The discovery by scientists from the South Australian Museum and the Universities of New England (UNE) and Adelaide that some ancient, primitive animals had highly-developed vision has brought new insights into the ‘Cambrian explosion’ of life half a billion years ago.
The evidence comes from exquisite fossils from Kangaroo Island in South Australia that are over 500 million years old, yet are so well-preserved that they look like eyes from a recently-swatted fly.
The discovery, revealed today in the prestigious journal
Nature, was made by an international team of palaeontologists led by Dr Mike Lee, from the South Australian Museum and University of Adelaide, and Dr John Paterson of UNE’s School of Environmental and Rural Science.
Dr Paterson explained that modern insects and crustaceans have ‘compound eyes’ consisting of hundreds or even thousands of lenses. They see their world as pixels, with each lens producing a pixel of vision. More lenses means more pixels and better visual resolution. The fossil compound eyes have over 3000 lenses, making them more powerful than anything previously known from that period and similar to the eyes found in many insects living now, such as robberflies.
‘These fossils represent the oldest complex eyes on the planet,’ Dr Paterson said. ‘The only others known in the fossil record from this time are either very simple trilobite eyes or circular blobs.’
‘The eyes we’ve discovered belonged to an arthropod – a big group which includes insects, crustaceans and spiders – and are preserved in a way that the optical design and lens arrangement is clearly visible, enabling to us to understand how Cambrian animals could see and perhaps how they lived.’
The arrangement and size of the lenses suggests that the eyes belonged to an active predator that was capable of seeing in dim light. The findings suggest that sharp vision must therefore have evolved very rapidly, soon after the first predators appeared during the ‘Cambrian explosion’ of life that began around 540 million years ago. It is possible that sophisticated vision in early predators drove a Cambrian ‘arms race’ when animals evolved armour (e.g. shells) at a rapid rate.
As the eyes were found isolated, their owner is unknown, but they possibly belonged to a large shrimp-like animal. The 515 million-year-old rocks containing the eyes also preserve a dazzling array of marine creatures, many new to science. They include primitive trilobite-like creatures, bizarre armoured worms, and large swimming predators with elaborate feeding appendages.
The discovery provides further evidence that the ‘Cambrian explosion’ involved rapid innovation in fine-scale anatomy as well as major body architecture in animals, and is consistent with the idea that the development of advanced vision helped to drive this great evolutionary event.
Dr Paterson added that although the find was not expected, excavation has been taking place at the Kangaroo Island site since 2007. ‘Early on we were finding eyes attached to other animals but with no detail, just blobs. But these took us by surprise: normally the lens detail isn’t there.’
A key question has been how the eyes came to be preserved and fossilized in such detail. ‘Arthropods shed their corneas, which are made of material called chitin, similar to the thin, semi-hard exoskeleton you find on shrimps today,’ Dr Paterson said. ‘We suspect that, at a very early stage of fossilization, a compound called calcium phosphate replaced the chitin. Very specific conditions are needed for this reaction to take place, and the sediments must have been deposited under very low oxygen conditions, which both slowed decay and allowed this chemical reaction to take place.’
Dr Paterson said that, subject to funding, further excavations at the Kangaroo Island site were planned.