Page 122 - The Errors the American National Academy of Sciences

The Errors of the American National Academy of Sciences

[It is] the rule rather than the exception that homologous structures
form from distinctly dissimilar initial states. 17
The evolutionary developmental biologist Rudolf Raff studied two
species of sea urchin which had reached almost identical forms by way

of very different paths, and expressed the same difficulty in 1999:
Homologous features in two related organisms should arise by sim-
ilar developmental processes . . . [but] features that we regard as ho-
mologous from morphological and phylogenetic criteria can arise in
different ways in development. 18
The incompatibility between the developmental pathway of ho-
mologous organs also applies to some vertebrate limbs.

Salamanders are one example of this. The development of the
digits of many vertebrate limbs is from the back to the front,
i.e., from the tail to the head. This is the case with frogs, for
instance. Yet, the manner of development of salaman-
ders—which, like frogs, are amphibians—is very
different. In salamanders, the development of
digits is in the exact opposite direction, from the
head to the tail.
Another instance of homologous organs that

do not pass through the same embryological stages
relates to the way in which these organs
generally start to develop in different
regions of the embryo. Research has
shown that similar organs in different
animals begin to be formed by different
groups of cells within the embryo. The

development of the alimentary canal is
one example of this; this fundamental

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