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Numerous hypothesis for why bipedalism evolved

  1. Oct 12, 2005 #1
    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 evolution 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 bipedality:
    1. Savannah hypotheses (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. aquatic ape 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.

    Chart that summarizes these
    webpage with a discussion of these

    - 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:
    Future Human Evolution
    This was linked to from the other one
    Science Friday interview with geologist Peter Ward's new book on the future; skip to 8 mins 15 sec to get the start of this interview.
    culture's influence
     
    Last edited: Oct 12, 2005
  2. jcsd
  3. Oct 12, 2005 #2
    Violent inter-group-competition and the evolution of bipedality

    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
     
  4. Oct 13, 2005 #3
    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.
     
  5. Oct 13, 2005 #4

    EnumaElish

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    Whatever bipedia did for the human male, it made childbirth far riskier for the human female.
     
  6. Oct 13, 2005 #5
    yeah, no kidding. If we weren't, it wouldn't be so painful! Female chimps can actually deliver their own babies, without much problem.
     
  7. Oct 13, 2005 #6
    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
     
  8. Oct 13, 2005 #7
    *scratching head* ummm, your welcome
     
  9. Oct 13, 2005 #8
    The 'grappling is an effective way to kill' urban legend

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

     
  10. Oct 15, 2005 #9
    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:smile: Human males compete by acquiring money and trying to be clever:wink:

    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?
     
    Last edited: Oct 15, 2005
  11. Oct 15, 2005 #10
    Last edited: Oct 15, 2005
  12. Oct 15, 2005 #11

    Phobos

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    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)
     
  13. Oct 15, 2005 #12

    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?
     
  14. Oct 16, 2005 #13
    Can't we run a hell of a lot faster than gorillas and chimps?
     
  15. Oct 16, 2005 #14
    yeah, but why would that matter?
     
  16. Oct 16, 2005 #15

    iansmith

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    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/
     
  17. Oct 16, 2005 #16
    Instant vs gradual evolution

    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.
     
  18. Oct 16, 2005 #17
    That would be the most vital advantage. I think it would also help alot in catching alot of smaller game.
     
  19. Oct 16, 2005 #18
    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.
     
  20. Oct 16, 2005 #19

    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.
     
    Last edited: Oct 16, 2005
  21. Oct 16, 2005 #20
    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.
     
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