The Human Eye and the Evolution of Vision—”The Fruit, The Tree, and The Serpent” by Lynne Isabell

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Human vision. Un Chien Andalou

The beginning scene of Bunuel’s Un Chien Andalou, in which the woman’s eye is cut with a razor

Although Michael Haneke may disagree, cinema is a visual art first and foremost. As such, the theme of visual perception and the human eye is widely explored in movies, such as Michael Snow’s Wavelength.

The highly complex vision is one of the distinctive features of humans. As anthropology, paleontology, and natural selection have proven, the reasons for the emergence of sophisticated visual system in humans are evolutionary. But what environmental pressures could have caused vision to become the overwhelming human sense? And what are the philosophical implications about the nature of the human eye and the human visual system?

In her book “The Fruit, The Tree, and The Serpent: Why We See So Well,” Lynne Isabell investigates the possible reasons for the highly complex visual system in humans, the unique color spectrum, the enlarged brain, and, interesting enough, the ubiquitous fear of snakes, called ophidiophobia. Isabell argues that the selective environmental pressure of snakes and their threat to humans was the driving factor for the expansion of the human visual system, which in turn was an important factor for the brain enlargement. The anthropologist analyzes the heavy presence of snakes in mythology, literature, and common culture, while pointing out the surprising scientific accuracy of the Old Testament story of the origin of humans:

Eve’s big mistake was that she noticed the Serpent. If she had not detected it, if she had not looked at it, she would not have entered into the conversation that tempted her to pick the fruit off the Tree of Knowledge[…] God punished Eve for noticing the Serpent and eating that fruit.

[…]This particular creation story invokes snakes, visual attention to them, trees, and consumption of fruits to explain not only women’s pain during childbirth, but also our curiosity, our morality, and our fear of snakes. The story of Eve is actually quite similar to what I believe the scientific evidence suggests for the origin of primates, including humans. We know today that pain during childbirth is the result of our extraordinarily bigheaded babies, the price we pay for having brains with a large neocortex. Because areas either directly or indirectly involved in vision occupy approximately half of the neocortex, the behaviors associated with vision and our large brains do indeed cause us to give birth in pain.

[…] I will argue that a […] preadaptation for acute vision and large brains was the enjoyment of a diet of fruits and nectar (Eve’s fruit). When snakes (the Serpent) appeared, a particularly powerful selective pressure that favored expansion of the visual sense, only ancestral mammals having a diet of fruits or nectar from flowers in that arboreal milieu could afford to expand their visual systems, as such expansion typically occurs at a cost to olfaction. Only those mammals that ate odoriferous foods such as fruits and flowers could afford to reduce their sense of smell and still find their food efficiently.

By combining geographic, neurological, and physical factors, “The Fruit, the Tree, and the Serpent” examines:

 Is it just a coincidence that poorer vision and smaller brains are found in those primates that have never coexisted with venomous snakes and that excellent vision and large brains are found in those primates that have always coexisted with venomous snakes? Perhaps, but the coincidence becomes more suspicious when we factor in that platyrrhine primates, the New World monkeys, have had more exposure to venomous snakes than Malagasy prosimians but less exposure than catarrhine primates, and that they also have visual systems that are intermediate between those of prosimians and catarrhines.

Starting with the first primates and analyzing in detail their evolution, Isabell outlines some common primate characteristics:

 Grasping hands and feet, nails on at least the first toe, eyes that are more or less directed to the front of the face, reliance on vision as the predominant sense, and enlarged brains.

Furthermore, Isabell points out the differences between the two suborders of primates—prosimians and simians. Prosimians are non-anthropoid primates such as lemurs and tarsiers. Simians are the anthropoid primates—bonobos, orangutans, gorillas, chimpanzees, the extinct human ancestors, and humans.

Humans and other anthropoid primates have smaller snouts than prosimians, rely less on smell, have more frontally directed orbits for the eyes, greater visual specialization, and larger brains.

Isabell argues that in order to understand why primates are the only order of mammals to specialize in vision as their main sense, the difference between primates and non-primates vision is the key.

After comparing, the author emphasizes the importance of the pulvinar, which is responsible for hand-eye coordination and selective visual attention—useful to detect predators. The pulvinar is small in all mammals but primates, and especially large in anthropoids. According to Andrew Parker’s “Light-Switch Theory” proposed in his book “In The Blink of an Eye: How Vision Sparked the Big Bang of Evolution,” the reason for the rapid life emergence during the Cambrian period was the emergence of eyes, which greatly increased the predator-prey importance of natural selection.

“The Fruit, the Tree, and the Serpent” points out the LGN visual system, and particularly the so-called P pathway, as the other key difference between primates and non-primates and the source of the biggest visual system expansion.

 Differences in the complexity of the LGN reveal that the P pathway has not only expanded more in primates than in other mammals but also has expanded more in anthropoid primates than in prosimians, and more in catarrhines than platyrrhines. The P pathway is largely responsible for our own excellent central vision, fine visual acuity, and our ability to see rich color, all of which help us to perceive objects in our environment

There are many objects in our environments that our brains detect but that we do not consciously see. Our conscious awareness of the objects  around us appears to result from visual processing mainly through the LGN’s P pathway, which emphasizes visual acuity,  color, and object recognition via the ventral stream, when time can be taken to assess objects in the environment. The P pathway is largely responsible for the LGN visual system’s great expansion in primates, and is ultimately responsible, therefore, for providing primates with the ability to be more deliberate in their assessment of objects of interest in the environment.

[…] The LGN visual system might even be viewed as the thinking animal’s vision, with greater complexity in the LGN visual system indicating greater capacity for assessment and deliberation. Perhaps the complexity of the LGN visual system can even be used as a quantitative correlate of “intelligence” between species. Intelligence is a characteristic that is hard to measure but that is evident when we are in its presence. Certainly primates are considered more intelligent than rodents or rabbits, and their intelligence is expressed partly by their curiosity, which involves scrutiny.

Lynne Isabell connects the human vision to intelligence, which seems plausible since a significant portion of the human brain’s encephalization is due to visual system expansion. “The Fruit, the Tree, and the Serpent” emphasizes the importance of color for the human eye. The human color spectrum is significantly broader than the spectrum of other animals—this BBC video, for example, gives a simple comparison between human and cat vision. Although at night the nocturnal cats can see a lot better than humans, humans’ ability to distinguish colors is superior.

The human eye cannot see everything, though, and human vision has its limits, which create visual defects and illusions. The supremacy of the human vision over the other senses, for example, can skew hearing perception, as in the McGurk effect.