VLDLR: Back to Quadrupedalism

A number of years ago, the children in a Turkish family were found to be obligate quadrupeds. All of a sudden, researchers were fascinated by what they saw as people having reverted the supposed primitive state of quadrupedalism. Since then, a few other families and individuals also have been found to exhibit quadrupedalism.

I'll disregard all the problems with that for a moment.

A recent study by Ozcelik et al (2008), published in PNAS, has now proposed VLDLR, a gene involved in lipoprotein reception, to be a gene necessary for human bipedalism.

And you know what? They're probably right. But they're wrong that this is evolutionarily interesting. As the authors note, VLDLR influences many phenotypes, and when nonfunctional results in cerebrocerebellar hypoplasia, specifically vermial hypoplasia, accompanied by mental retardation, and dysarthric speech all in addition to quadrupedalism. It is quite possible that some of these mental defects may be what is causing the quadrupedal gait, and so any gene that influences that portion of the brain will induce quadrupedalism.

As Herz et al (2008) note:

Clinical, evolutionary, and molecular evidence refutes their hypothesis. Quadrupedal locomotion is more likely an adaptation to the severe truncal ataxia present in all VLDLR-deficient patients, resulting from a combination of uneven, rough surfaces in rural areas, imitation of affected siblings, and lack of supportive therapy. Conclusions about the role of VLDLR in the transition from quadrupedal to bipedal locomotion are uninterpretable and untenable in the presence of such extensive structural cerebellar defects.

So the authors, then, have found one gene among many that, when changed will produce a negative effect on many traits including quadrupedalism. But they have no idea what the mechanism for that relationship is.

I will say that they do note that they want to look into families B and C, of which a mutation in VLDLR does not exist but do exhibit quadrupedalism to see what other neurological mechanisms may influence gait. But will it really tell us that much about locomotion? Not likely.

Least favorite sentence:

Quadrupedal locomotion was first reported when Tan (9, 10) described a large consanguineous family exhibiting Unertan syndrome, an autosomal recessive neurodevelopmental condition with cerebellar and cortical hypoplasia accompanied by mental retardation, primitive and dysarthric speech, and, most notably, quadrupedal locomotion.

Why is "quadrupedal locomotion" the most notable of these disorders? Because it is the easiest to have a metric for? Because it is a rarer disorder? Because it is the one they are covering in this paper?