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Zihlman’s ‘pygmy chimpanzee hypothesis’


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What the hell, something else from the archives. So much for plans to publish new stuff (such as the long-awaited take on the recent Dinosaur Art event, and on the book). Anyway, the article here first appeared on Tet Zoo ver 2 in November 2009 and resulted in quite a few comments. I’ve made no effort to update it, sorry.

Produced by Adrienne Zihlman, the picture shown here has been used to support Zihlman’s ‘pygmy chimpanzee hypothesis’ (Zihlman et al. 1978): this being the idea that the Bonobo Pan paniscus is “the best prototype for the common ancestor of humans and [other] African apes” (Zihlman 1984, p. 39).

Many recent discoveries have shown that at least some australopithecines really were more chimp-like than used to be thought, and the old idea that fossil hominins were just prototype versions of Homo is now very much dead. However, fossils like Sahelanthropus and Ardipithecus have also highlighted the fact that chimps, bonobos and other modern great apes are anatomically specialised too (e.g., Lovejoy et al. 2009), and are not relicts that necessarily reflect an ‘ancestral’ morphology.

So… is the ‘pygmy chimpanzee hypothesis’ still viable, or is it defunct? One doesn’t see it getting much discussion these days, and the proposal that fossil African hominids were closely similar to the bonobo in proportions and so on has not been supported; so I think it is no more. But, hey, I’m no palaeoanthropologist.

Refs – -

Lovejoy, C. O, Suwa, G., Simpson, S. W., Matternes, J. H. & White, T. D. 2009. The great divides: Ardipithecus ramidus reveals the postcrania of our last common ancestors with African apes. Science 326, 100-106.

Zihlman, A. 1984. Pygmy chimps, people, and the pundits. New Scientist 104 (1430), 39-40.

- ., Cronin, J. E., Cramer, D. L. & Sarich, V. M. 1978. Pygmy chimpanzee as a possible prototype for the common ancestor of humans, chimpanzees and gorillas. Nature 275, 744-746.

Darren Naish About the Author: Darren Naish is a science writer, technical editor and palaeozoologist (affiliated with the University of Southampton, UK). He mostly works on Cretaceous dinosaurs and pterosaurs but has an avid interest in all things tetrapod. His publications can be downloaded at darrennaish.wordpress.com. He has been blogging at Tetrapod Zoology since 2006. Check out the Tet Zoo podcast at tetzoo.com!

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Follow on Twitter @TetZoo.

The views expressed are those of the author and are not necessarily those of Scientific American.



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  1. 1. Tayo Bethel 10:47 pm 10/20/2012

    This might be a little off topic but …

    If all modern apes are specialized, how do we get an idea of what a generalized ape might have been like?

    Link to this
  2. 2. Andreas Johansson 5:00 am 10/21/2012

    I’ve run across a few people online who, while not buying the initial bipedalism “theory” that’s been discussed on TetZoo before (the one with bipedal aquatic apes as the ancestral vertebrates), insist that the human-chimp LCA was bipedal. Is this garden-variety Internet crackpottery, or is there any evidence to support it?

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  3. 3. naishd 7:36 am 10/21/2012

    Tayo – good question. By looking at characters that are shared across hominids, and ignoring those that are specific to such lineages as pongines and hominines, we can imagine what stem-hominids were like.. and we have fossils that mostly conform to our predictions – animals like the dryopithecines and (controversially) Ouranopithecus and Pierolapithecus.

    Andreas: it isn’t internet crackpottery; in this case there are good reasons for thinking that bipedality (of a sort) was primitive for hominids. Hominoids that are outside of Homininae, like orangs and gibbons, are good bipedal locomotors both arboreally and terrestrially, and fossil stem-hominids, most notably Morotopithecus, appear to have had some bipedal adaptations. The hypothesis that bipedality arose early in hominids is also supported by the fact that the quadrupedal adaptations seen in pongines and hominines seemingly evolved independently. I previously discussed this issue here at ver 2.

