fours. Humans evolved from apes, which, as you point out, already have a propensity for upright movement.

 

You want to look at modern savanna dwelling primates to demonstrate that savanna adapted primates are always quadrupedal. Well, look at water-adapted primates. Talapoin monkeys (Miopithecus talapoin), Allen's swamp monkey (Allenopithecus nigroviridis), macaques, and proboscis monkeys (Nasalis larvatus) all swim frequently. All are quadrupeds or partial brachiators and all show adaptations like webbed feet missing in humans.

 

Of the animals you listed, only penguins actually walk upright on land. I'm really not sure where you got seals from. And penguins walk upright principally because they're birds. Have you ever seen a quadrupedal bird?

[Rev Blair]

What prompted primitive man to become bipedal?

 

Primitive women?

 

My guess is that it was a combination of factors.

 

I think we were likely knuckle walkers, like the chimps. We likely used some very basic tools, again like the chimps, to feed ourselves. Somebody who could carry a stick a little further would have a better chance of passing on their genes.

 

Cooling likely had a bit to do with it too. Being able to stand up catch the breeze is a pretty cool thing (har) to do in a hot environment.

[iNow]

I'm not sure if this theory is valid any longer, but I remember some years ago reading that, since we were on vast grassy savanna's, being able to stand up allowed you to see farther, hence be better at evading predators and hunting prey.

 

 

EDIT: Looking more closely at CDarwin's post, perhaps it allowed us to move beyond the forest's edge, and increase our potential resources. Instead of being locked into the forest, those that could stand would have more opportunities by venturing into the savannah. Those that went into the savannah prior to being upright would have suffered more (on average) from an inability to avoid predators and find prey than those standing taller.

[CDarwin]

I'm not sure if this theory is valid any longer, but I remember some years ago reading that, since we were on vast grassy savanna's, being able to stand up allowed you to see farther, hence be better at evading predators and hunting prey.

 

 

EDIT: Looking more closely at CDarwin's post, perhaps it allowed us to move beyond the forest's edge, and increase our potential resources. Instead of being locked into the forest, those that could stand would have more opportunities by venturing into the savannah. Those that went into the savannah prior to being upright would have suffered more (on average) from an inability to avoid predators and find prey than those standing taller.

 

But that doesn't really explain permanent bipedal movement. Standing above tall grass to look out for predators is fine, bit it also means you're much more visible to predators themselves. Walking upright makes you constantly so. Chimpanzees will simply stand up in place for short periods to see over grass (vervet monkeys have a much more amusing tactic, they bound through tall grass so that can see when they're up on their arc), and that serves them fine, so it would seem as though watching for predators couldn't be the prime motivation for evolving a whole new way of moving around.

[iNow]

I accept your point, but there is a difference in opportunity.

 

Those that stand all of the time still can duck so as not to be seen.

However, those that are on all fours most of the time are still more frequently surprised by attack from (and eaten by) predators.

 

Over time, those that were more upright more of the time had a selective advantage.

 

 

I'm speculating here. Let me make that clear. If I'm wrong, I'm okay with that, but I'd like to know why.

[Rev Blair]

Standing may also make you appear larger though, so some predators may decide not to attack you. There's a trade-off there in any case, but if standing upright allowed some other things, like tool use and more efficient cooling, then those would act with being able to see better and appearing larger to encourage standing.

[iNow]

Hmm... Sort of like how dogs raise their hair on end, arch their back, and raise their tail in the face of opposition to make themselves appear as more of a threat. That's an interesting point I wasn't considering, Rev.

 

It makes them perceptually bigger, and hence will demotivate a greater percentage of attack. Not likely a key evolutionary pressure, but certainly one to include in our considerations.

[CDarwin]

I accept your point, but there is a difference in opportunity.

 

Those that stand all of the time still can duck so as not to be seen.

However, those that are on all fours most of the time are still more frequently surprised by attack from (and eaten by) predators.

