Creationist Arguments: Semicircular Canals

A number of creationists (Gish 1995; Lubenow 1996; Mehlert 1996; Wieland 1994) have cited studies of the semicircular canals as evidence of a lack of transitional forms leading from apes to humans. These claims are based on the work of Fred Spoor and his colleagues (Spoor et al. 1994; Shipman 1994).

The semicircular canals are three small, intricate structures in the inner ear, arranged roughly at right angles to each other, which give us our sense of balance and allow us to orient ourselves. Hoping that their structure might reveal something about hominid evolution, Spoor studied the canals of many living primates, including humans, and compared them with some hominid fossils. Because the canals are so small and buried in a bony part of the skull, it was necessary to use CT (computerized tomography) scanning to examine the canals without destroying the fossils.

The results proved interesting. The canals in Australopithecus africanus and robustus skulls were most similar to the great apes. Spoor et al. found this consistent with the commonly-held view that australopithecines were partly arboreal and partly bipedal. They did not conclude that australopithecines were quadrupedal, as most creationists imply or claim. For example:

Also, CAT scans of australopithecine inner ear canals (reflecting posture and balance) by anatomist Dr. Fred Spoor and his colleagues at University College, London, showed they did not walk habitually upright. (Catchpoole 2000

) Spoor et al. did not say that australopithecines did not walk upright. Rather, they interpreted their results as showing that they were not obligatory bipeds, as we are, but part-time bipeds, and not as accomplished at bipedalism as humans are. Claiming that australopithecines were still partly bipedal is not a desperate attempt to retain an intermediate status for them, as Catchpoole implies, but a recognition of the fact that the evidence for bipedality in australopithecines is extremely strong.

The Homo erectus skulls all had a very humanlike pattern. This would be expected, since erectus was fully adapted to bipedality. Another skull, SK 847, which has been attributed to both H. erectus and H. habilis, proved to have canals like those of erectus and sapiens.

Most interesting were the results for Stw 53, which is often classified as H. habilis. The morphology of this skull was unlike both apes or humans, and most closely resembled that of some large monkeys. Spoor et al. suggested that this meant that Stw 53 relied less on bipedality than did the australopithecines. This would argue against Stw 53 being ancestral to humans, which would be consistent with another study done on the partial skeleton OH 62 (Hartwig-Scherer and Martin 1991) which concluded that its limb proportions were more apelike than those of Lucy. However since it is widely thought that more than one species is represented in all the fossils that have been assigned to habilis, this result does not necessarily apply to all habilines. (Spoor himself thinks that Stw 53 is unlikely to belong to Homo habilis.) Further studies are clearly needed on the other habiline fossils to work out what is going on here.

Interestingly, one of the Homo erectus skulls studied was Sangiran 2, found on Java. Gish (1995) points out that Sangiran 2's semicircular canals have modern humanlike proportions, obviously implying that it is merely a modern human. However this partial skull is virtually identical to the original Java Man skullcap, which Gish considers an ape. So not only does Gish classify two very similar skulls as an ape and a human respectively, he classifies the smaller one, the 815 cc Sangiran 2, as a human, and the larger 940 cc Java Man skullcap as an ape!

More recently, Hublin et al. (1996) show that the semicircular canals in Neandertals are different from those of modern humans. The significance of this finding is unclear, since there is no doubt that Neandertals were fully bipedal, but they suggest it might be related to neck and eye motility. However it does indicate that Neandertals are not particularly closely related to modern humans, and gives some support to those who believe that they should be considered a separate species, Homo neanderthalensis, rather than a subspecies of Homo sapiens. It is not a result that can be easily explained by creationists, who have always argued that Neandertals are little more than a racial variant of modern humans. This unexpected result suggests that the link between locomotion and the structure of the semicircular canals is complex and not well understood. Indeed, Graf and Vidal (1996) have argued that there is no relationship between the shape of the semicircular canals and locomotion, although Spoor et al. (1996) dispute this.

More recently, in a study which compared the bony labyrinth of humans, apes, and other primates, Spoor and Zonneveld admitted that the issue of inner ear morphology is too complex, both phylogenetically and functionally, to allow simple conclusions to be drawn, or even to easily distinguish between bipedal and quadrupedal behavior:

"It is concluded that any link between the characteristic dimensions of the human canals and locomotion will be more complex than a simple association with the broad categories of quadrupedal vs. bipedal behavior." (Spoor and Zonneveld 1998)

While these early results have not shown any clear evidence of transitional types of semicircular canals, neither are they, with the exception of Stw 53, enough out of line with evolutionary expectations to cause much surprise. Moreover some results of these studies are problematic for creationists. The human-like canals of Sangiran 2 are a serious problem for the many creationists who claim Java Man is an ape, while the distinctive canals of Neandertals suggests a greater difference between them and modern humans than most creationists are likely to be happy with.

References

Gish D.T. (1995): Evolution: the fossils still say no! El Cajon, CA: Institute for Creation Research. (an updated version of Gish 1985)

Graf W. and Vidal P. (1996): Semicircular canal size and upright stance are not interrelated. Journal of Human Evolution, 30:175-81.

Hartwig-Scherer S. and Martin R.D. (1991): Was "Lucy" more human than her "child"? Observations on early hominid postcranial skeletons. Journal of Human Evolution, 21:439-49. (a comparison of Lucy and OH 62)

Hublin J., Spoor F., Braun M., Zonneveld F., and Condemi S. (1996): A late neanderthal associated with upper palaeolithic artefacts. Nature, 381:224-6.

Lubenow M.L. (1996): Palaeoanthropology in review. Creation Ex Nihilo Technical Journal, 10:10-7.

Mehlert A.W. (1996): Australopithecus and Homo habilis - pre-human ancestors? Creation Ex Nihilo Technical Journal, 10:219-40.

Shipman P. (1994): Those ears were made for walking. New Scientist, (July 30, 1994) 143:26-9.

Spoor F., Wood B.A., and Zonneveld F. (1994): Implications of early hominid labyrinthine morphology for evolution of human bipedal locomotion. Nature, 369:645-8.

Spoor F., Wood B.A., and Zonneveld F. (1996): Evidence for a link between human semicircular canal size and bipedal behaviour. Journal of Human Evolution, 30:183-7.

Spoor F. and Zonneveld F. (1998): Comparative review of the human bony labyrinth. Yearbook of Physical Anthropology, 41:211-51.

Wieland C. (1994): New evidence: only people ever walked really upright. Creation Ex Nihilo Technical Journal, 8:127-8.

Standing (and Walking) Alone: The Vestibular System and Its Role in Theories of Human Evolution, by John Woodmorappe (creationist article)

New evidence: Lucy was a knuckle-walker, by David Catchpoole (creationist article)

Commentary on the Arcy-sur-Cure Neandertal

This page is part of the Fossil Hominids FAQ at the talk.origins Archive.