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As the worm turns
Natural History, Feb, 1997 by Stephen Jay Gould
How can you evolve a vertebrate from an invertebrate? Invert it.
When Hamlet, in the most celebrated soliloquy of English literature, weighs the relative values of life and death, he describes suicide ("not to be") as an escape from active insults, including "the oppressor's wrong, the proud man's contumely." But writers and intellectuals worry far more about an opposite fate on life's potential "sea of troubles"--erasure and oblivion, the pain of being simply ignored. Samuel Johnson, as recorded by Boswell, expressed this silent arrow of outrageous fortune in a famous aphorism: "I would rather be attacked than unnoticed. For the worst thing you can do to an author is to be silent as to his works."
I therefore felt a special poignancy when I recently read an anecdote about the last years of a great English physiologist, Walter H. Gaskell (1847-1914). After a distinguished career of solid experimental work on the function of the heart and nervous system, Gaskell switched gears and devoted the entire second half of his professional life (from 1888 on) to promoting and defending his idiosyncratic theory for the origin of vertebrates. The last paragraph of Gerald L. Geison's long article on Gaskell in the Dictionary of Scientific Biography reads:
His final years were clouded . . . by a
feeling that his deeply loved theory of the
origin of vertebrates was not receiving a
fair hearing. Even at Cambridge, where
Gaskell lectured on the topic until his
death, his audience decreased over the
years until, near the end, the poignant
scene is drawn of Gaskell closing his
course by shaking hands with a lone
remaining auditor.
We may grieve for Gaskell's personal fate as an intellectual pariah, but, truth to tell, he had been pushing a pretty nutty theory for the origin of vertebrates. Gaskell believed with all his soul, and with a striking absence of critical questioning, that the evolution of animal life must follow a single pathway of progressive advance mediated by an increasing elaboration of the brain and nervous system. Gaskell wrote in his major work of 1908, The Origin of Vertebrates (the source of all quotes from Gaskell in this essay):
We can trace without a break, always
following out the same law, the evolution
of man from the mammal, the mammal
from the reptile, the reptile from the
amphibian, the amphibian from the fish,
the fish from the arthropod [insects and
their allies], the arthropod from the
annelid [segmented worms], and we may
be hopeful that the same law will enable
us to arrange in orderly sequence all the
groups in the animal kingdom.
Gaskell identified this controlling principle of linear advance as the "law of the paramount importance of the development of the central nervous system for all upward progress." In a rhetorical flourish, he then inverted the preacher's famous argument (Ecclesiastes 9:11) for randomness and aimless change without direction: "The law of progress is this--The race is not to the swift, nor to the strong, but to the wise."
Advocates for a single line of progress encounter their greatest stumbling block when they try to find a smooth link between the apparently disparate designs of invertebrates and vertebrates. In addressing this old problem, Gaskell adopted the standard strategy of linear progress theorists from time immemorial: identify the most complex invertebrate and attempt to forge a link with the simplest vertebrate. Gaskell, again following tradition, selected arthropods as the invertebrate pillar for his bridge, and then tried to build the span under his law of neurological complexification:
This consideration points directly to the
origin of vertebrates from the most highly
organized invertebrate group--the
Arthropoda--for among all the groups of
animals living on the earth in the present
day they alone possess a central nervous
system closely comparable in design with
that of vertebrates.
So far, so conventional. Gaskell's theory becomes idiosyncratic and a bit bizarre in his chosen mode for forging the improbable link of arthropod to vertebrate. Among the plethora of prominent differences between these phyla, one central contrast has always served as a focus for discussion and a chief impediment to any linear scheme. Arthropods and vertebrates share some broad features of general organization--elongated, bilaterally symmetrical bodies with sensory organs up front, excretory structures in the back, and some form of segmentation along the major axis. But the geometry of major internal organs could hardly be more different, thus posing the classical problem that has motivated several hundred years of dispute and despair among zoologists.
Arthropods concentrate their nervous system on their ventral (belly) side as two major cords running along the bottom surface of the animal. The mouth also opens on the ventral side, with the esophagus passing between the two nerve cords and the stomach and remainder of the digestive tube running along the body above the nerve cords. In vertebrates, and with maximal contrast, the central nervous system runs along the dorsal (top) surface as a single tube culminating in a bulbous brain at the front end. The entire digestive system then runs along the body axis below the nerve cord. But could evolution (or a sensible, divine Creator, for that matter) turn an arthropod with belly above nerve cords into a superior vertebrate with brains on top and a gut below?
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