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Thursday, August 11, 2011

Early Cambrian eyes

Eyes have evolved independently in different lineages up to a hundred times during the course of life on Earth.  Until recently, there has been little evidence of complex eyes dating back to the early Cambrian, which began 540 million years ago.  Now, Australian paleontologists Michael Lee and John Paterson and their colleagues have found evidence of extremely complex compound eyes dating to about 515 million years ago.

A half-billion-year-old fossil compound eye, showing exquisite detail of the visual surface (the individual lenses can be seen as darker spots).
Credit: Photo by John Paterson (University of New England).

Most of the major phyla present today first appeared during the Cambrian ‘explosion’.  Although many of these early organisms would have had eyes, few of the eyes have been preserved, with the exception of the biomineralized eyes of trilobites. In contrast, the recently discovered fossils were of compound eyes made up of over 3000 lenses (shown as 'pixels' below).  In comparison, living horseshoe crabs have compound eyes made up of 1000 lenses, whereas dragonflies are the visual champions with 28,000 lenses.

The researchers are not sure to which creature the eyes belonged, but suspect that it was an active predator, given that many of its contemporaries, such as trilobites, had eyes with only 100 or so lenses.  You can see what a difference this makes below:

Images as seen by (left to right) Cambrian trilobite, newly found fossilized eye, modern dragonfly.
Credit: Thierry Laperousaz (South Australian Museum) and Mike Lee (South Australian Museum/University of Adelaide).

Notice that, contrary to a commonly held myth, each lens does not provide a separate picture of the whole visual field but rather a tiny section of the visual field.  The more lenses (or pixels) available, the higher the picture resolution.  This makes sense if you think about it.  Seeing one image in greater resolution could give an animal a clear advantage. It's hard to imagine what advantage would be gained by seeing 28,000 tiny separate images.