homoplasy

[insert clever quip about australopithecus hips]

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A week and a half ago, Kibii and colleagues (2011) published reconstructions and re-analyses of two hips belonging to the 1.98 million-year old Australopithecus sediba. As with many fossil discoveries, these additions to the fossil record raise more questions than they answer. Unless the question was, “did A. sediba have a pelvis?” It did. Here’s a good summary from the paper itself:

Thus, Au. sediba is australopith-like in having a long superior pubic ramus and an anteriorly positioned and indistinctly developed iliac pillar…[and] Homo-like in having vertically oriented and sigmoid shaped iliac blades, more robust ilia, and a narrow tuberoacetabular sulcus…and the pubic body is upwardly rotated as in Homo. (p. 1410, emphases mine)

So far as I can tell, the main way the hips are ‘advanced’ toward a more human-like condition is that the iliac blades are more upright and sweep forward more than in earlier known hominid hips. Here’s the figure 2 from the paper (more sweet pics of the fossils are available here). NB that in both A. sediba hips much of the upper portions of the iliac blades are missing (reconstructed in white; this region is missing in lots of fossils), so it’s possible they were more flaring like the australopith in the center photo.

The authors’ bottom-line, take-home point is that the A. sediba pelvis has features traditionally associated with large-brained Homo – but belonged to a small-brained species (based solely on the ~430 cc MH1 endocast). They argue that this means that many of these unique pelvic features did not evolve in the context of birthing large-brained babies, as has often been thought. They state that these features are thus “most parsimoniously attributed to altered biomechanical demands on the pelvis in locomotion,” and suggest that this hypothetical locomotion was mostly bipedalism but with a good degree of climbing. Maybe, maybe not. This interpretation is consistent with the analysis of the A. sediba foot/ankle (Zipfel et al. 2011).

The weird mix of ancient (australopith-like) and newer (Homo-like) pelvic features in A. sediba really raises the question of how australopithecines moved around. More intriguing is that the A. sediba pelvis has different Homo-like features than the ~1 million year old Busidima pelvis (Simpson et al. 2008), which has been attributed to Homo erectus (largely in aspects of the iliac blades). This raises the question of whether A. sediba is really pertinent to the origins of the genus Homo, and whether the Busidima pelvis belongs to Homo erectus or a late-surviving robust australopithecus (e.g. boisei, Ruff 2010).

Also interesting is that the subpubic angle (in the pic above, the upside-down “V” created by the pubic bones just above the red labels) is pretty low in MH2. This is curious because modern human males and females differ in how large this angle is – females tend to have a large angle which contributes to an enlarged birth canal, whereas males have a low angle like MH2. But MH2 is considered female based on skeletal and dental size. This raises the additional questions of whether human-like sexual dimorphism had not evolved in hominids prior to 1.9 million years ago, and whether the sex of MH2 was accurately described.

Finally, though the authors did a great job comparing this pelvis with those from other hominids, I think a major, more comprehensive comparative review of hominid pelves is in order. How does the older A. afarensis hip from Woranso (Haile-Selassie et al. 2010) inform australopithecine pelvic evolution? What about the possibly-contemporary-maybe-later hip from the nearby site of Drimolen (Gommery et al. 2002)? Given the subadult status of the MH1 individual, it would be interesting to compare with the WT 15000 Homo erectus fossils, or A. africanus subadults from Makapansgat, to examine the evolution of pelvic growth.

ResearchBlogging.org

Lots of interesting questions arise from these fascinating new fossils. “The more you know,” right?

References
Gommery, D. (2002). Description d’un bassin fragmentaire de Paranthropus robustus du site Plio-Pléistocène de Drimolen (Afrique du Sud)A fragmentary pelvis of Paranthropus robustus of the Plio-Pleistocene site of Drimolen (Republic of South Africa) Geobios, 35 (2), 265-281 DOI: 10.1016/S0016-6995(02)00022-0

Haile-Selassie Y, Latimer BM, Alene M, Deino AL, Gibert L, Melillo SM, Saylor BZ, Scott GR, & Lovejoy CO (2010). An early Australopithecus afarensis postcranium from Woranso-Mille, Ethiopia. Proceedings of the National Academy of Sciences of the United States of America, 107 (27), 12121-6 PMID: 20566837

