Numerous hypothesis for why bipedalism evolved

[0TheSwerve0]

There have been numerous hypothesis for why bipedalism evolved, some more silly than others, but it seems that none are capable of explaining it. Beyond it's own importance, I'm also interested in getting at where http://en.wikipedia.org/wiki/Future_evolution_of_humans" is going for us humans. A teacher of mine mentioned a Berekley study that found there are already adaptations for aquatic living in the population. Anyone heard of this? He mentioned that it had to do with evidence in our dentition, how a part of the population has jaws that pop...it was evidence for something, can't recall.

Any links on this are welcome too.

Hypothesis for the evolution of http://www.pbs.org/wgbh/evolution/library/07/1/l_071_02.html" :
1. http://www.pbs.org/wgbh/evolution/library/07/1/text_pop/l_071_04.html" (with several sub-hypothesis)
These hypotheses assume that humans evolved in savannahs

• increased visual range: proposes that apes moved out into the savannah and developed bipedality to counter predation by look over the grass.

problem with this: there are primates that live in open areas, eg patas monkeys, that are able to stand on their hind legs to keep an eye out for predators that do not develop bipedality.

• thermoregulation: proposes that humans evolved bipedality to decrease exposure of the body at the hottest time of day (noon) in savannahs, with the added benefit of reaching cool breezes.

problem: what about the rest of the time when the sun would reach more of the body than a quadruped? Plus, there are quadrupeds that don't evolve bipedality to deal with the sun.

Problem with savannah hypotheses in general - the fossil record shows that humans evolved in a forested habitat, not a savannah.

2. hunting/tool hypothesis: proposes that apes were hunting with tools and evolved bipedality to hunt more efficiently.

problem: it is clear from the fossil record that evolution occurred in this order: bipedality - increased brain size - tool use. Also, there is no evidence for tool use in early hominins, only later do we get the level of tool sophistication that characterizes the leap in cognition. Plus, chimpanzees hunt (cooperatively) without the use of tools (tho they use unmodified materials as tools to get other food). Again, tool use would be a side benefit of bipedality, not a cause.

3. home-base hypothesis: proposes that early ancestors practiced pair-bonding and that the males provisioned females and offspring who would stay at a home base to conserve energy and take care of offspring.

problem: this only explains male bipedality. Also, there is no evidence for monogamy - fossils indicate a level of sexual dimorphism characterized by polygynous mating systems. Plus, there is no evidence for a home base, it appears that these early hominins were on the move.

4. postural feeding: proposes that apes developed bipedality to reach food higher up.
problem: there are quadrupedal animals (eg the antelope like gerenuk) that can stand on hind legs to do this, but have no need to travel bipedally. Plus, apes are arboreal and wouldn't need to get any higher, they'd be living in the middle of their food.

5. http://en.wikipedia.org/wiki/Aquatic_ape_theory" Too much to outline here, most anthropologists find that it isn't likely.

6. locomotor efficiency: bipedality is 50% more efficient than the knuckle-walking of apes, thus they possibly were able to forage further, because the climate was cooling and forests were receding. This would mean that the group would be able to sustain more individuals rather than following the fission-fusion social pattern of chimpanzees (they fuse when food is plentiful, fission when it's scarce). So, the ones that took the bipedal route would become humans.

problem: knuckle-walking and bipedality are almost equally inefficient modes of locomotion for apes. Also. early humans didn't have the same morphology as anatomically modern humans do, so it wasn't drastically more efficient for them either. However, the difference could tip the scale for development of bipedality. Freed up hands would be a side benefit once bipedality evolved.

http://www.riverapes.com/AHAH/ComparativeBiology/Bipedalism/Bipedalism.htm"
http://jeb.biologists.org/cgi/content/full/204/18/3235"

- this is the one my teacher say is the best bet so far. Any others I missed or comments on the above?

Links on the future of human evolution:
http://www.human-evolution.org/future.php"
http://msnbc.msn.com/id/7103668/" was linked to from the other one
Science Friday http://www.sciencefriday.com/pages/2002/Mar/hour2_032202.html" with geologist Peter Ward's new book on the future; skip to 8 mins 15 sec to get the start of this interview.
http://observer.guardian.co.uk/international/story/0,6903,644002,00.html"

 

[hitssquad]

Violent inter-group-competition and the evolution of bipedality

Any others I missed

Violent inter-group-competition. Killing with weapons is more effective if the weapon-wielder is standing on two legs and holding his weaponry with two free limbs.

