Historical Constraints and the Evolution of Development 1121
together, offer strong support for a modern recasting of Geoffroy's old theory of
inversion.
These studies have aroused considerable excitement and controversy, and a
substantial set of alternative interpretations has been proposed. But, in my judgment,
some of these objections attack the wrong target (Patten's hypothesis of direct
evolutionary transition, not Geoffroy's argument for common structural design),
while others raise legitimate questions of an interesting and fundamental nature,
although not yet resolvable by information now in hand. To cite a most cogent
example in each category:
Jacobs et al. (1998) compare the neural organization of arthropods and
vertebrates to a platyhelminth outgroup bearing a potentially plesiomorphic design:
"The flatworm nervous system is often conceived of as having an anterior nerve ring
with four major nerves emanating posteriorly from it" (1998, p. 348). They then point
out, citing Bier (1997), that "the default condition of the ectoderm is neurectoderm"
(p. 349), and that the development of non-neural ectoderm therefore requires
additional and apomorphic down-regulation, now largely accomplished, in
vertebrates and arthropods, by the chd/sog and dpp/BMP- 4 systems described above.
Jacobs et al. (1998) therefore interpret the relatively inverted systems of neural
development in arthropods and vertebrates as two different specializations from the
plesiomorphic (flatworm) condition of four major nerves extending posteriorly in
radial symmetry around the anterior ring. Thus, arthropods retain the two ventral
cords (and suppress dorsal neurectoderm by dpp action described above), whereas
vertebrates keep the plesiomorphic state in a dorsal position and suppress ventral
neurectoderm by the action of BMP-4. Jacobs et al. (1998, pp. 349-350) conclude:
"The bilaterian central nervous system would be the product of concentrating the
nervous organization in part of the ectoderm, by eliminating it from other regions ...
If this were the case, then the ventral nervous system in protostomes could derive
from the ventral pair of nerves in the orthogon [the fourfold system of flatworms] and
the dorsal system in vertebrates from the dorsal pair."
So far so good, and so reasonable. But Jacobs et al. (1998, p. 350) then make a
false inference about Geoffroy's views: "The above scenario explains the available
data without invoking an instantaneous dorsoventral inversion as envisioned in the
transcendental scheme of Geoffroy." But Jacobs et al., while correctly criticizing
Patten's theory of flipover in direct phyletic transition, misattribute this view to
Geoffroy. In fact, the scenario of Jacobs et al. fits splendidly with Geoffroy's actual
hypothesis of separate and different transformation, constrained by structural rules of
growth, from a common archetype—in this particular case, suppression of either the
two dorsal, or the two ventral, nerve cords in an originally radially symmetrical
circlet of four. (Gerhart, 2000, questions the inversion hypothesis with an alternative
strikingly similar to the proposal of Jacobs et al., but equally, and truly, consonant
with Geoffroy's actual claim.)
In a different potential criticism, Bang et al. (2000) accept the description