Editorial: An eye opener

Published in The Hindu on December 22, 2011

If complex eyes could evolve rapidly despite being an organ of extreme perfection, there is no way of confirming this. The soft nature of the organ worked against it being well preserved even in the best of depositional conditions. With the fossil record offering inadequate evidence, our understanding of the early evolution of eyes has been rather sketchy. This is true even in the case of 500-million-year-old Cambrian Burgess-Shale-type Konservat-Lagerstatten deposits in the Canadian Rocky Mountains, known for their exceptional preservation of soft body parts. But a discovery reported in June 2011 of a 515-million-year-old fossil of a fearsome predator, an Early Cambrian Anomalocaris, with well-preserved compound eyes provided the much-needed opportunity to study the early evolution of the visual organs. Each compound eye of the arthropod possessed about 3,000 individual lenses. The latest discovery of another Anomalocaris from the same Emu Bay Shale deposits (about 515 million years ago) in South Australia is even more exciting. According to a paper published recently in Nature (“Acute vision in the giant Cambrian predator Anomalocaris and the origin of compound eyes,” by John R. Paterson et al.,), each pear-shaped eye measures 2 cm to 3 cm in length and 1 cm to 2 cm in width, and contains nearly 16,700 hexagonal lenses. The number of lenses is a multiple of what some modern-day arthropods have — 3,200 in the case of common houseflies and fewer than 1,000 in most ants. With each lens behaving like a pixel, vision clarity improves as the number of lenses increases. Even the regular packing of the hexagonal lenses is a surprise, considering that the present-day horseshoe crab Limulus lacks it.

The specimens are unambiguously Anomalocaris and hence have a definite affinity with arthropods. The complex nature of the eyes is not surprising as the latest find has all the morphological features of a top predator — about a metre-long body with spine-bearing front appendages, and a mouth with teeth or tooth-like projections. But the fully armed predator lacks a hardened exoskeleton. Since another anomalocaridid discovered in 2009 has compound eyes and an exoskeleton, the researchers argue that compound eyes in arthropods must have evolved prior to the development of hardened exoskeletons. If this is true, the recent find must be considered a more primitive member (basal) than the one discovered in 2009. This has a major implication — the perfect hexagonal packing of the lenses seen in the most recently discovered fossil “pushes the origin of compound eyes further down” the arthropod evolutionary tree.