Tuesday, 31 July 2012

On the Evolution of the Mammalian Middle Ear

This article was originally published on Evolution News & Views.

A correspondent recently asked me about the evolution of the mammalian middle ear in relation to the fossil record. Based on data gathered from embryology, it is widely thought that the bones of the mammalian middle ear (the region just inside the eardrum) evolved from bones of the reptilian lower jaw joint. Besides the paleontological data, this hypothesis is based on the fact that, in mammals, Meckel's cartilage plays a role in forming the middle ear bones and mandible before subsequently disappearing. In reptiles, it ossifies to become part of the jaw.

Two of the three bones that comprise the mammalian middle ear are located in the lower jaw joint of reptiles. During the transition of mammals to reptiles, therefore, it is supposed that the quadrate and articular (as well as pre-articular) bones became separated from the posterior lower jaw and evolved into the incus and malleus, two of the bones associated with the mammalian ear.

Last year, a paper was published in Nature, reporting on the discovery of "the first unambiguous ectotympanic (angular), malleus (articular and prearticular) and incus (quadrate) of an Early Cretaceous eutriconodont mammal from the Jehol Biota, Liaoning, China" (Meng et al., 2011). Called Liaoconodon hui and dating to the Cretaceous (~145 mya to ~65 mya), the last period of the Mesozoic era, the fossil (which itself dates from 125 to 122 mya) exhibits the so-called "transitional middle ear." The malleus, incus, and ectotympanic (i.e. the bones associated with hearing in mammals) are decoupled from the lower jaw and are hypothesized to be held in place by an ossified cartilage which, in turn, rests on a groove on the lower jaw (although cartilage is itself obviously not preserved in the fossil record).

There are a few points that are worth raising here. Firstly, even supposing that the hypothesis of common ancestry is valid, this lends little traction to neo-Darwinism (one has to distinguish between pattern and process) and it does nothing to undermine the hypothesis of design. ID, in its purest sense, has nothing to say about common ancestry. ID does, however, open up the possibility that universal hereditary continuity may be false, perhaps radically so. Many of us Darwin critics, therefore, also happen to be skeptical of common ancestry. But it would not invalidate our position on ID if common ancestry turned out to be true.
Secondly, if one reads the paper carefully, it is curious that (as noted by the authors), "Given the phylogeny, the [definitive mammalian middle ear] evolved several times independently." An earlier paper in Nature, published in 2007, reported on the discovery of a fossil of a eutriconodont mammal species called Yanoconodon (Luo et al., 2007). Curiously, as explained by this editor's summary of the paper,
"The situation is not as clear-cut as it seems. The evolutionary relationships of the fossil suggest that either the "modern" middle ear evolved twice, independently or that it evolved and was then lost in at least one ancient lineage."
It thus appears to be the case that the middle ear evolved independently at least twice: in monotremes and in placentals and marsupials. Multiple occurrences of difficult evolutionary trajectories is something that is not easy to square with the standard neo-Darwinian narrative.

Furthermore, remember that a major prediction of the evolutionary narrative is the "nested hierarchy" phylogenetic pattern. Indeed, the supposed presence of this pattern is hailed by many as among the strongest lines of evidence pointing to common ancestry. But it's not quite as straightforward as that -- strong convergence (that is to say, the independent appearance of highly similar traits across multiple lineages) is a widespread phenomenon in biology. The methodology for inferring common descent is thus dependent on circular reasoning. If similarity can be accounted for with reference to common descent, then it is taken as evidence for common descent. On the other hand, if similarity cannot be accounted for with reference to common descent, it is evidence for convergent evolution. It's a typical "heads-I-win, tails-you-lose" tactic. I have written about this topic more generally here.

Moreover, Meng et al. (2011) report that Liaconodon's middle ear "differs from that of Yanoconodon." The supplementary information document supplied with the paper covers this in more detail, noting:
"In general, what have been interpreted as ear ossicles in Yanoconodon differ significantly from the middle ear elements of Liaoconodon. In Yanoconodon all ossicles are broken, fractured and displaced. These elements are embedded in the matrix and only their broken sections and impressions anre visible.

The element identified as the incus of Yanoconodon was considered similar to modern mammals in having a crus longum (stapedial process) and a crus breve (for basicranial articulation). This element is proportionally much smaller than that of Liaoconodon and shows a different morphology. The articular facet for the malleus is not clearly shown. Because only the impression of the element in Yanoconodon was illustrated, it is difficult to make any detail comparison with the incus of Liaoconodon.

The ectotympanic and its impression are probably preserved more completely in Yanoconodon; the outline of the element is similar to that of Liaoconodon. The dorsal crus of the ectotympanic and the prearticular element of the malleus in Yanoconodon was interpreted as being fused to each other and both are further connected (fused) anteriorly with the fossified Meckel's cartilage. This differs from the condition of Liaoconodon in which the ectotympanic, the malleus and the OMC are clearly not fused."
Finally, in the absence of a viable materialistic mechanism to account for the transition in question, the supposition that one can slap these different structures down on a table and draw arrows between them seems highly suspect. The methodology is circular -- it assumes that these structures are connected by descent. When one's entire interpretative framework presupposes common ancestry at the outset, it is no wonder that any and every observation is taken as supportive of that paradigm. When some of the most impressively documented transitions in the fossil record (such as the evolution of the whale) occur far too quickly to be feasibly attributed to a naturalistic mechanism, do we not have reason to question whether that interpretative framework is actually valid?

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