 

As individuals, maybe, but early hominids almost certainly moved in groups. In a group it works fine to have individuals periodically pop up, scan and then go back down as you're moving. I believe in the army they call that bounding reconnaissance. Maybe you can catch me on this, but I'd think the selective preference would be to remain inconspicuous over remaining vigilant. If you're a primate out on the grassland and you see a lion, what are you going to do? Run for the tree-line? Try and scare it off? Either might work but I'd say you'd be better off if the lion never saw you at all.

 

There's also a question of how far into grasslands early hominids really ventured. As long as you stick to open woodland, then best way to survey your environment is always going to be scampering up a tree. Some of the earliest hominids that have been discovered, like Ardipithecus ramidus, were discovered in what appear to have been full-blown forests, so savanna-based explanations are becoming more tenuous as open environments seem to have been at most only a part of the range occupied by early hominids.

 

Standing may also make you appear larger though, so some predators may decide not to attack you. There's a trade-off there in any case, but if standing upright allowed some other things, like tool use and more efficient cooling, then those would act with being able to see better and appearing larger to encourage standing.

 

That's another solution that's been proposed, but again it doesn't really explain efficient bipedal movement. A lot of primates can stand up, but not a lot can move that way for long distances.

 

I'm sorry I'm being mostly negative in this thread, but I don't think that bipedalism really something that has a simple, positive, clean, answer. It just kind of happened at some point toward the end of the Miocene, probably beginning in the forest, and then suddenly became the object of selective refinement.

Edited June 28, 2008 by CDarwin
multiple post merged

[iNow]

Could it be something as simple as, "the girls just liked taller guys better?"

[Mr Skeptic]

Standing may also make you appear larger though, so some predators may decide not to attack you. There's a trade-off there in any case, but if standing upright allowed some other things, like tool use and more efficient cooling, then those would act with being able to see better and appearing larger to encourage standing.

 

That's a good thought. I know that young children will overestimate the size of something tall and skinny compared to something short and wider. Even to the extent that they think you made more milk appear when pouring a short, wide glass of milk into a tall, skinny one. It would not surprise me if animals do that too even to adulthood. However, I doubt that would be the primary reason we became bipedal.

[Rev Blair]

I don't think that bipedalism really something that has a simple, positive, clean, answer.

 

I doubt anything in evolution has a simple, clean, positive answer. It's messy and not very efficient.

 

That's why I propose a bunch of things...I think that it's likely multiple pressures that push any given evolutionary trait.

 

As for whether we developed bipedalism in the forest or on the savannah, I don't see why it can't be both. If populations either migrated between the forest and the plains seasonally, or loosely aligned groups split but still intermingled, isn't it possible that bipedalism gave an advantage in both environments?

 

Think about it...it's easier to reach branches if you are standing, and I very clearly remember standing on one branch and holding onto another when picking saskatoons when I was a kid.

 

So if we lived in both environments and standing up and walking was beneficial in both, that would drive it too.

 

Could it be something as simple as, "the girls just liked taller guys better?"

 

That's pretty much all of evolution though, isn't it? Simple sexual selection. Some of that selection is based on survival needs, but I've been around enough barnyards to know that some other animals just don't like some other animals for reasons I've never seen explained.

[iNow]

That's pretty much all of evolution though, isn't it? Simple sexual selection.

While I appreciate your point, the answer really is "no." Evolution has much more going on than just sexual selection. Like, being eaten, or falling off a cliff, or being sterile, but I think you already know this, so I won't beat a dead horse.

 

CDarwin has raised a lot of really good points, and I'm inclined to pivot to his knowledge on the topic. He's brought up some really good points, and raised questions to which I don't personally have ready answers.

