More anatomical similarities? The first pharyngeal arch, which as already stated, develops into the jaw bones in all gnathostomes (jawed vertebrates) which is innervated by the 5th cranial nerve (the trigeminal) which connects the brain to the jaws and parts of the face. (Shubin 2009, p.92) Another nerve of interest is 10th cranial or vagus nerve. In “fish” this comes out of the base of the brain and connects to various organs in the body occasionally sending off branches along the way. One of these branches, the 4th, innervates the gills of the 6th pharyngeal arch in “fish”; which is a pretty straight path from the top (dorsal) part of the “fish” to the bottom (ventral) part (see the picture below). Coyne 2009, p.83 However, as we saw earlier the in mammals the 4th and 6th pharyngeal arches fuse—after the transitory 5th arch is reabsorbed. The 4th/6th arch, modified by development then contributes to the formation of the larynx. The trick is that when the extended neck evolved in amniotes and the 4th branch of the vagus nerve—now called the recurrent laryngeal nerve—got hooked by the 6th aortic arch and dragged down into the chest, changing what was a fairly straight shot along the 6th gill arch in “fish”, into a circuitous route wherein the vagus nerve come out of the base of the brain travels down into the chest, loops around the pulmonary artery and then goes back up the neck into the larynx. This has led to a laryngeal nerve of comic proportions in giraffes. If you’re not squeamish and you’d like to see this for yourself, the excellent British television show Inside Nature’s Giants did a dissection of a giraffe in their 4th episode, highlighting its recurrent laryngeal nerve; I cannot recommend this program highly enough. Perhaps it’s not surprising then, that giraffes don’t vocalize very much. By the time the signal to make a noise travels from their brains down their necks and back up again they’ve forgotten what they were making noises about! In any case, there is no obvious sense to this state of affairs from a designed from scratch, engineering point of view, but it makes perfect sense if amniotes are opportunistically modified “fish”. Physiology, Genetics Comparisons between pharyngeal arches in “fish” and amniotes extend beyond their gross anatomy; there are also shared physiological and genetic similarities. For example, while it is universally admitted that amniote embryos don’t use their “gill slits” to breath, there is at least one other physiological function originating in the pharyngeal apparatus that is conserved between “fish” and amniotes; regulating calcium salts (a major component of vertebrate bones). “Fish” regulate the amount of calcium in their bodies using their gills. Amniotes do this by secreting hormones from their parathyroid glands which just happens to develop from the 3rd and/or 4th embryonic pharyngeal pouches (Okabe & Graham 2004). For more see: Deep homologies in the pharyngeal arches on Pharyngula Human parathyroid probably evolved from gills on biochemist.org At the genetic level there are two significant families of genes (Hox & Dlx) which control the development of the pharyngeal region of all vertebrates. A nested set of Hox genes control the development of the pharyngeal structures head to tail (anterior to posterior) from just after the first pharyngeal arch (Hunt et al. 1991) (Prince et al. 1998) (Kuratani 2004). And likewise a nested set of Dlx gene are expressed in the development of the pharyngeal structures in the front to back (ventral/dorsal) direction (Schilling 2003) (MacDonald et al., 2010). pigeonchess/2012/05/31/gill-slits-by-any-other-name/
Posted on: Sun, 04 Jan 2015 15:12:19 +0000
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