See Cattell:
http://www.efn.org/~callen/ToCremove.htm

 

[0TheSwerve0]

I actually proposed this in class, but my teacher said it wasn't likely. It doesn't actually give the bipedal ape more of an advantage. I'll try it up in my notes later.

 

[EnumaElish]

Whatever bipedia did for the human male, it made childbirth far riskier for the human female.

 

[0TheSwerve0]

yeah, no kidding. If we weren't, it wouldn't be so painful! Female chimps can actually deliver their own babies, without much problem.

 

[Leah]

Thanks for the ride 0TheSwerve0...my puter got most of it, but needed some upgrades to be able to see the rest. But, I got the sense! I took your love, I took your land, I took where I cannot stand. Good one! Thanks! Leah

 

[0TheSwerve0]

*scratching head* ummm, your welcome

 

[hitssquad]

The 'grappling is an effective way to kill' urban legend

I actually proposed this in class, but my teacher said it wasn't likely. [Standing] doesn't actually give the bipedal ape more of an advantage [when fighting with handheld weapons].

Street-fighting expert Marc "The Animal" MacYoung begs to differ:
http://www.nononsenseselfdefense.com/grappling.html

While it is important to know how to keep your head when you go to the ground, let's start by saying that if groundfighting was all that effective, armies would lie down when they fought. As a matter of fact, they wouldn't carry weapons, instead they'd use submission holds and mounting positions to defeat the other army's soldiers.

Since that is not the case, we must assume that grappling is not as universally effective as its proponents would claim.

 

[0TheSwerve0]

Yeah, but the evidence says that bipedalism evolved first, then tool use and greater intelligence. Seems like weaponry falls under the ability to imagine a tool in mind and then create it. Plus, chimpanzees already have weapons...large, sexually dimorphic canines. I did a study on sexual dimorphism in hominins and one study (Plavcan, et al) said that they found high body dimorphism and low canine dimorphism, which isn't found in any other primate. They proposed that males could have increased in size to protect their group/troop from predators. However, they wonder why canines would not also be useful for this. They also critique the weapon's replacement hypothesis since stone tools don't show up in the archaeological record until a million years after this. Another concern would be why weapon replacement wouldn't also lead to a reduction in body size. Also, other australopithecines would also be under predation pressure but they don't all show this pattern of dimorphism.

And, our ancestors were (as we are) a fraternal group, so they would compete as chimps do - indirectly through sperm count. That's why chimps have bigger chaones for their size than we do Human males compete by acquiring money and trying to be clever

As for attacking other groups, they usu have raids with large numbers and don't need any weapons; they attack lone foreign chimps who are easily overwhelmed and in that way they destroy entire neighboring groups.

btw, doesn't this also only explain bipedality in males?

 

[0TheSwerve0]

See Cattell:
http://www.efn.org/~callen/ToCremove.htm

Where in this shoud I look?

PS http://www.world.honda.com/HDTV/ASIMO/200412-run/index.html" Might help us imagine the advantages

 

[Phobos]

some thoughts...
(1) How about a combination of factors? (e.g., savannah effect + a pinch of thermoregulation + a dose of locomotive efficiency)
(2) Behavioral mutation to find bipedalism "sexy"? (half-kidding)
(3) Partial-bipedalism developed while still arboreal (I seem to recall seeing video of orangutans moving through branches/vines in an upright position...pseudo-tightrope walking style)

 

[0TheSwerve0]

some thoughts...
(1) How about a combination of factors? (e.g., savannah effect + a pinch of thermoregulation + a dose of locomotive efficiency)
(2) Behavioral mutation to find bipedalism "sexy"? (half-kidding)
(3) Partial-bipedalism developed while still arboreal (I seem to recall seeing video of orangutans moving through branches/vines in an upright position...pseudo-tightrope walking style)

first off, hominins didn't evolve in the savannah, the evidence shows that they evolved in a forested habitat. Perhaps, tho, they evovled in a mosaic of forest patches and grasslands. Locomotive efficiency was the best one I found, so that's possible, but works in a mosaic landscape, not the savannah.

as for number 3, I've been reading about that one. A lot of people are considering that one, but it just seems to not fit. How would being fully bipedal help you feed in a tree? Primate seem to get along just fine without being habitual and obligate bipeds. How would going all the way really improve your fitness?

 

[zoobyshoe]

Can't we run a hell of a lot faster than gorillas and chimps?

 

[0TheSwerve0]

yeah, but why would that matter?

 

[iansmith]

yeah, but why would that matter?

If there something is running and trying to eat you and a chimp. Who's going to be alive to pass down its gene?

