Saturday, August 29, 2009

In-Cahoots FAIL

Two papers in the current issue of American Journal of Physical Anthropology on Homo floresiensis, our dear friend the supposed hobbit. I can't help but present one sentence from each paper, out of context:

"Insofar as we can tell, LB1, who lived to be a mature adult female, did not suffer from LS [Laron Syndrome] and was not pathological." (Falk et al. 2009, p. 61, my emphasis)

"As asymmetric craniofacial deformities are not typically seen in a skull with intentional artificial deformation, the aforementioned asymmetric deformities [of LB1] are likely to have been caused by some pathological or abnormal condition." (Kaifu et al. 2009, p. 180, my emphasis)

Just a little funny that two independent papers supporting the new-species status of H. floresiensis seem to say opposite things. To be fair, when Falk et al. state that LB1 "was not pathological," they mean that there is no convincing argument that a pathology or disease could cause a modern human skull to look like that of LB1. And Kaifu et al. are referring to the condition of "posterior deformational plagiocephaly," a not uncommon and presumably non-debilitating phenomenon in recent and historic modern humans, as a result of a persistent application of non-uniform pressure to a child's developing cranium (like consistently sleeping with one's head in a certain position). But the ambiguity of words like "pathology" allow the two sentences above to be juxtaposed to make it look like each paper makes contrary arguments.

Nevertheless, the papers do underscore a problem with fossils selected to be the holotypes of new species. Clearly, there is something pathological about LB1, but the extent to which a given pathology undermines the species-status of H. floresiensis is debated. And selection of pathological specimens as holotypes has occurred before: the holotype of Ardipithecus kadabba, one of the earliest and more questionable early hominins, has a first molar socket which "is extensively altered by pathology" (Haile-Selassie et al. 2009, p. 164). And a lot of species, and even genera, have been erected around subadult holotypes, too, which can be problematic.

This is just one of myriad issues we get to deal with in human paleontology.

Falk D, Hildebolt C, Smith K, Jungers W, Larson S, Morwood M, et al. 2009. The type specimen (LB1) of Homo floresiensis did not have Laron Syndrome. Am J Phys Anthropol 140: 52-63.

Haile-Selassie Y, Suwa G and White T. 2009. "Hominidae." In Ardipithecus kadabba: Late Miocene Evidence from the Middle Awash, Ethiopia, Haile-Selassie and Wolde-Gabiriel, eds.

Kaifu Y, Baba H, Kurniawan I, Sutikna T, Saptomo EW, and Jatmiko. 2009. Brief communication: "Pathological" deformation in the skull of LB1, the type specimen of Homo floresiensis. Am J Phys Anthropol 140: 177-185.

Thursday, August 20, 2009

Australopithecus afarensis: A mythical creation of Jim Henson?

DIK-1-1 is a nearly complete juvenile Australopithecus afarensis skeleton, from the site of Dikika in Ethiopia (Alemseged et al. 2006). The spectacular skeleton is approximately 3.3 million years old. Such a rare find is great news for paleoanthropologists, since its completeness provides much-needed information about growth and development, juvenile morphology, and even bones that rarely or almost never preserve well from hominins, including a scapula (part of the shoulder) and hyoid bone (sits in the middle of the throat, unwilling to be friends with any other bones). All in all, it's a very interesting specimen, whose feet show that it was adapted for bipedalism. But its "gorilla-like" scapula may indicate some degree of climbing behavior. The find made the cover of Nature, and here's part of Figure 1 from the paper:
Now compare this to Jen, a gelfling from the 1982 Jim Henson film The Dark Crystal.
Creepy. But the resemblance is dead-on, just look at the prognathic faces of DIK-1-1, above, and Jen here.

So what do we learn? Most probably A. afarensis is ancestral to the gelflings, as well as later, more well-known hominins like A. africanus, robustus, boisei, and our genus, Homo. I suppose the gelflings were an evolutionary 'side-branch.' And since DIK-1-1 is a juvenile while this gelfling is an adult, we have documented here a case of paedomorphosis, an evolutionary phenomenon in which the adults of the descendant taxon appear more similar to the juveniles of their ancestors (for a real-life example of this, see the axolotl).

Also, Alemseged et al. posit that the gorilla-like morphology of the Dikika scapula may reflect climbing behavior. Well, if we remember The Dark Crystal, we'll recall that Jen climbed Aughra's model solar system with gusto when the bad guys came and messed the place up. So the functional interpretation of the fossil shoulder is corroborated with behavioral data from the animatronic puppet. Oh, also I think the gelflings lived in a wooded, perhaps even forest environment. Such environments likely characterized the habitats of earlier hominins, but isotopic and relative abundances of different kinds of other fossil animals suggest that Dikika may have been a bit more open (Wynn et al. 2006).

Alemseged Z, Spoor F, Kimbel W, Bobe R, Geraads D, Reed D, and Wynn J. 2006. A juvenile early hominin skeleton from Dikika, Ethiopia. Nature 443: 296-301.

Wynn J, Alemseged Z, Bobe R, Geraads D, Reed D, and Roman D. 2006. Geological and paleontological context of a Pliocene juvenile hominin at Dikika, Ethiopia. Nature 443: 332-336.

Wednesday, August 12, 2009

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

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).

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.

Wednesday, August 5, 2009

Ancient Arboreality and Convergent Evolution

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.