    Darren

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  4. 4. naishd 7:39 am 10/21/2012

    I should add, though, that you shouldn’t imagine stem-hominids as human-like striders and runners – look at terrestrial orangutans and gibbons to see what sort of bipedality we’re talking about. It can involve stiff-legged striding and most likely evolved within an arboreal context, not a terrestrial one.

    Darren

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  5. 5. BrianL 7:50 am 10/21/2012

    @Andreas Johansson:

    I’m far from a paleoanthropologist myself, but I don’t think that particular idea is too far out. If I remember correctly, some of the supporting circumstantial evidence is this:
    - Chimpanzees and gorillas do not knucklewalk in the same way, suggesting that they evolved it independently from each other. This would suggest that their concestor was not a knucklewalker itself, in which case bipedalism would seem more likely than plantigrade quadrupedalism, given that no ape is known to use its hands for quadrupedal terrestrial walking.
    - Hominids in general have a vertical column that is most suitable to holding the body in a vertical position. Also the long arms, hands and fingers allow for upright movements in trees. Thus, the idea is that apes would be ancestrally bipedal in trees and terrestrial bipedality be an exaptation of that.
    - Orangutans and gibbons are proficient and habitual bipedal walkers, both on the ground and in trees. (And as has been discussed on Tet Zoo 2 in the past, orangutans at least seem to be quite efficient at it.)
    - *Orrorin* (I believe) is possibly a biped and thus more human-like than chimpanzee- or gorilla-like in locomotion despite being possibly close to the concestor of both humans and at least chimpanzees, suggesting said concestor was more bipedal than chimpanzees.
    - Terrestrial bipedalism is not unique to humans and their kin among apes, as at least *Oreopithecus* appears to have been one too, even if its particular style would have been odd (with highly splayed toes, if I remember correctly). Also, *Gigantopithecus* and *Lufengpithecus* have been considered bipedal by some and as far as I know, the jury is still out.

    So in conclusion, it all amounts to: Apes move upright in trees, *Oreopithecus* moved upright on the ground, gibbons move upright on the ground, orangutans move upright on the ground (as would their relatives *Gigantopithecus* and *Lufengpithecus*),gorillas and chimpanzees normally do not and appear to have evolved knucklewalking independently, African fossil hominids possibly close to the gorilla-chimpanzee-human concestor moved upright on the ground and Hominini move upright on the ground therefore hominids in general may ancestrally move upright on the ground.

    Like I said, though, I’m far from an expert and have undoubtedly presented a somewhat distorted version of the arguments. This is just from memory.

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  6. 6. BrianL 7:51 am 10/21/2012

    I seem to have crossposted with Darren who is of course more knowledgeable on the subject than I am. Oh well, I tried my best. :)

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  7. 7. Andreas Johansson 2:21 am 10/22/2012

    @Darrn & Brian: Thanks.

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  8. 8. cccampbell38 9:22 pm 10/22/2012

    We may not be descended from Bonobos but we might do well to emulate their social behavior.

    Make love-not war!

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  9. 9. vdinets 10:35 am 10/23/2012

    cccampbell38: making love not war is not necessarily a good evolutionary strategy. Note that chimps occur over half of equatorial Africa, while bonobos are stuck in one small isolated area of swamp forest in Congo-Zaire, and will likely go extinct if chimps ever get introduced there (which is entirely possible considering the level of animal trafficing in Africa).

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  10. 10. David Marjanović 10:57 pm 10/23/2012

    Note that chimps occur over half of equatorial Africa, while bonobos are stuck in one small isolated area of swamp forest in Congo-Zaire

    I thought it was simply an issue of chimps occurring north and bonobos south of the Congo (a very wide river over most of its length)?

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  11. 11. vdinets 11:39 pm 10/23/2012

    David: bonobos are limited to the inside of a large bend made by Congo. Chimps live all over the other side of the river, as well as east from its basin. They used to occur in savanna woodlands as well, but that part of their range has mostly been lost by now.

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  12. 12. Heteromeles 1:27 am 10/24/2012

    Ummm, to ask a really dumb question: why do we think that the last common ancestor of apes and hominids had to be a “generalized ape?” I’m not aware of any general polarity in evolution that says that primitive is automatically more generalist, while derived is automatically specialized.