Kibii, J., Churchill, S., Schmid, P., Carlson, K., Reed, N., de Ruiter, D., & Berger, L. (2011). A Partial Pelvis of Australopithecus sediba Science, 333 (6048), 1407-1411 DOI: 10.1126/science.1202521

Ruff, C. (2010). Body size and body shape in early hominins – implications of the Gona Pelvis Journal of Human Evolution, 58 (2), 166-178 DOI: 10.1016/j.jhevol.2009.10.003

Simpson, S., Quade, J., Levin, N., Butler, R., Dupont-Nivet, G., Everett, M., & Semaw, S. (2008). A Female Homo erectus Pelvis from Gona, Ethiopia Science, 322 (5904), 1089-1092 DOI: 10.1126/science.1163592

Zipfel, B., DeSilva, J., Kidd, R., Carlson, K., Churchill, S., & Berger, L. (2011). The Foot and Ankle of Australopithecus sediba Science, 333 (6048), 1417-1420 DOI: 10.1126/science.1202703

What big teeth you have, indeed

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If our friend Little Red Riding Hood was dumb enough to’ve thought a wolf in babushka threads was her grandma, well, she probably would have played Bingo with a grandmother-mimicking Australopithecus anamensis.
Australopithecus anamensis is the earliest undisputed hominid, found in deposits ranging from 4.2 – 3.9 million years ago in Ethiopia and Kenya (Leakey et al. 1995, White et al. 2006). Now, hominids are allegedly distinguished from other apes by having relatively short canine teeth distinguished by having relatively tall ‘shoulders,’ creating a diamond-shape in front view. Nevertheless, compared with humans these early australopiths had pretty murdersome canines, within the range of female chimpanzee species. (my dictionary is trying to tell me ‘murdersome’ isn’t a word, but I learned long ago not to learn right and wrong from a book)
Such canine form – relatively small with tall shoulders – was important in diagnosing Ardipithecus ramidus (> 4.4 million years) as a hominid back in the roaring 1990s (White et al. 1994). Of course, we learned in the 1980s that many ancient fossil apes looked superficially like hominids because of dental similarities, the result of either parallel evolution or hominids’ retention of primitive features. Indeed, even in light of the recently described Ardipithecus ramidus skull and skeleton, the main similarities with later, undisputed hominids are dental.
With this in mind, I’m struck by the canine of Nakalipithecus nakayamai, an ape from Kenya dating to nearly 10 million years ago (Kunimatsu et al. 2007). This is ape was a pretty important discovery because it began to fill in a rather lonesome Late Miocene ape fossil record in Africa. So, below is a picture of Nakali and anamensis canines, which I’ve tried to properly scale with the cutting-edge techniques of Microsoft Powerpoint (that is absolutely not a plug for Microsoft). On the left is Nakalipithecus, and the 2 on the right are Au. anamensis. The middle one is anamensis from Asa Issie in Ethiopia, and is the largest canine found of any hominid, ever I think. On the right is anamensis from Kanapoi in Kenya, not as big but sharp as shi…
…sh kabob skewers. Well crap, the “hominid feature” of short canine crown with nice shoulders is found in this 10 million year-old ape!
Two mutually exclusive scenarios could explain this similarity: [1] this canine morphology truly is a shared-derived feature of hominids, but hominids and Nakalipithecus just happened to evolve the same morphology independently for no better reason than, say, ennui. [2] This morphology is the ancestral condition for hominids (and chimpanzees and possibly gorillas). The fanciest cladistic methods won’t resolve this issue, only the discover of more badass fossils will. But if [2] is correct, that would deal a tough blow to the case of Ar. ramidus (and Sahelanthropus) behing a hominid. Really, it seems like the distinguishing feature of early hominids was their deplorable lack of distinguishing features.
ResearchBlogging.org
Oy, if bones and teeth are prone to homoplasy (similarity due to parallel evolution and not because of common ancestry), could paleoanthropologists have a special proclivity for it, too (that is, in naming dental hominids)?


