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Oldest eye?

Metacanthina issoumourensis

For many years, the first confirmed eye was thought to be that of a trilobite, Olenellus. The eyes of trilobites fossilize easily because they are composed of a form of calcium carbonate, called calcite. (The white cliffs of Dover are composed of calcium carbonate.) Calcite is a crystalline form, which, under certain conditions, can form a clear lens that is perfectly able to focus light. Coincidentally, this crystal also fossilizes so well that sometimes such lenses can be used today even though they are hundreds of millions of years old.  Trilobites evolved the capability to make such lenses as the focusing portion of their eyes. When the trilobite fossilized, so did the eye, allowing us to document this eye, even knowing its refractive status.

This probably wasn’t the first eye because it is too evolved to have appeared de novo. Other less derived eyes likely appeared many times before this one. Since soft tissue does not fossilize well, often not at all, we don’t know as much about earlier eyes.

Recent reports from Kangaroo Island off the southern coast of Australia, however, provide definite evidence of an eye that was at least contemporary and perhaps older than that of Olenellus. It is derived enough that it could not have been the first eye either, but now it becomes one of the oldest known eyes, if not the oldest.

In a recent report in Nature by Lee et al (Lee, MSY, et al:  Modern optics in exceptionally preserved eyes of Early Cambrian arthropods from Australia. Nature 30 June 2011. 474; 631-634), a new candidate for the first eye, or at least a very early eye, is another arthropod. The creature was disarticulated and hence is known only from the eye, and may be from an animal already known though other fossils. Consequently, it is unnamed at present until more is known about its body.  Although the animal species is not known, the find appears to be a portion of the carapace discarded during ecdysis (shedding). If that is the case, the shed imprints of the eye resembles those of some extant insects or alternatively long-bodied decapods, such as a lobsters or shrimp.  It has a compound eye with perhaps 3000 ommatidia (the individual units of a compound eye, like a fly’s eye; it is derived from the Greek for “little eyes.”).  This animal, whatever it was, is an important step in understanding the evolutionary development of eyes. This newly discovered remnant of an eye is more derived and better organized than that of Olenellus although the Olenellus specimen is probably a bit older. Analysis of the multiple fragments of these specimens leads these investigators to believe the creature was a relatively fast-moving predator with binocular vision, capable of activity in dim light, including hunting.

Whatever animal is the source of this fossil, it is important and illustrates that eyes were probably well developed before the Cambrian and/or the speed of development of eyes was potentially quite rapid.