Humans are visual creatures, relying heavily on our eyes to navigate, find food and spot danger. That said, we are not exactly ocular pioneers compared to some other organisms. Let's take a look at...
The Earliest EyesEyes are generally absent from the fossil record (they are usually soft tissue after all), but researchers think that the earliest examples were little more than light-sensitive spots on the surface of a single-cell organism. Cyanobacteria possessing these photoreceptors would only be able to tell if it was light or dark - a significant advantage since the organism relied upon photosynthesis for energy.
At some point a mutation occurred placed those photoreceptors in a "dip" on the surface of the cell. This allowed the direction of light to be detected - picture it like a shaft of light entering a cave or shed. Since this version provided more information, it was an advantage and natural selection favored those organisms with the "dip" mutation.
The first eye from the fossil record known to have a lens belongs to the trilobite Olenellus fowleri. This creature lived between 550 and 600 million years before the Cambrian explosion, and the lenses found in the fossils consisted of calcite crystals as opposed to the protein-based crystallins favored by modern animals.
It is thought that lenses are a result of a mutation. Certain specialized proteins called crystallins can be found thorough the body, absorbing heat-shock for other proteins. This helps prevent other proteins from denaturing when overheating, allowing them to remain functioning. Crystallins are also transparent - and scientists believe that a mutation could have caused them to grow over the eye, where they could act as a rudimentary lens. Further mutation refined their function, allowing them to become the advanced lenses we see today... though they are also responsible for the growth of cataracts in later life!
The Worm With Eye-Covered TentaclesThe alien-looking fan worms live in tubes on the sea floor. Like many other marine organisms, they are filter feeders that extend tentacles into the water to catch particles of food drifting in the current. Unlike many of their aquatic companions, their tentacles are festooned with compound eyes that allow them to detect shadows. These eyes seem to have evolved independently from visual systems in other animals - but they serve a similar purpose, allowing the worm to detect a predator and hide before becoming lunch.
Octopi also have better coverage in their visual field. The eyes of vertebrates grow out of the brain and are connected by a nerve that punches right through the retina - this leaves a small but significant blind spot in our vision. Octopus eyes lack this problem - they grow separately from the brain and connect later.
Humans have three types of photoreceptors in our eyes, allowing us to perceive the world in color... but the octopus only has a single kind or receptor that detects blue light. Despite this, octopuses seem to be capable of seeing in full color by using their shaped pupils to split light into red, blue and green. The octopus can then move their eyes to build up a colorized picture of the world.
There is another potential trick up many sleeves of the octopus. Their tentacles are loaded with chromatophores that allow them to change color and match their surroundings. Since the neurons of the octopus extend into their arms, it is possible that the color-sensitive chromatophores could act as auxiliary eyes for the mollusk. Color-based information could be passed to and processed by the neurons of their tentacles, letting them see with their arms!
- The earliest eyes
- The Cambrian eye explosion
- The four-eyed fish
- The worm with eye-covered tentacles
- The 24-eyed jellyfish
- How octopus eyes outclass ours
 |
| The earliest known lenses were found in a trilobite... (Wes Warren) |
The Earliest EyesEyes are generally absent from the fossil record (they are usually soft tissue after all), but researchers think that the earliest examples were little more than light-sensitive spots on the surface of a single-cell organism. Cyanobacteria possessing these photoreceptors would only be able to tell if it was light or dark - a significant advantage since the organism relied upon photosynthesis for energy.
At some point a mutation occurred placed those photoreceptors in a "dip" on the surface of the cell. This allowed the direction of light to be detected - picture it like a shaft of light entering a cave or shed. Since this version provided more information, it was an advantage and natural selection favored those organisms with the "dip" mutation.
The first eye from the fossil record known to have a lens belongs to the trilobite Olenellus fowleri. This creature lived between 550 and 600 million years before the Cambrian explosion, and the lenses found in the fossils consisted of calcite crystals as opposed to the protein-based crystallins favored by modern animals.
The Cambrian Eye Explosion
The Cambrian explosion (505-543 million years ago) coincided with an explosion of eyes. Certain worms began to develop compound eyes (that use multiple "eyes" to create vision) while mollusks begin to show a wide variety of eye structures. Finally the hagfish and lamprey emerged as some of the earliest vertebrates. These primitive jawless fish possessed basic camera-style eyes.It is thought that lenses are a result of a mutation. Certain specialized proteins called crystallins can be found thorough the body, absorbing heat-shock for other proteins. This helps prevent other proteins from denaturing when overheating, allowing them to remain functioning. Crystallins are also transparent - and scientists believe that a mutation could have caused them to grow over the eye, where they could act as a rudimentary lens. Further mutation refined their function, allowing them to become the advanced lenses we see today... though they are also responsible for the growth of cataracts in later life!
The Four-Eyed Fish
A contender for the strangest looking fish, Anableps anableps has huge googly eyes that it pokes above the surface of the water, allowing it to see tasty insects and predatory birds. This would put it at something of a disadvantage against other predatory fish or aquatic insects, so evolution has resulted in a rather unique solution... have a second pupil in each eye, one for above the waterline and one for below! |
| Octopuses may be able to see with their tentacles... (glucosala) |
The Worm With Eye-Covered TentaclesThe alien-looking fan worms live in tubes on the sea floor. Like many other marine organisms, they are filter feeders that extend tentacles into the water to catch particles of food drifting in the current. Unlike many of their aquatic companions, their tentacles are festooned with compound eyes that allow them to detect shadows. These eyes seem to have evolved independently from visual systems in other animals - but they serve a similar purpose, allowing the worm to detect a predator and hide before becoming lunch.
The 24-Eyed Jellyfish
Could you picture a jellyfish with eyes? Box jellyfish have a complicated visual array comprising of 24 eyes. These are mounted on four separate structures, each of which has a pair of simple "pit" eyes, a pair of slit eyes, a small upper lens eye and a larger lower lens eye. The jelly uses these to avoid obstacles and orient itself to light to maximize feeding time!How Octopus Eyes Outclass Ours
Octopuses have camera style eyes like a human, but there are a few key differences. For one thing, octopuses actually move the lens of their eye to focus on a target. This means that they can both rapidly lock on - and can compensate for any flaws they develop in their vision. Humans cannot do this - hence why we need corrective glasses.Octopi also have better coverage in their visual field. The eyes of vertebrates grow out of the brain and are connected by a nerve that punches right through the retina - this leaves a small but significant blind spot in our vision. Octopus eyes lack this problem - they grow separately from the brain and connect later.
Humans have three types of photoreceptors in our eyes, allowing us to perceive the world in color... but the octopus only has a single kind or receptor that detects blue light. Despite this, octopuses seem to be capable of seeing in full color by using their shaped pupils to split light into red, blue and green. The octopus can then move their eyes to build up a colorized picture of the world.
There is another potential trick up many sleeves of the octopus. Their tentacles are loaded with chromatophores that allow them to change color and match their surroundings. Since the neurons of the octopus extend into their arms, it is possible that the color-sensitive chromatophores could act as auxiliary eyes for the mollusk. Color-based information could be passed to and processed by the neurons of their tentacles, letting them see with their arms!
Thanks for reading - for more, try...