    So couldn’t our last common ancestor be a specialist that got lucky to have more generalist descendents?

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  13. 13. David Kelly 3:10 am 10/24/2012

    Darren

    Have you been watching Prehistoric Autopsy on the BBC this week?

    I was expecting some comment from you on it.

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  14. 14. Dartian 5:45 am 10/24/2012

    Regarding the anatomical and behavioural peculiarities of bonobos: it is probably relevant that bonobos – unlike common chimpanzees – are nowhere sympatric with gorillas*. Thus, assuming that the immediate ancestor of the bonobo was rather more chimpanzee-like than the present-day paniscus is, it seems that when the bonobo ancestors spread south of the Congo River they, at least ecologically speaking, partially expanded into the empty gorilla niche. The bonobo certainly seems to be much less frugivorous (and apparently less meat-eating too) than the common chimpanzee is, and thus be more like the gorilla in this regard. As for why release from gorilla competition would have favoured the evolution of ‘peaceful’ behaviour in the bonobo lineage is less clear, but Richard Wrangham has suggested (for example, in his and Dale Peterson’s book Demonic Males) that chimpanzee – and human – intraspecific aggressiveness can at least partially be explained by their utilisation of patchily distributed, valuable food resources (such as fruit-bearing trees) that must be defended against conspecifics.

    * There is, of course, the caveat here that the fossil history of gorillas (and bonobos too, for that matter) is still pretty much completely unknown.

    Heteromeles:
    to ask a really dumb question: why do we think that the last common ancestor of apes and hominids had to be a “generalized ape?”

    It’s not really a dumb question at all; quite often it’s just a combination of sloppy thinking and preconceived notions. (The traditional idea that quadrupedalism is primitive and bipedalism is derived in the great ape and human lineage is a good example of this.)

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  15. 15. John Scanlon FCD 7:23 am 10/24/2012

    Quite a long time ago (OK, it was 1985), while I was looking up info to resolve the troglodytes-paniscus-sapiens trichotomy (it wasn’t obvious, and took a bit of work back then), I found and read Hans Kummer’s book Primate Societies (1971). In it he argued that a huge amount of the variation in social behaviour (group size, mating systems etc) in primates depends on the size and spatial distribution of critical resources (usually food, but also refugia). Since then, almost every book, paper and online discussion about primate evolution touches on this, and many people almost seem to get it, but I rarely see Kummer credited for this. I did actually find a citation in one of Wrangham’s books, but it wasn’t prominent.
    I guess that powerful explanatory theories in primatology are quite thin on the ground, and tend to be strongly defended against previous owners as well as newcomers by whoever gets hold of them…

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  16. 16. David Marjanović 10:28 am 10/24/2012

    I’m not aware of any general polarity in evolution that says that primitive is automatically more generalist, while derived is automatically specialized.

    Bingo! It used to be a common assumption that such a polarity exists, but numerous exceptions are now known. (Alligators come from a long line of specialist durophages; limbed vertebrates come from a long line of specialist pike-like near-top predators…) There really aren’t any forces in evolution other than mutation, selection, and drift.

    I guess that powerful explanatory theories in primatology are quite thin on the ground, and tend to be strongly defended against previous owners as well as newcomers by whoever gets hold of them…

    So full of win!

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  17. 17. Christopher Taylor 5:55 pm 10/24/2012

    I’m not aware of any general polarity in evolution that says that primitive is automatically more generalist, while derived is automatically specialized.

    That said, an assumption of a generalist ancestor does still offer a useful null hypothesis in cases where we don’t have direct evidence for the ancestral morphology. As Dartian and David said, this doesn’t mean that it’s actually right, just that it provides the simplest theoretical point in comparison when discussing possibilities.

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  18. 18. Heteromeles 6:26 pm 10/24/2012

    The problem I have with the idea of a generalist ancestor for chimps and humans is, basically, what the heck does generalist mean? Our ancestor wasn’t some sort of humanzee intermix, created by averaging humans and chimps, nor were they beings who had only the characteristics chimps and humans share and nothing else. That shared set of characters is insufficient for life, and it leaves too many gaps. For example, saying that our ancestors were non-bipedal non-knuckle walkers doesn’t say much about what they did do to get around.

