Historical Constraints and the Evolution of Development 1109
They also constitute, in the earliest agnathan vertebrates, the major functional aspect
and structural extent of the organism's segmental anatomy—and not just a small
portion of the anterior end. The region of the agnathan gill slits occupied more than
half the body's length in many early forms. Moreover, the pharyngeal clefts
functioned not only in breathing, but also, as the branchial basket, in gathering and
filtering food. In fact, these earliest vertebrates may have fed in the manner of many
arthropods, by passing food along a series of segments and their appendages, from
posterior to anterior towards the jawless mouth (rather than in the reverse direction
that we know so well from our own experience!). For many of the earliest agnathan
vertebrates, and without gross exaggeration, one might be tempted to regard the
posterior vertebral column (behind the branchial basket) as an add-on and
afterthought. In this historical sense, if insect metameres are homologs of
rhombomeres in the developing hindbrain of vertebrates, then segmental homology
between the two phyla governs the major primordial system of vertebrate
segmentation, even if most later gnathostome clades deemphasized this anterior
system and strengthened the somites of the subsequent and posterior vertebral
column.
- MORE EXTENSIVE HOMOLOGIES THROUGHOUT THE DEVELOPING
SOMITES. If homologies based on the Hox code place vertebrate rhombomeres into
phylogenetic union with arthropod metameres, must we conclude that the far more
prominent somites of the gnathostome vertebral column bear no relationship of
homology with arthropod segments? Such a conclusion need not follow, for the
obvious reason that development and specification of arthropod segments requires the
operation of several genetic systems prior to and beyond the activation of Hox genes.
The Hox genes, after all, do not regulate the formation, number and timing of
segments, and
10 - 16. Schematic diagram from Raff, 1996, showing that each rhombomere in the developing
embryonic hindbrain of vertebrates correlates directly with the pharyngeal arches developing
just alongside, with each arch corresponding to two rhombomeres.