Insect leg development
Evolution out on a limb
by Pierre Jerlström
Hidden unity
The body plans of vertebrates and insects differ greatly in their size and shape,
and in the type and number of appendages. Nevertheless, there is a hidden unity
in the genes and the genetic system that control their development. Cells along
the main body axis of vertebrates, and of insects such as fruit flies, 'know' their
position as well as what type of appendage they will develop into from the level
of expression of the homeotic selector genes (Hox) inside their nuclei.1
The role of specific Hox genes in insect limb development has recently been studied.
At a certain stage of insect larva growth the Distal-less (Dll) gene switches
on, causing some of its cells to organize into legs. Switching off Dll
on the other hand, results in only stumps forming.2 In the early 1990s,
scientists were astounded to find almost identical copies of this gene in vertebrates,
and to find that as with insects, these genes switch on during leg development.
This was surprising because vertebrates and insects have completely different limbs:
bugs have their muscles on the inside of a protective exoskeleton, whereas in animals
muscle covers the bone. And, according to evolutionary belief, insects and vertebrates
are only distantly related to a limbless flatworm that lived perhaps a billion years
ago. They believe that limbs and the genes for their development have evolved independently
in these two lineages.2
Scientists further looked at other 'distant relatives' of the flatworm such as velvet
worms, sea urchins and sea squirts, which also have limb-like appendages. They found
that Dll-like genes were active in the developing appendages in each of
these animals. 2
Looking at the evidence within a Biblical framework, it is easy to recognise this
hidden unity in limb development as the work of one Creator who used a highly successful,
basic blueprint to design appendages for movement for the various created kinds.
By analogy, the wheels of bicycles, cars, trains, etc., have not arisen by accident,
but are all variants of a basic engineering design. In this light, it is not surprising
to find that similar molecular information (Hox genes) in the genetic code of different
animals gives rise to analogous leg structures.
Mutant study
Two other Hox genes, Ultrabithorax (Ubx) and abdominal-A
(abd-A), also have distinct functions in some insects. In the red flour
beetle, Tribolium castaneum, abd-A determines whether or not a
limb grows in the abdomen by acting on Dll, while Ubx tells the
cells what type of limb they should become.3,4 When scientists inactivated
these two genes they found that flour beetle larvae sprouted 16 legs on their abdomen.
This has been hailed as supporting evidence for the idea that insects and arthropods
(animals without backbones) evolved 400 million years ago from animals resembling
centipedes and millipedes, which have many non-specialized body segments, each with
its own pair of legs. During the supposed evolution of insects, groups of segments
fused together to form the head, the thorax and a legless abdomen. Leg-making genes
also switched off, giving rise to more agile six-legged insects.4,5
Although a beetle larva can be persuaded to produce legs on its abdomen, this is
hardly support for evolution. It only confirms the role of particular genes in leg
development—it is well known that in insects every segment has the potential
to form a limb.5 But the type of limb, or whether or not it forms, is
determined by the individual Hox genes—in the fruit fly Drosophila melanogaster,
a particular appendage (leg type or antenna) in a segment is specified by a pair
of Hox genes.
Conclusion
Even the idea of mere insect evolution is inconsistent. Evolutionists are perplexed
as to how evolution could have produced such huge morphological variation among
insects, especially considering how highly conserved Hox gene expression is within
this lineage.3 The Scriptures plainly describe that all creeping things,
which includes insects, were created complete on the same 6th Day of
Creation to multiply after their own kind.
Evolutionary dogma interprets similarity as phylogeny. The genome of the fruit fly
Drosophila has recently been sequenced.6 With the elucidation
of the complete DNA sequence of more insects in the future, the lack of phylogeny
will become clearly evident, as has been recently documented among microorganisms.7,8
This will result in the further collapse of the ailing evolutionary 'tree of life',
as insects believed to be lower or higher in the tree are seen not to be connected
by consistent lines of descent. One wonders whether multiple origins of insects
('Creationist orchard' again) will also be proposed as this unfolds.8
References
- Alberts, B., Bray, D., Lewis, J., Raff, M., Roberts, K. and Watson, J. Molecular
Biology of the Cell, Third Ed., Garland Publishing Inc., New York, p. 1106,
1994.
- Zimmer, C., Hidden Unity, Discover Magazine 19(1), 1998.
<http://www.discover. com/cover_story/9801-3.html>.
- Lewis, D.L., DeCamillis, M. and Bennett, R., Distinct roles of the homeotic genes
Ubx and abd-A in beetle embryonic abdominal appendage development, Proc. Nat. Acad.
Sci. USA 97(9):4504–4509, 2000.
- Hecht, J., The magic number, New Scientist 166(2237):20,
2000.
- Shubin, N., Tabin, C. and Carroll, S., Fossils, genes and the evolution of animal
limbs, Nature 388:639–648, 1997.
- Adams, M.D. et al., The genome sequence of Drosophila melanogaster,
Science 287(5461):2185–2195, 2000.
- Jerlström, P., Shaky tree of life, CEN Tech. J. 13(1):10–11,
1999.
- Jerlström, P., Is the evolutionary tree turning into a creationist orchard?
CEN Tech. J. 14(2):11–13, 2000.
- Flour Beetle, Merit Students Encyclopedia, Cayne, B.S.(Ed.), Crowell-Collier
Educational Corporation, USA, 7:160, 1968.
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