Further reading!
Kunimatsu, Y., Nakatsukasa, M., Sawada, Y., Sakai, T., Hyodo, M., Hyodo, H., Itaya, T., Nakaya, H., Saegusa, H., Mazurier, A., Saneyoshi, M., Tsujikawa, H., Yamamoto, A., & Mbua, E. (2007). A new Late Miocene great ape from Kenya and its implications for the origins of African great apes and humans Proceedings of the National Academy of Sciences, 104 (49), 19220-19225 DOI: 10.1073/pnas.0706190104
Leakey, M., Feibel, C., McDougall, I., & Walker, A. (1995). New four-million-year-old hominid species from Kanapoi and Allia Bay, Kenya Nature, 376 (6541), 565-571 DOI: 10.1038/376565a0
Ward, C. (2001). Morphology of Australopithecus anamensis from Kanapoi and Allia Bay, Kenya Journal of Human Evolution, 41 (4), 255-368 DOI: 10.1006/jhev.2001.0507
White, T., Suwa, G., & Asfaw, B. (1994). Australopithecus ramidus, a new species of early hominid from Aramis, Ethiopia Nature, 371 (6495), 306-312 DOI: 10.1038/371306a0
White, T., WoldeGabriel, G., Asfaw, B., Ambrose, S., Beyene, Y., Bernor, R., Boisserie, J., Currie, B., Gilbert, H., Haile-Selassie, Y., Hart, W., Hlusko, L., Howell, F., Kono, R., Lehmann, T., Louchart, A., Lovejoy, C., Renne, P., Saegusa, H., Vrba, E., Wesselman, H., & Suwa, G. (2006). Asa Issie, Aramis and the origin of Australopithecus Nature, 440 (7086), 883-889 DOI: 10.1038/nature04629

More convergence and arboreality: Knuckle-walking and the African apes

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As long as we’re on the topic of homoplasy, a recent study suggests that knuckle-walking evolved independently in chimpanzees (Pan) and gorillas (Gorilla). If true, this suggests that hominins did not evolve from a knuckle-walking ancestor. Interesting.

Take-home points from the paper include:

  • Many purported ‘knuckle-walking’ features of the hominoid wrist might rather indicate arboreal wrist postures
  • Knuckle-walking in Pan and Gorilla are biomechanically distinct, and may thus have evolved independently in each lineage
  • More tentatively: Humans may not have evolved from a knuckle-walking ancestor, lending further credence to the idea that Pan is not a great model for the Pan-human common ancestor
  • This may be another example of one of Futuyma’s Principles of Evolution: HOMOPLASY IS COMMON IN EVOLUTION

“Knuckle-walking” refers to the mode of locomotion employed by most Pan and Gorilla when on the ground. Whereas most terrestrially quadrupdal primates use either the palmar surfaces of their ‘fingers’ or their palms to contact the ground, Pan and Gorilla‘s hands contact the ground with the back surface of the middle of their fingers (their intermediate phalanges, in technical terms). It is a very unusual posture–so far as I know, among all animals it is unique to these apes. So, it is perfectly sensible to assume that that knuckle-walking in chimpanzees and gorillas is homologous, represents the ancestral posture in African apes, and that humans evolved from a knuckle-walking ancestor.

But Tracey Kivell and Dan Schmitt present evidence from the wrist that suggests knuckle-walking in Pan and Gorilla are biomechanically and developmentally distinct. They point to several features of the wrist bones (carpals) that have traditionally been assumed to reflect knuckle-walking behavior. The expression of these features does not fit expectations given size and maturation differences between the two African apes. In fact, most of the features are more common/pronounced in Pan, and sometimes even other primates, more so than in Gorilla. The authors thus posit that many of the hitherto-knuckle-walking features of the wrist are actually indicative of arboreal wrist postures, and not knuckle-walking.

That authors acknowledge that it is possible that the wrist differences between Pan and do not necessarily preclude the possibility that knuckle-walking in the two apes has a common, ancestral origin, and that the differences accumulated after the evolutionary split between Gorilla on the one hand and Pan-humans on the other. That is to say, the behavior in the apes is homologous (as in common ancestry) but non-identical. Another possibility, which would also indicate that humans did evolve from a knuckle-walking ancestor, is that the behavior evolved separately in the Gorilla lineage, and in the Pan-hominin lineage before the split between Pan on the one hand and hominins on the other. The only way to test such a hypothesis is with fossils, fossils which so far as I know we do not have (yet).