 

 

 

~~~~~~~~~~~~~~~~~~~~

 

As individuals, maybe, but early hominids almost certainly moved in groups. In a group it works fine to have individuals periodically pop up, scan and then go back down as you're moving. I believe in the army they call that bounding reconnaissance.

That's a good point. The only thing I can think to say is that there would still be some members in the group more likely to survive than others. With that said, however, your point about stealth seems to negate my point suggesting a "scare crow" effect.

 

 

 

Maybe you can catch me on this, but I'd think the selective preference would be to remain inconspicuous over remaining vigilant. If you're a primate out on the grassland and you see a lion, what are you going to do? Run for the tree-line?

 

<...>

 

There's also a question of how far into grasslands early hominids really ventured. As long as you stick to open woodland, then best way to survey your environment is always going to be scampering up a tree.

 

This triggered in me a new thought. Bipedalism frees the hands for weapon use. Instead of just running away from predators, just "climbing up a tree," those that were bipedal could travel while carrying weapons, and use those weapons against predators. They then could kill the predator instead of simply fleeing from it. If they killed it, it would also bring another source of food.

 

Just a thought. Being on all fours doesn't really lend to carrying or using weapons, and these guys didn't exactly have back packs or thigh holsters.

Edited June 29, 2008 by iNow
multiple post merged

[SkepticLance]

My personal theory is that bipedalism followed the retention of tools and weapons. Our ape ancestors made use of tools. Nothing too surprising there, since chimps do that in the wild today. However, what made our ancestors different is that they got into the habit of holding onto and retaining superior tools. There is a clear adaptive advantage to NOT throwing away a tool or weapon that is particularly good.

 

Of course, the only way to hold onto a superior tool, in the days before clothing or the making of containers, is to use a hand to do it. Possibly two hands if there are two tools. Bipedal locomotion suddenly becomes highly advantageous.

 

Note that I am talking about a time well before the MAKING of stone tools. These would be sticks and stones that were naturally good tools.

[CDarwin]

That's why I propose a bunch of things...I think that it's likely multiple pressures that push any given evolutionary trait.

 

As for whether we developed bipedalism in the forest or on the savannah, I don't see why it can't be both. If populations either migrated between the forest and the plains seasonally, or loosely aligned groups split but still intermingled, isn't it possible that bipedalism gave an advantage in both environments?

 

But there's not really any evidence of that. Early hominids (like Australopithecus) didn't even venture out into plains as such, only reasonably open woodland. It seems much more likely that bipedalism evolved primarily in a forest environment, being as that's where the Miocene apes lived and that's where the very earliest hominids have been found.

 

And you're right, there were probably lots of little things. I had this clearer in my mind yesterday as I was walking the dog but I didn't have a computer then to share my brilliant thoughts with the world, but basically... You can think up this little selection pressure and that little selection pressure and any of them may very well have contributed to the evolution of bipedalism (although savanna based notions are somewhat suspect), but note that any of them should apply to any species.

 

That should tell us that what really matters in the evolution of bipedalism, what really made the difference, were specific and serendipitous ecological conditions in the late Miocene and the group of apes that just happened to be best preadapted to take advantage of them by strengthening bipedal movement. What came before bipedalism is much more important to understanding it than what was happening during its evolution.

 

My personal theory is that bipedalism followed the retention of tools and weapons. Our ape ancestors made use of tools. Nothing too surprising there, since chimps do that in the wild today. However, what made our ancestors different is that they got into the habit of holding onto and retaining superior tools. There is a clear adaptive advantage to NOT throwing away a tool or weapon that is particularly good.

 

Chimps will hang on to tools, actually. They'll carry them in their mouths or in one hand. It hasn't made them bipedal yet, probably because they don't really move that much, and even if you loose a really great stick, a pretty great one will do the job about as well.

 

That's a good point. The only thing I can think to say is that there would still be some members in the group more likely to survive than others. With that said, however, your point about stealth seems to negate my point suggesting a "scare crow" effect.

 