    The problem we have is that our common ancestors most likely had traits that have since been lost in both humans and chimps, and they also lived in a world that’s sufficiently distant in time that it has to be reconstructed from fossils–there are no precise analogs in Africa to guide us.

    This isn’t a counsel of despair. I’m rather more interested in rattling this conventional cage hard enough that people will start thinking outside it a bit. Since we haven’t found the fossils of our common ancestor, I’d suggest it’s possible that we need a bit of outside-the-box thinking to figure out what habitat they may have occupied, so we can go looking for it.

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  19. 19. Dartian 12:41 am 10/25/2012

    Speaking of bonobos and their peaceful behaviour; it’s interesting to note that in the recent reboot of the Planet of the Apes franchise, the filmmakers seem to have gone for a deliberate inversion of the bonobos’ popular image. In Rise of the Planet of the Apes, Koba, the most brutal and nasty of the (non-human) apes, is a bonobo.

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  20. 20. naishd 4:28 am 10/25/2012

    Interesting comments – really loving the discussion. BBC2′s Prehistoric Autopsy series was mentioned up-thread: I saw two of the three episodes and thought it was really good. I live-tweeted thoughts while watching it, in fact (I tweet as @TetZoo: see also #TetZoo).

    Darren

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  21. 21. gentle lemur 7:41 am 10/26/2012

    Dartian’s point is a good one, and bonobo violence is not just fictional. I once watched bonobos fight in a zoo and it was utterly brutal and shocking. The popular accounts of their ‘make love not war’ lifestyle are not the complete story by any means.

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  22. 22. David Marjanović 1:49 pm 10/26/2012

    in the recent reboot of the Planet of the Apes franchise, the filmmakers seem to have gone for a deliberate inversion of the bonobos’ popular image

    Either that, or they simply didn’t know and took the kind of “chimp” that was dark in the face for the role of evil.

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  23. 23. Dartian 6:19 am 10/27/2012

    David:
    Either that, or they simply didn’t know and took the kind of “chimp” that was dark in the face for the role of evil.

    Oh, I think it’s safe to assume that the filmmakers did their homework and knew exactly what they were doing. Personally, I was for the most part quite impressed by the attention to subtle detail in the behaviour of the apes in this movie. Surely they knew their paniscus from their troglodytes.

    (It also think it was a nice touch to give the meanest ape the name ‘Koba’. A fairly clever way of hinting at a coming power struggle between him and Caesar in the inevitable sequel(s) to Rise.)

    In any case, let’s keep in mind that this was a Hollywood movie. They have no problem with characters that resolve conflicts with violence. Sex, on the other hand… ;)

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  24. 24. David Marjanović 11:37 am 10/27/2012

    It also think it was a nice touch to give the meanest ape the name ‘Koba’. A fairly clever way of hinting at a coming power struggle between him and Caesar

    Please explain.

    In any case, let’s keep in mind that this was a Hollywood movie. They have no problem with characters that resolve conflicts with violence. Sex, on the other hand…

    …Yeah.

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  25. 25. Dartian 12:12 pm 10/27/2012

    Please explain.”

    ‘Koba’ was an earlier nickname of this jolly old chap.

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  26. 26. Margaret Pye 11:25 pm 12/13/2012

    Does anyone here have much idea how important same-species conflict is as a cause of bonobo death? It seems plausible that they could be “less violent” than the other two species for whatever reason, but the idea that they rarely or never kill each other… well, I don’t know much about bonobos, but it sounds like wishful thinking.

    And what’s with the assumption that bonobos’ non-violence is connected to the fact that they’re less carnivorous? Is it something about dead animals being easy to defend and worth fighting for and/or the fact that predation and warfare require a fairly similar skill set, or is it a silly kneejerk response from the “eating red meat equals MASCULINE, and masculine is BAD and VIOLENT and HAS NEGATIVE ENERGY!” team?

    Link to this

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