Monday, November 21, 2011

Evolution's Witness: How Eyes Evolved

(This essay was written as a book review on about my friend and professional colleague Professor Ivan Schwab's textbook Evolution's Witness: How Eyes Evolved.  The 
review was published on November 21, 2011)

  Episodically a book is written of such profound importance that it not only encapsulates and defines a subject, but transforms our under-standing of it as well.

Fossilized Ammonite
    Such is "Evolution's Witness," by Professor Ivan Schwab M.D., at once both magisterial treatise and kaleidoscopic visual treat on the subject of the evolution of the eye. Dr. Schwab has managed, apart from being a Professor of Ophthalmology at the University of California at Davis where he is an ophthalmologic surgeon and well-published and internationally recognized basic and clinical research scientist, to have found the time, focus and diligence to comb the world's literature, communicate with leading authorities in the fields of paleontology, invertebrate and vertebrate biology, comparative anatomy and physiology, collate hundreds of splendid color photographs, microphotographs and histopathology images, and bring to bear an enormous quantity of wide-ranging scientific discourse on how eyes have evolved over the eons, and in so doing, to explicate how the eye is indeed the "visual witness" of the story of evolution.

   For years, it had been argued that the complexity of the eye was a reason for some kind of intelligent design in its creation,that the eye could not have developed simply from a combination of time and natural selection. This book, much as Richard Dawkins' seminal work The Blind Watchmaker, is implicitly a paean to and defense of the concept which Darwin championed, that over an impossible-to-comprehend period of time measured in the billions of years and often by only subtle mutations and changes responding to natural selection, nature actually could, and did, design, refine and re-design the eye as a response to predation, biochemistry, environment, natural catastrophes and extinctions. In Doctor Schwab's words, "Sensory systems, especially visual systems, will continue to be among the premier forces of evolution because of the advantage they afford species."

   Dr. Schwab begins at the beginning, as it were, with the formation of life itself, around 3.75 billion years ago. What is clear after only a few chapters into this richly illustrated book is how important and early in the history of evolution is the special sense of vision, and how crucial it was for the development of the most primitive organisms to today.

   He augments the chronology by marvelously and engagingly telling the reader many stories along the way - of the lowly Euglena, the protist whom we all met in high school biology, and how its flagellum may be powered by a variant of rhodopsin, a primitive form of the same visual pigment we have in our own human eyes; that the trilobite, birthed in the Cambrian Era 543 million years ago likely from an even older pre-Cambran (Ediacaran) forebear, might have possessed the "first eye"; that the Cambrian explosion begat many of the last common ancestors of present-day organisms and their unimaginably diverse eyes that have evolved and radiated over time ; the world of the misnamed mantis shrimp with its sixteen visual pigments (as opposed to three in the human) and its "superman"-like qualities in its particular ecosystem; that the scallop has 40-100 eyes, resembling tiny "blueberries" attached to its mantle; that the shark may have a form of synesthesia as it is able to use the electromagnetic as well as the visible spectrum to navigate waters; why birds may have the most complicated and highest visual system; and the evolution of the primate eye, culminating in our magnificent and beautiful human organ of vision.
Trilobite eye (Metacanthina sp.)
Note the numerous ommatidia.
Trilobite eyes fossilize easily because
 they contain calcite.
   Dr. Schwab masterfully engages the reader in this wondrous chronology and by augmenting the narrative with these stories, replete with vivid color illustrations, he does so compellingly well, as opposed to robotically listing a myriad organisms and examining their visual apparati. And in reality, he tells two interweaving stories: the story of evolution as seen through the specific lens of ocular evolution, and the story of many of the world's creatures, and the development of their unimaginably divergent and fascinating eyes, which evolved in response to Darwinian natural selection.

    Dr. Schwab's writing style is effortlessly brilliant - erudite but self-effacing, meticulous to a fault but in the service of accuracy and science, clever and witty but always focused on providing information accretively and understandably. The book is divided into chapters for each eon or era, with representative examples of both last common ancestors and current examplars of these radiations of life. Colored margins on each page of text correlate to a color-coded timeline which keeps the reader on task synthesizing the plethora of knowledge of geology, paleontology, anatomy, biology and physiology that the author explicates.

Taienura lymma
The blue-spotted ray
with its pupillary operculum
   This book is hard to put down. It will be both a wonderful tome to pick up frequently and select a chapter at random to read on the remarkable and colorful story of life and vision, as well as a serious, deeply referenced academic resource for ophthalmologists, visual scientists and readers of all stripes who are intrigued by how it is that our eyes, and the eyes of countless creatures both present and extinct, have come to be.

Copyright 2012 Vincent de Luise MD A Musical Vision

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