Patas monkeys actually do that intentionally. The males are big and red and generally pretty conspicuous, but they're also the fastest primates in the world. Their function in the group is to catch the attention of predators and run like, well, a patas monkey, to draw them away from the females and young hiding in the grass. I'm not really sure what the mortality rate is.

 

EDIT: Actually, when I think about what a scarecrow actually is, patas monkeys don't do that... But it's still an interesting anecdote from the world of primatologica.

 

This triggered in me a new thought. Bipedalism frees the hands for weapon use. Instead of just running away from predators, just "climbing up a tree," those that were bipedal could travel while carrying weapons, and use those weapons against predators. They then could kill the predator instead of simply fleeing from it. If they killed it, it would also bring another source of food.

 

I don't know, early hominids weren't that big or that smart. I certainly don't think I'd want to go up against a lion with a pointy stick, even if I did have some buddies.

 

That was Raymond Dart's idea, the discoverer of Australopithecus africanus, by the way. So you're not in bad company.

Edited June 29, 2008 by CDarwin
multiple post merged

[SkepticLance]

To CDarwin

 

Chimps do not hang onto tools for long, and the moment they need to get moving at any speed, they drop those tools and do not look back. And if you think there is no significant difference between a mediocre tool and a great tool, then you are not a tool user! Our early ancestors did not make tools. They picked up what was lying around. Now and again, they would pick up a beautifully shaped piece of flint, or a wonderfully pointed length of hardwood. The difference in utility compared to your average stick and stone is utterly worlds apart.

 

Tool use would have to precede any movement onto the savannah. If you look at the physical bodies of the early hominids, you will see that their natural defenses were negligible. A lion or similar predator would regard a troop of Australopithecus as merely smorgasbord. For those early ancestors to survive on the plains, they would need both social clustering and weapons. A lion or hyena would hesitate to attack 200 apes armed with long pointed sticks as spears. Especially after a few hundred thousands years evolving as tool and weapon users!

[bombus]

Early humans didn't live on the savanna, they lived on the forest edge, and they didn't evolve from baboons who are highly specialized to move on all fours on the ground as efficiently as they do and evolved from ancestors who also moved on all fours. Humans evolved from apes, which, as you point out, already have a propensity for upright movement.

 

I know humans didn't evolve from baboons, but they did evolve from the common ancestor of humans and chimps. I'd bet they were very chimp-like and walked on four limbs most of the time. Why would they walk on two legs when they moved out to the savannah if four legs is better and evolution could have taken them either way?

 

 

You want to look at modern savanna dwelling primates to demonstrate that savanna adapted primates are always quadrupedal. Well, look at water-adapted primates. Talapoin monkeys (Miopithecus talapoin), Allen's swamp monkey (Allenopithecus nigroviridis), macaques, and proboscis monkeys (Nasalis larvatus) all swim frequently. All are quadrupeds or partial brachiators and all show adaptations like webbed feet missing in humans.

 

Proboscis monkeys are thye best swimmers, and regularly walk upright. They have also developed a nose flap that prevents water going straight up the nostrils - like humans maybe?

 

 

Of the animals you listed, only penguins actually walk upright on land. I'm really not sure where you got seals from. And penguins walk upright principally because they're birds. Have you ever seen a quadrupedal bird?

 

I know that only penguins actually walk upright, but the others all have a body aligned in a similar way, it's just that they don't walk upright as they can't! Maybe you miss the point?

 

Penguins btw are the only truly upright animal apart from humans. Other birds are not truly upright.

 

I accept your point, but there is a difference in opportunity.

 

Those that stand all of the time still can duck so as not to be seen.

However, those that are on all fours most of the time are still more frequently surprised by attack from (and eaten by) predators.

 

Over time, those that were more upright more of the time had a selective advantage.

 

 

I'm speculating here. Let me make that clear. If I'm wrong, I'm okay with that, but I'd like to know why.

 

Meerkats stand upright to look out for predators, but still run on all fours.

 

It's swimming that did it. One day I'll be proven right...

Edited June 29, 2008 by bombus
multiple post merged

[CDarwin]

To CDarwin

 

Chimps do not hang onto tools for long, and the moment they need to get moving at any speed, they drop those tools and do not look back. And if you think there is no significant difference between a mediocre tool and a great tool, then you are not a tool user! Our early ancestors did not make tools. They picked up what was lying around. Now and again, they would pick up a beautifully shaped piece of flint, or a wonderfully pointed length of hardwood. The difference in utility compared to your average stick and stone is utterly worlds apart.

 

