Why are our eyes not located on the sides of the head, but looking forward? This is partly due to the need to perceive 3D images, but BBC Future found other reasons as well.
Have you ever noticed that most of the animals in a zoo fall into one of two groups? Some have eyes on the sides of the head (these are chickens, cows, horses, zebras), while others have them set closer and located in front (this group includes monkeys, tigers, owls and wolves). The visitors to the zoo themselves - people - obviously belong to the second group. What is the reason for this difference?
The location of the eyes is always a compromise. When the eyes are in front, each of them sends an image to the brain from its own angle of view, and by superimposing these images on top of each other, a person perceives depth. Animals with eyes on the sides are not able to see the third dimension, but their view is much wider.
Probably, the position of the eyes was formed differently in different animals. For example, some turtles have eyes on the sides, but the brain processes visual information as if their eyes were looking forward - perhaps this is due to the fact that when the turtles pull their head under the shell, their eyes perceive light only from the front. as if they are located in front of the head. But why did our branch of the evolutionary tree - primates - have eyes in front? There are many explanations for this.
In 1922, British ophthalmologist Edward Treacher Collins wrote that early primates needed vision that "would allow them to swing and jump accurately from branch to branch … grab food with your hands and bring it to your mouth." Therefore, the scientist decided, in the process of evolution, they developed the ability to estimate distance.
In the following decades, Collins's hypothesis was repeatedly revised and refined, but its essence remained unchanged for a long time: in the process of evolution, the eyes of our ancestors moved forward to accurately estimate the distance when jumping from tree to tree. The cost of error in determining the distance between trees was indeed considerable. “The penalty for the miscalculation was a fall from a height of several meters to a land teeming with carnivores,” wrote visual therapist Christopher Tyler in 1991.
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The weak point of Collins' hypothesis is that many animals that live in trees - for example, squirrels - have eyes on the sides. Therefore, in 2005, the American biologist and anthropologist Matt Cartmill proposed another hypothesis, based on the features of the vision of predators, which are able to very well estimate distance. According to Cartmill, this allows them to track and catch prey, be it a leopard creeping after a gazelle, a hawk clinging to a hare's claws, or one of the primates grabbing an insect from a branch.
The scientist found this explanation very elegant, since it allowed understanding other evolutionary changes characteristic of primates. For example, early primates rely on sight rather than smell to hunt. Cartmill decided that the deterioration in his sense of smell was a side effect of the eye convergence: there was simply not much room left for the nose and for the nerves connecting it to the brain - all the space was occupied by the eyes.
American neuroscientist John Allman took up Cartmill's hypothesis and refined it based on information about nocturnal predators - after all, not all predatory animals have eyes in front. In cats, primates and owls, they are indeed in the front of the head, and in mongooses, tupai and flycatchers - on the sides. Allman's contribution to the development of this hypothesis consists in the assumption that such vision is necessary for those who hunt at night - for example, cats and owls - because the eyes perceive light better in front than on the sides. The early primates hunted at night and, perhaps, it is precisely because of this addiction to night hunting that all their descendants, including humans, have eyes located in the front.
The American theoretical neuroscientist Mark Changizi had another explanation. In 2008, he published an article in the Journal of Theoretical Biology (USA) on "X-ray vision," suggesting that the eyes in front allowed our forested ancestors to see through dense foliage and tightly intertwined branches.
The loud name "X-ray vision" comes from a curious phenomenon described by Changizi: "If you keep your finger in front of your eyes in an upright position, fixing your gaze on some object located behind the finger, two images of the finger will enter the brain, and both of them will be transparent." Thus, it turns out that a person can "see through" a finger, as with the help of X-rays.
The pile of trees in the forest makes it difficult to see only large animals, such as primates. Smaller squirrels, such as squirrels, do not have this difficulty, since their small head can easily squeeze between branches and leaves. Large animals that do not live in the forest also have enough eyes that are located on the sides.
Thus, the reason that our eyes are in front has not yet been established. Each hypothesis has its own strengths and weaknesses. But no matter why we needed such vision - to jump from branch to branch, catch tasty bugs, or see through foliage - it is obvious that this eye position is associated with life among the trees.