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Crystal Eyes
Natural History, Oct, 2000 by Richard Fortey
Five hundred million years ago, trilobites looked at the world through clear calcite glasses.
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One of the most difficult jobs I ever attempted was to count the number of lenses in a fossil trilobite eye. I had been fascinated by these long-extinct arthropods as a child and have since spent years studying them as a paleontologist. For the task at hand, I took several photographs from different angles of the eye (which was about the size of a grain of rice) and then made enormous prints that allowed me to see the individual lenses (which were microscopic). Eventually I hit upon the notion of pricking each counted lens on the photograph with a pin so that it wasn't counted twice. But when I moved on to the next photo, I would be unsure of the last lens I'd counted and how the tiny hexagons linked up from one picture to the other. Was the last lens the one with the little scratch, or that one a mite larger than its neighbor? The work was undeniably suitable for an obsessive with insomnia, but it was worth it. For trilobites have the first really well-preserved visual systems in the fossil record. Furthermore, their eyes are unique in that they are made of the mineral calcite. The trilobites have many living, if remote, arthropod relatives--and trilobites have been extinct for 250 million years--but no other has chosen the trilobite way to see the world.
Calcite is one of the most abundant minerals. The white cliffs of Dover are calcite; so are the bluffs along the Mississippi River. Surely one could expect no surprises from a substance so common and so familiar. But calcite has some unusual properties. Because of its natural impurities it has long provided builders with colorful stone and decorative slabs. The purest examples of calcite, however, are transparent, with perfect crystal form and clarity. The chemical composition, Ca[CO.sub.3], is simple as minerals go. As the crystal grows, the constituent atoms stack together in a lopsided way and do not allow other, stray atoms to intrude to cloud the crystal's mineral exactitude. The clearest calcite crystal, transparent as a toddler's motives, is Iceland spar. Look into a crystal of Iceland spar and you can see the secret of the trilobite's vision. While most other arthropods have lenses made of relatively soft, unmineralized cuticle, similar to that of the rest of their exoskeleton, trilobites used the transparency of clear calcite as a means of transmitting light. The trilobite eye is in continuity with the rest of its shelly armor. It sits on top of the animal's cheek, an en suite eyeglass, tough as a clamshell.
Clear calcite is optically complex. If you break a large piece of crystalline calcite, you are left with a regular, six-sided chunk of the mineral--a rhomb--which treats light in a peculiar way. If a beam of light is shone at the sides of the rhomb, the beam splits in two, a phenomenon known as double refraction. The course of the two rays is determined, as is the shape of the rhomb, by the stacking of the individual atoms. There is one direction, and one direction only, in which light does not indulge in this optical split: when a ray of light approaches along what is termed the c axis of the crystal, it is afforded free passage. Like a VIP at an international airport, this privileged ray passes straight through. If a crystal is elongated in parallel to the c axis, into the shape of a long prism, light entering from most angles will be split and the dual rays will in turn be deflected to reach the edge of the prism and will not be "seen" by a receptor cell located at the base of the prism. But light shining along the prism's long axis will still pass unrefracted. This is how most trilobite eyes are constructed. We know that the first trilobites already had a well-developed visual system. Indeed, the large eyes found in the genus Fallotaspis, from Morocco, prove that sophisticated vision goes back at least 540 million years to the Cambrian period.
Recent laboratory work has led to the discovery of the pervasive influence of genes that control the sequence of development of the various organs as animals grow from embryo to adult. These are genes so deeply embedded in the body plans of organisms that the memory of their origin is lost far back in Precambrian history. We can never, ever, directly sample the genetic code of the trilobite, but we can be sure that its development was under the control of the same kinds of genes we recognize in living animals. Development inexorably follows a blueprint originally drawn up in the most ancient times. It is rather wonderful to imagine this distant manifesto at work on the growing trilobite, directing the brain to be enclosed within the head and, of course, issuing instructions for the growth and development of eyes.
For eyes are part of this ancient list of instructions. It seems that the making of an eye is the same impulse in fish or fly or man. Eyes are under the control of a gene called Pax6. The end product may be very different, but the instruction "Make eyes" may be common to all animals. The deep language of the genes is an Esperanto of biological design that can be understood by all creatures that have light-sensitive organs. Trilobites offer visible evidence of the halfway point in optical history. We can feel a bond with the trilobite that would not have been apparent when nineteenth-century investigators first gazed upon the animal's stony eyes. "Look into my eyes," the trilobite now seems to say, "and you will see the vestiges of your own history."
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