A stick is still pretty much a stick, and they aren't really rare enough so that the perfect form is going to be worth evolving a whole new mode of locomotion just to hang onto for a few extra months.

 

Tool use would have to precede any movement onto the savannah. If you look at the physical bodies of the early hominids, you will see that their natural defenses were negligible. A lion or similar predator would regard a troop of Australopithecus as merely smorgasbord. For those early ancestors to survive on the plains, they would need both social clustering and weapons. A lion or hyena would hesitate to attack 200 apes armed with long pointed sticks as spears. Especially after a few hundred thousands years evolving as tool and weapon users!

 

Good thing they didn't move out onto the savanna until Homo erectus.

 

I know humans didn't evolve from baboons, but they did evolve from the common ancestor of humans and chimps. I'd bet they were very chimp-like and walked on four limbs most of the time. Why would they walk on two legs when they moved out to the savannah if four legs is better and evolution could have taken them either way?

 

Apes don't walk like baboons, though. Baboons stride much like cats or dogs, putting a preponderance of their weight on the front limbs. Apes put most of their weight on their hind limbs and live a surprising amount of their time upright in trees. And it's amongst the trees that bipedalism probably evolved.

 

Proboscis monkeys are thye best swimmers, and regularly walk upright. They have also developed a nose flap that prevents water going straight up the nostrils - like humans maybe?

 

Male proboscis monkeys have an pendulous nose, probably as a result of sexual selection. If this is an adaptation for swimming, females seemed to have missed the boat. Or rather not missed the boat, I suppose. I've never seen anything about them regularly walking upright. They have a fairly upright posture in the trees, but so do all colobine monkeys. It would seem much more likely that being upright was a preadaptation for swimming, rather than swimming was a preadaptation for being upright. I'm also not sure what structure of the human nose you're referring too.

 

I know that only penguins actually walk upright, but the others all have a body aligned in a similar way, it's just that they don't walk upright as they can't! Maybe you miss the point?

 

I get that all of them have hind-limbs that point more-or-less back away from the body (otters not so much). But the point is that even though they have this body arrangement, they all still walk quadrupedally on land. We're not swimming around typing on this message board, we're on land. So if aquatic ape is going to be able to refer to the body arrangements of truly aquatic animals as evidence, those arrangements are going to have to translate into a locomotion vaguely human-like while on land. Instead, we have seals that flop, otters that run like the weasels they are, cetaceans that just die, and penguins that waddle inefficiently (even if they are 'upright').

 

It's swimming that did it. One day I'll be proven right...

 

You (or someone, rather) will have to find some actual fossil or archaeological evidence. And then you would refer to it, and I would feel very sheepish and bemused. But not today! Which I can say with confidence since you probably won't post again today.

Edited June 30, 2008 by CDarwin
multiple post merged

[Mr Skeptic]

It's almost certain that there were multiple reasons for our upright posture. I'll throw out a few potential ones:

 

Efficient walking. Most primates walk rather awkwardly and inefficiently.

 

Carrying. It's much easier to carry stuff, be it food, tools, or weapons.

 

Reaching. It's easier to reach upwards if you are upright. Sometimes useful in trees, but mostly for reaching from the ground. Would be the ideal position in an environment with low fruit trees spread out enough that monkeys can't jump from one to the other.

 

Tool Use. An upright position gives you two free hands to use for tools. You might think that four hands is better than two for tool use, but consider: 1) If you also use hands for support, those hands are unavailable for using tools anyways. Quadrupeds would have only one free hand if they are not sitting, and using it would severely compromise their gait. 2) Hands are complicated things; a huge portion of our brain is dedicated to ours. Two more dexterous hands are more important than four hands. Consider how clumsy you can be if you use your left hand for a task you usually do with your right.

 

Throwing. When throwing from an upright position, you can use all the large muscles in your body to throw.

 

More sensitive hands. Less wear and tear and stepping on sharp things could mean the hands can be more sensitive. Not having to bear the weight of the body might also mean they can sacrifice strength for dexterity. This would apply to the wrists as especially -- consider how many skills are "all in the wrists".

 

Kicking. A kick is far more powerful than a punch precisely because the leg is heavier and stronger due to carrying all our weight. Also has the advantage of keeping your head and neck far from the enemy.

 

