How did 3D vision develop?
The law of Newton-Müller-Gudden
In 1682, Isaac Newton proposed that the human and primate optic nerve, in terms of how it stretches from the eye to the brain, has a very particular architecture compared to that of other animals. Almost half of the nerve fibres from the human retina display hemi-decussation, that is, they project to the brain hemisphere on the same side as the eye from which they originate. In most animals, all - or almost all - nerve fibres cross to the opposite side of the brain. Newton’s theories of binocular vision provided the basis for succeeding examinations of the visual system by early anatomists and physiologists. Eventually, it led to a widely accepted concept - that the degree of optic fibre decussation in the optic chiasm is inversely related to frontal orientation of the optical axes of the eyes, which is the law of Newton-Müller-Gudden. A controversial aspect of the Newton-Müller-Gudden law is the considerable interspecific variation in ipsilateral (same sided) visual projections, particularly in non-mammalian species. This variation is not related to an overlap of visual fields, mode of life, or taxonomic position. In other words, it's an evolutionary mystery.
The general assumption among researchers has been that the arrangement of nerve fibres in the optic chiasm in primates and humans primarily is intended to create accurate depth perception, also known as stereopsis, i.e. the eyes perceive an object from slightly different angles and the difference in angle helps the brain to estimate distance.
The new eye-forelimb hypothesis challenges the idea of stereopsis as well as the Newton-Müller-Gudden law. It says that stereopsis might be no more than spinoff in a more essential evolutionary process. The eye-forelimb hypothesis suggests that the architecture of the retina, as well as the optic chiasm, is shaped to help us and other animals to steer the forelimbs (hands, claws, wings or fins).
With the primate variant of the optic chiasm, nerve cells that control right hand movement, nerve cells that receive sensory impressions from the hand, and nerve cells that receive visual information about the hand, will end up in the same (left) brain hemisphere. The opposite applies to the left hand. Felines (cats) and tree-climbing marsupials have similar arrangements with 30 to 45 % uncrossed nerve pathways and forward pointing eyes. Again, that aids their eyes to be in service of the paw i.e. visual information of the forelimb will reach the appropriate hemisphere.
There is evidence that there have been small, gradual changes to the direction of the nerve pathways in the optic chiasm. The direction of these pathways may change in either direction. Mice have lateral eyes and few crossings in the optic chiasm. Since mice’ paws mainly work in the lateral visual field, the neural architecture of mice aids the mice eye to be in service of the paw. The list of suitable examples from the animal kingdom is almost endless. The eye-forelimb hypothesis applies to essentially all vertebrates while the older theory (on depth perception) generally only applies to mammals, and even then there are important exceptions. For example, predatory dolphins have only uncrossed pathways.
It is commonly claimed that predatory animals generally have frontally-placed eyes to enable them to estimate the distance to their prey, while animals preyed-upon have laterally-positioned eyes, which allow them to scan their surroundings and detect the enemy in time. There are however flaws to this logic; most predatory animals may also become prey to other predators, and many predatory animals, for example the crocodile, have laterally situated eyes. The crocodile only has crossed nerve pathways, and under the new eye-forelimb hypothesis, this optic chiasm architecture would have evolved to provide short nerve connections and optimal control of the crocodile's front foot.
The eye-forelimb hypothesis may solve another scientific problem in the evolution of visual pathways. Snakes, cyclostomes and other animals that lack extremities have many uncrossed pathways in the optic chiasm. This can be explained by the fact that they have no hands, paws, fins or wings to coordinate for the eye. Moreover left and right body parts of such animals cannot move independently of each other; when a snake curls clockwise, the left eye will only see the left body-part and vice versa in anti-clock-wise position the left eye will see the right body-part. Hence it seems functional for snake-like animals to have some uncrossed pathways in the optic chiasm, improving visual steering of the right and left body-part. As mentioned, the direction of the nerve pathways in the optic chiasm may change in either direction. Cyclostome descendants that eventually ceased to curl and instead developed forelimbs would be favoured by achieving completely crossed pathways as long as forelimbs were primarily occupied in lateral direction. In contrast, reptiles such as snakes that lost their limbs, would gain by retaining a bunch of uncrossed fibres over their evolution. Indeed, that is what happened, providing further support for the eye-forelimb hypothesis.
The traditional theory on depth perception is problematic in more senses than one. For instance, birds, most of which have laterally situated eyes, have a good ability to estimate distance and they usually manage to fly through a dense wood without crashing. But wait a minute! Owls have frontal eyes and a primate-like visual system due to double crossing of neural pathways! And since owls do not use wings for manipulation of objects, their elaborate neural substrate for binocular vision appears to be at odds with the EF hypothesis. But the decussation and binocular vision of owls may in theory improve eye-lower-limb coordination. Raptors normally take prey with their feet, approaching the target with feet brought up into the visual field just prior to capture. Moreover, several owl species have been observed foraging on foot.
So was Newton, and others, totally wrong? The eye-forelimb hypothesis does not exclude a significant role of stereopsis, but suggests that primates evolved superb depth perception to be in service of the hand. The eye-forelimb hypothesis may provide us with a better understanding of how humans' excellent ability to estimate distance has developed. Stereopsis is an amazing aesthetical experience. It helps us to construct virtual realities, which might be more and more important in future generations. How and why stereopsis evolved in primates is another story. Most likely, the particular architecture of the primate visual system evolved to establish rapid neural pathways between neurons involved in hand coordination, assisting the hand in gripping the accurate branch and other vital objects.