Sunday, August 07, 2005

Bipedal Locomotion and Semicircular Canals

I am currently working on a post for Transitions involving the molecular evidence for the common ancestry of apes and humans (Note: this is slow going because I am having trouble finding websites with age appropriate discussions of this material - if you know of any let me know as I hate doing posts without giving links for people to get further information). While searching the internet I kept stumbling across a lot of creationists sites disputing this relationship. An incidental finding was an argument concerning bipedality in Australopithicines based on CAT scans of the inner ear morphology. Which is what this post is about.

An example of the creationist argument:

High-resolution computed tomography, known as CT or Cat Scan, was used to scan the inner ear labyrinth of 53 humans, a few dozen apes consisting of pygmy chimps, chimps, gorillas, orangutans, and other species. They also scanned fossil humans (early Homo and H. erectus), Australopithecus and Paranthropus. Reporting in Nature, Wood and his coworkers made height and width measurements of the arc of each semicircular canal from the CT scans. From these measurements, they calculated the radius of the arc's curvature. Among the living specimens, they correlated the arc size of the three semicircular canals with the body mass. Taking body mass into account, modern humans have larger anterior and posterior canals and a smaller lateral canal than the great apes. According to Wood, Homo erectus is the earliest fossil hominid to demonstrate the modern human morphology of the inner ear. The dimensions of Australopithecus and Paranthropus inner ears resemble those of living great apes. Wood says,

"Modern human locomotor behaviour [walking] makes particular demands on the vestibular apparatus for it involves the maintenance of an upright body posture by balancing on very small areas of support."

In other words, humans have the correct inner ear configuration for obligatory upright walking, while apes, australopithecines, and Paranthropus do not.

Finally, Wood concludes his report by saying,

"This study demonstrates that the morphology of the bony labyrinth has the potential to provide information about both the locomotor behaviour and the phylogenetic relationships of early hominids." [Wood, 1994]

Another example:

Some interesting new work has also helped to demolish the idea that the australopithecines habitually walked upright. Computerized X-ray scans are able to reveal the bony structure of the inner ear. The shape of this has been shown to directly reflect patterns of movement. Understandably, humans (the only creatures alive that walk habitually upright) have an inner ear structure which stands out from the rest. When this analysis is carried out on fossil skulls, the results are completely in line with modern creationist expectations. So-called Homo erectus (which even some evolutionists are saying should be reclassified as Homo sapiens) has an inner ear structure just like ours; whereas that of all australopithecines (and habilines) studied are 'decidedly ape-like'.

There is also this Intelligent Design version:

Other recent studies have found that the handbones of Lucy are similar to those of a knucklewalking ape, and that their inner ear canals, responsible for balance and related to locomotion, resemble small inner-ear canals of the great apes rather than larger canals found in humans and other members of the genus Homo.
(Note: I may do something on this article as it purports to lay out an ID theory of human evolution.)

So what does it all mean? Let's start with some basic anatomy. The picture below is a cross section of the inner ear.

The anterior and posterior semicircular canals are labeled "B" (the horizontal canal is not labeled). Note that both of these travel through the bone. Below is a picture showing the semicircular canals in relation to blood vessals and nerves, note also the two organs called utricle and saccule.

The function of this complex is to help maintain balance. The Utricle and saccule respond to linear acceleration (turning your head left or right, for example)and the orientation of the head relative to gravity (tilting your head left or right for example). The horizontal semicircular canal also has a role in the vstibulo-ocular reflex - which allows you to keep you gaze fixed on an object while your head is moving (trying to read a sign in a moving car for example). The semicircular canals respond to angular accelerations.

In humans the horizontal semicircular canal and utricle are at an angle to the naso-occipital plane.

When a person walks or runs they tilt their head forward causing the plane of the horizontal semicircular canal and utricle to be parallel to the earth horizontal and perpendicular to gravity. The anterior and posterior semicircular canals are positioned verticall in the skull and at angles to each other.

Note that the anterior on one side is oriented in the same fashion as the posterior on the other side.

Back to the creationists. Two studies are usually cited by creationists in order to claim that Australopithecines were apes. The first is by Spoor. The second is by Spoor et al and is available here. Spoor et al used CT scans to examine the morphology of the inner ear of 31 extent primates and 12 fossil hominids. They used height and width measurements to compute the radius of curvature of the arc. One of the findings of the study was that arc size of each of the three semicircular canals scaled with body mass. When body size is taken into consideration, humans have larger anterior and posterior and smaller horizontal semicircular canals than apes. They also discovered that the naso-occipital and sagittal orientation of the semi circular canals was the same in apes, humans and fossil hominids (this is not surprising since inner vestibular anatomy is a highly conservative trait in evolution). Spoor et al state that this implies that the ape condition was the likely ancestral condition and that the functional consequences of the elarging semicircular canal arc are not fully understood. They also point out that:

"...if the enlargment of the anterior and posterior canals is functionally related to modern human-like obligatory bipedalis, then at least in this respect the vestibular apparatus of the australopithicines was not adapted to this type of locomotor behavior(Spoor et al 1994, Nature 369:645-648)"

Basically then, what Spoor et al concludes is that australopithicine semicircular canals were somewhat different (in size but not orientation) from humans but they aren't sure what the functional meaning of the differences are. Which isn't quite what the creationists were arguing. One other note, Australopithecus afarensis was not included in the study.

For more on semicircular canals go here