Weapon use. Two free hands means you can carry a weapon and use it more effectively.

 

Swimming. I'd think that would be a minor side effect, but it could have contributed. I'd need to see a comparison of swimming speed and energy efficiency for several primates to be convinced though (with adjustments for webbed hands/feet).

 

Extra visibility. We can see better, especially in tall grass. Even on flatish land, being higher off the ground gives us a more bird's eye view of a larger area. Also, we could see predators from farther away, but they could see us too. Overall, it would seem like the predators would get the better end of the bargain for that aspect, but then again we don't have that many predators.

 

Looking bigger. Young kids overestimate the size of a tall object. It would seem reasonable that animals do too considering how some of them stand up to try to intimidate other animals. This would only matter when already being upright would prevent a confrontation though.

 

That's all I can think of for now. Most of these points are about tool use, walking, and walking with tools.

[bombus]

Apes don't walk like baboons, though. Baboons stride much like cats or dogs, putting a preponderance of their weight on the front limbs. Apes put most of their weight on their hind limbs and live a surprising amount of their time upright in trees. And it's amongst the trees that bipedalism probably evolved.

 

There is hardly any difference between them.. I feel you are clutching at straws!

 

Male proboscis monkeys have an pendulous nose, probably as a result of sexual selection. If this is an adaptation for swimming, females seemed to have missed the boat. Or rather not missed the boat, I suppose. I've never seen anything about them regularly walking upright. They have a fairly upright posture in the trees, but so do all colobine monkeys. It would seem much more likely that being upright was a preadaptation for swimming, rather than swimming was a preadaptation for being upright. I'm also not sure what structure of the human nose you're referring too.

 

The females still have longer noses than most monkeys. The human nose flap I refer to is, well, the human nose. C/f other primates noses! I may be clutching at straws here...

 

I get that all of them have hind-limbs that point more-or-less back away from the body (otters not so much). But the point is that even though they have this body arrangement, they all still walk quadrupedally on land. We're not swimming around typing on this message board, we're on land. So if aquatic ape is going to be able to refer to the body arrangements of truly aquatic animals as evidence, those arrangements are going to have to translate into a locomotion vaguely human-like while on land. Instead, we have seals that flop, otters that run like the weasels they are, cetaceans that just die, and penguins that waddle inefficiently (even if they are 'upright').

 

Penguins don't walk quadrupally on land. The others have no choice, but the balance and ability to stand kinda upright shown by sealions, and otters for that matter, are abilities that an aquatic or semi-aquatic lifestyle can bring. Homonids had long legs to start with though, so these would have been used in a fashion similar to frogs, which are also swimmers with long legs. So, our body form would become elongated and 'straight' effectively pre-adapting us for walking upright. Also look at our feet - they are flipper shaped.

 

 

 

You (or someone, rather) will have to find some actual fossil or archaeological evidence. And then you would refer to it, and I would feel very sheepish and bemused. But not today! Which I can say with confidence since you probably won't post again today.

 

Fossil evidence is already there - it just depends on the interpretation. It's amazing how hostile people are to the aquatic theory despite it answering ALL the questions about human evolution (hair loss, sub cutaneous fat, babies swimming as soon as they are born, tears, dive reflex etc etc etc) - unlike the savannah theory.

 

It is the truth, and I will one day be proven right, and everyone will pretend they agreed with me all the while...

[pioneer]

It may have been simply due to the practical needs for migration. Standing upright offers a better way to vent heat.

 

What walking vertical also increases the vertical distance between the heart and brain. This increases the blood pressure to the legs. When we sleep the vertical differential is close to zero. Walking on all fours has the brain-heart differential somewhere between sleep and awake slouching on the sofa. This is where you daydream instead of get to business. This slight lowering of the blood pressure to the brain, relative to ape design parameters, would slightly cool the brain during continuous exercise.

 

Another simple possibility was the arms got shorter, the legs got longer or the torso stretched. Any of these will all start to tip the body so the head is the lower than the heart making heat loss even worse.

 

The transition critters that try to stick to old school will over heat first and fall by the wayside. Selective advantage goes to those who figure out how to orientate the body in a way to keep the body and brain cooler.