Evolution of Jaws: A Hox on Storytelling

Lampreys are jawless fish, unlike Jaws and his kin.  M.J. Cohn found that Hox genes are expressed in a lamprey in the first pharangeal arch.  Noting that fish with jaws do not express Hox genes in the first pharangeal arch [PA1], from which the jaws develop, Cohn hypothesized that jaw evolution proceeded with a retreat of Hox expression from this arch.  But an international team publishing in Nature¹ found another lamprey with no Hox expression in the arch.  They conclude, “Cohn’s finding is not a general feature within the lamprey group and is therefore unlikely to be related to jawlessness.”  Instead, “ the lack of Hox expression in the lamprey PA1 may reflect the fact that in both lampreys and gnathostomes [jawed creatures] the rostral-most pharyngeal arch forms highly specialized structures that are morphologically distinct from those of more posterior arches.”  The presence or absence of Hox expression in PA1 is “therefore not functionally relevant to jawlessness,” they say.

¹Takio et al., “Evolutionary biology: Lamprey Hox genes and the evolution of jaws,” Nature (20 May 2004); doi:10.1038/nature02616.

Let us understand that evolution cannot advance by losses.  A lamprey is not going to get a jaw by turning off genes.  Is Cohn asking us to believe that there were jaw-making genes that the lamprey decided to turn off?  Clearly not.  But to evolve by losses is like the merchant who lost money on every sale but thought he could make it up in volume.  A jaw is a “highly specialized structure” made of many parts that must fit together.  Cohn’s hypothesis was just a tall tale based on circumstantial evidence and belief in evolution.  At least he was ambidextrous; he could wave his hands and whistle in the dark at the same time.
    It’s getting monotonous to report evolutionary just-so stories that have proven false.  Can we get some positive evidence for a change, instead of jawboning?