ENHS Nature Trails Mar. 98 Article 2

President's Column: Of Flies and Men
by Nathan Tublitz

The 1820s was a decade of change around the world. Europeans continued their colonialist tendencies in Asia and Africa while losing control of much of South America. Friction between the Greeks and the Turks reached the boiling point yet again, resulting in the horrific Reign of Terror. The US, still in its infancy, flexed its growing political muscle with the announcement of the Monroe Doctrine which forever closed the American continent to European expansionism. And our neighbor to the south, Mexico, finally became an independent republic. To E. Gouvion St. Hilaire, however, none of this was of any concern. What mattered, what was much more pressing than global affairs, was the world unfolding under his newly acquired, high-quality microscope.

St. Hilaire, one of the two great French biologists of that era, was an enthusiastic disciple of comparative biology, a field which in that pre-Darwinian time centered on understanding the relationships between different species. Comparative biology was in its heyday in the 1820s, primarily championed by that other luminous Frenchman, Georges Cuvier, who spent most of his scientific career collecting and grouping animal and plant species according to their morphological and physiological characteristics. In the early 1820s, Cuvier published an expansive treatise on animal phylogeny in which he set down a well-reasoned argument that arthropods and mammals clearly must have arisen independently since they had little in common. After all, insects with an exoskeleton, 6 legs, multifaceted eyes, and antennae sticking out of its head, were no closer to humans than the French were to the English. Mon dieu, c'est evidente!

Cuvier's charisma and logical reasoning on this issue overwhelmed the European community of biologists. He was hailed as the world's pre-eminent biologist during a triumphal lecture tour across Europe. But St. Hilaire was not among his supporters. St. Hilaire spent countless hours looking in great detail at the bodies of numerous organisms, including insects, lobsters, and humans. His observations led to a conclusion radically different from his colleague, the great Cuvier. Where Cuvier saw differences, St. Hilaire saw similarities. He observed that both arthropods and mammals had jointed appendages emerging from the belly, distinct body regions, eyes on the head, and a nervous sytem consisting of a brain and a nerve cord. He even look at the development of arthropods and mammals and determined that both had segmented embryonic forms. The only difference was that the organization of the internal organs were upside down in insects: the heart was on the back and nerve cord ran along the belly which was opposite from their arrangement in mammals. He posited that insects were just inverted mammals! St. Hilaire coalesced these observations into a unified theory on the relationship between different species, known as his "Theory of unity of plan in organic composition". Based on these observations, St. Hilaire concluded that arthropods and mammals must have arisen from a common ancestor.

But Cuvier would have none of this. In Cuvier's mind, St. Hilaire's theory was totally unsubstantiated by any real evidence. Scientific discourse in the 1820s was mainly through oral rather than written channels, and Cuvier took his attack on St. Hilaire on the road, stopping at most of the major institutions of higher learning to deliver a scathing lecture against St. Hilaire's theory. St. Hilaire was immediately put on the defensive and spent the next 10 years defending his theory and his reputation. But it was to little avail - in the forum of public opinion, Cuvier, through his eloquence and sharp wit, prevailed on the scientific community. St. Hilaire's theory, although not completely disgraced, effectively disappeared from the forefront of scientific thought, and after many years of depression over the failure of his theory to take hold, St. Hilaire died.

But good ideas never die - they just go into intellectual diapause, waiting for the proper moment to emerge again in a mature form. In this instance, St. Hilaire's theory hibernated undisturbed for over 170 years, re-surfacing only recently as a result of the explosion of exciting new results in developmental biology. Over the past 10 years many new molecules involved in forming the adult body plan have been isolated from a number of organisms, including worms, flies, fish, frogs and mice. Surprisingly, many of these molecules are used in similar manners by all of these organisms to set up the final body plan both in the head-tail and belly-back axis. One of the most recent findings implicate a specific molecule, known as decapentaplegic which has been implicated in forming the belly-back axis in invertebrates such as flies, and also in vertebrates such as frogs and mice. These data suggest that the same set of molecular signals organize body pattern formation in both flies and vertebrates, and that the exact nature of the pattern in any given species is determined by species-specific differences in the expression of these molecules. This and other emerging evidence from comparative developmental studies is finally beginning to vindicate St. Hilaire's long-lost theory after 170 years in disrepute. If St. Hilaire was alive today, you might hear him proclaim, "Vive la difference, mais vive la similarite aussi!"

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