July 29, 2013
For Your Eyes Only: Understanding How Sight Evolved
Brett Smith for redOrbit.com - Your Universe Online
The ability to look out for predators or see a distant source of water has allowed humans to get where we are today, but how did our sense of vision evolve throughout time?In a newly published research review, Trevor Lamb, a neuroscientist at the Australian National University, decided to look into several questions surrounding the evolution of the human eye.
"There are profound questions about the eye which are still not easy to answer because it appeared so very long ago," he said. "Why did the eye develop? Why are there many different kinds of eye, including one for insects and crustaceans -- and one for vertebrates like us?"
In his review, which was published in the journal Progress in Retinal and Eye Research, Lamb examined a wide range of studies that traced back 700 million years, to when the first light-sensitive chemicals known as opsins began to appear in simple, single-celled organisms. While primordial organisms already had some signaling pathways, opsins enabled them to sense light for the first time.
"But these animals were tiny, and had no nervous system to process signals from their light sensors," Lamb said.
During the following 200 million years, evolutionary pressures allowed for emerging organisms to develop more sensitive and more reliable vision. Around 500 million years ago, many organisms had developed something that resembled the cone cells found in our eyes.
"The first true eyes, consisting of clumps of light-sensing cells, only start to show up in the Cambrian, about 500 million years ago -- and represent a huge leap in the evolutionary arms race," Lamb said. "Creatures that could see clearly had the jump on those that couldn't."
"For example there is Anomalocaris, a meter-long predator like a giant scorpion - the "Jaws" of its day - which had eyes the size of marbles, with which to navigate the ancient seas and locate its prey," Lamb continued. "This beast, which employed the 'insect eye' model with many facets, had no fewer than 16,000 facets containing vision cells, in each eye."
According to the Australian neurologist, these emerging eyes generated an enormous amount of signaling information, known as optic flow, which was sent across an ancient creature's nervous system.
"This all has to be processed, so we also begin to see the rapid development of a central nervous system able to cope with such immense amounts of data, continually provided by the eyes and other sensory organs from the world around the animal," Lamb said. "For the first time animals begin to 'see' the complex landscape which they inhabit."
While the insect eye was allowing for some creatures to navigate their environment, an early precursor of our own eye was also developing in ancient sea creatures. Lampreys came on the scene around 500 million years ago with a set of "camera-style" eyes that looked very much like our own.
"From this we can say that the vertebrate-style eye has been around at least 500 million years -- and although its light-sensors and signaling systems are very similar to those of insects and other invertebrates, its optical system evolved quite independently from the insect-style eye with its many facets," Lamb said.
From that point, the vertebrate eye became refined and specialized by various organisms, including fish, reptiles and mammals.
"The advent of spatial vision provided immense survival value to the creature that had it -- but the process occurred slowly, over countless steps, with the transition from a simple eye spot to the vertebrate-style camera eye possibly taking as long as 100 million years," he concluded.