Reference
Kivell T and Schmitt D. Independent evolution of knuckle-walking in African apes shows that humans did not evolve from a knuckle-walking ancestor. Proceedings of the National Academy of Sciences, in press.

Ancient Arboreality and Convergent Evolution

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Nature has a short blurb about Suminia getmanovi, a 260 million year old fossil that is the earliest evidence of an arboreal vertebrate. The blurb doesn’t tell too much, although it does have a very sweet picture. Apparently, evidence for arboreal behavior in this fossil includes elongated limbs, [something unelaborated upon about] its digits, and a long tail. It may have had a prehensile tail (like the Neotropical Ateline primates, including howler monkeys and spider monkeys), and possibly an opposable thumb (like in all true primates).

Given the fossil’s great antiquity and its potentially primate-like anatomy, one may ask, ‘Is this the common ancestor of all primates–are primates as ancient as the Permian Therapsids (a bad-ass group of ancient animals, also known as “mammal-like reptiles”)?’ Of course not. Rather, it is a great lesson in evolution: if there’s a niche to be filled, something will fill it, which means convergent evolution has probably been pretty common in the history of life. Flying evolved independently in dinosaurs, birds and mammals. Arboreal predation evolved in parallel in the marsupial Caluromys and (possibly) the first true Primates–there are several examples of convergent evolution in marsupials (Metatherians) and Eutherian (like us!) mammals. In fact, gliding evolved independently in the Jurassic mammal Volaticotherium, the (modern) marsupial sugar gliders (Petaurus), and the (modern) Eutherian colugos. Oh, and then there’s the ‘single-lens camera eye’ that evolved independently in cephalopods and vertebrates.

So, where there are similar niches to be filled, there’s a good chance that different animals will independently evolve similar adaptations to fill these niches. To quote one of D. Futuyma’s Principles of Evolution: homoplasy is common in evolution.

As promised, malaria resistance in baboons

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Last week I started to mention a recent paper in Nature on the evolution of malaria resistance in baboons, but then went out and partied instead. Not wanting to be a bastard, I’d better make good. While I’ll try to pull a good lesson from this, be warned that I’m about to discuss a topic about which I am no expert.

Malaria sucks, you don’t want to get it. There are anti-malaria medications out there, but I understand that they can make you insane, or at least have crazy dreams. Fortunately for millions of humans, there is a genetic basis for malaria resistance, so they don’t have to buy the anti-malaria crazy pills. Now, the paper tells me that a polymorphism in part of the FY gene turns the gene off in red blood cells, and that individuals with this variant are then strongly protected from malaria. No Lariam for these folks.

Jenny Tung and colleagues analyzed the homologous region of the FY gene in almost 200 yellow baboons (Papio cynocephalus) from Kenya, as well as tested these baboons for Hepatocystis parasites–relatives of Plasmodium vivax, which don’t cause malaria in baboons, but does really suck for them. And wouldn’t you know it–this same region on the baboon FY gene is also associated with Hepatocystis infection, where individuals with certain genetic variants have a lower susceptability to infection!

Now, the underlying genetic architecture and subsequent mechanisms of infection resistance are not exactly the same. But here’s the take home message from the paper:

“These results suggest that the genetic basis of phenotypic variation in different primate species can exhibit a remarkable degree of parallelism. In this case, not only are the similarities present at the molecular level . . . but they also extend to the mechanism that links molecular and phenotypic variation”

In other words, closely related species are equiped with very similar (or often the same) genetic or developmental “hardware,” and so evolution can cause them to come up with similar solutions to the same problem. In this case, there’s a similar genetic basis underlying infection resistance in humans and baboons. But I think this is a lesson that can be extended to, or at least kept in mind when considering, phenotypic evolution generally.

I’ve always (well, for the past three and a half years since I’ve been studying physical anthropology) thought that such a situation might characterize the “robust” australopithecines of East and South Africa. It is possible that these groups are not each others’ closest relatives, but that they evolved many craniodental characters in parallel, in response to selection for a heavy-chewing diet. This becomes even more plausible if it should turn out that many of these cranial and dental features are morphologically integrated–something I’m working on at the moment (if anyone reads this and scoops me, you will pay).

So, interesting paper. Reference

Tung, J. et al. Evolution of a malaria resistance gene in wild primates. Nature, in press.