To add on the subject of running and bipedalism, the following article is interresting
http://pharyngula.org/index/weblog/comments/marathon_man/

 

[hitssquad]

Instant vs gradual evolution

Can't we run a hell of a lot faster than gorillas and chimps?

If there something is running and trying to eat you and a chimp. Who's going to be alive to pass down its gene?

This is explained in Point #6 in the first post in this thread:

problem: knuckle-walking and bipedality are almost equally inefficient modes of locomotion for apes

I.e.: the fact of humans being able to run a lot faster than chimps is not relevant because we are not talking about chimp ancestors transforming instantly into humans.

 

[zoobyshoe]

If there something is running and trying to eat you and a chimp. Who's going to be alive to pass down its gene?

That would be the most vital advantage. I think it would also help a lot in catching a lot of smaller game.

 

[zoobyshoe]

This is explained in Point #6 in the first post in this thread:
I.e.: the fact of humans being able to run a lot faster than chimps is not relevant because we are not talking about chimp ancestors transforming instantly into humans.

I don't think he meant it that way. His point, as I took it, was that the faster members of a band of homonids are going to escape a running predator, or any fast moving danger, while the slower ones will be the first to get caught.

 

[0TheSwerve0]

If there something is running and trying to eat you and a chimp. Who's going to be alive to pass down its gene?
To add on the subject of running and bipedalism, the following article is interresting
http://pharyngula.org/index/weblog/comments/marathon_man/

If you're a chimp or the arboreal proto-hominin, you escape into the trees. Running doesn't do much good in a forest. I don't know about endurance running, but it seems likely that the ability to travel longer distances efficiently would be a good possibility for the mosaic landscape and patches of food model we have for their environment. But if you can't outrun predators or chase down prey, why develop bipdedality for endurance running instead of just walking? It seems that ER is just a side-effect. Maybe later hominins adapted to take advantage of it, tho.

 

[zoobyshoe]

If you're a chimp or the arboreal proto-hominin, you escape into the trees. Running doesn't do much good in a forest.

Sure it does. And if you're the guy with the bipedal mutation while your pals aren't you'll get home to breed while they'll be eaten.

 

[0TheSwerve0]

I don't think he meant it that way. His point, as I took it, was that the faster members of a band of homonids are going to escape a running predator, or any fast moving danger, while the slower ones will be the first to get caught.

I addressed this in the above post - being bipedal doesn't do much for predator evastion when you're a quadruped. You can't knuckle-walk in the trees; rather apes are built for suspensory locomotion, ie brachiation. True, they are then in a vertical postition, and can walk bipedally for short bouts before tiring, but that's going to work against your getaway in the trees. Seems having longer and stronger arms would help you much more. Imagine a human (ape sized) trying to run across branches to evade a predator - wouldn't the dense habitat slow them, trip them up, and test their balance? Also, since we have evidence that early hominins were still partially arboreal, wouldn't it make more sense that bipedality evolved for terrestrial use?

 

[0TheSwerve0]

Sure it does. And if you're the guy with the bipedal mutation while your pals aren't you'll get home to breed while they'll be eaten.

It doesn't make is so just because you said it. Can you explain why running through a dense forest would help you? It may have helped in the mosaic habitat, ie if they've evolved bipedality to travel more efficiently (check my 1st post to see other benefits), then being able to run from a predator would also help because you're in an open space. But, it wouldn't be the main drive for bipedality because the reason they'd be out of the forest and traveling in open spaces in the first place would be to get food. Anthropologists think this to be a good theory because the evolution of bipedality coincided with major climate and habitat changes, ie the recession of forests (where chimps live). They think this huge change opened up a new niche. Otherwise, why wouldn't bipedality have evolved earlier?

 

[zoobyshoe]

It doesn't make is so just because you said it. Can you explain why running through a dense forest would help you?

To escape the predator! As I explained before, if you're the guy with the bipedal mutation, you always get away when the leopard shows up, while your band members get picked off each time. You run, they climb a tree. The leopard climbs up the tree after them. You live to breed. they don't.
Also, people and animals who live in forests, or go into them frequently, don't bushwhack every time. They develop complex systems of paths.

 

[0TheSwerve0]

Hmmm, possible but I'd still need to look into primate studies to confirm this; I still don't know that much about primate predators. But that stills asks the question why wouldn't bipedality have evolved earlier? Should we discount the fact that it coincided with the recession of forests? I'm taking classes from a primatologist, maybe she'd know. I'll ask her on Tues.