Eyes provide us with most information about the surrounding world. Yet, the eye, especially that of a human, enables not only to receive stimuli but also to transfer this information outside. Eye tracking, which is the technology of tracking the glance has been used for multiple years in medicine, entertainment and numerous areas of research just because our eye may talk about us more than we would like to admit to ourselves. And what do we know about it? The human eye has extremely white sclera, which very clearly contrasts with a darker iris. This is a very crucial characteristic of the human kind. Not long ago it was considered a feature which distinguished humans from other primates. 25 years ago, the magazine Nature published a breaking through dissertation by Japanese scientists (Kobayashi and Koshima) which postulated that our eyes were special because they are white and wide. Those scientists, however, ha dan access to much fewer and less precise pictures that those we have now.
- New research does not confirm the previous thesis – says Dr. habil. Sławomir Wacewicz, NCU professor from Center of Language Evolution Studies (CLES) in the Faculty of Humanities of Nicolaus Copernicus University. – Now we know that this is a much more complex issue, and that the versatility is much bigger than that postulated by the Japanese researchers.
In fact, many species of anthropoids have almost an entirely dark sclera (e.g., Bornean orangutan), others a surprisingly white one (e.g., Sumatran orangutan), and among the bonobo monkeys there are huge differences. There are individuals with dark and fair spots, or with one eye much in color from the other. Yet, it goes without saying that among humans this characteristic is very strong: due to the shape of the eye and a big surface of the exposed sclera its white is very clear, in both eyes and in all people. Obviously, there exists small variability between populations – some people will have a darker, and more yellowish sclera. In general, however, these are not big differences.
Aggression in the eyes
Experiments show that if in photographs the color of the sclera is delicately dyed red or yellow, the perception of the person in the photo changes. The person is considered to be older, less attractive, less healthy. Such changes in color really happen and they are caused by micro injuries or the accumulation of fat particles in the eye.
What is it like in animals? The scientist would like to find out if the color of this part of the eye has anything to do with the level aggression and cooperation. – A group of scientists from Switzerland corelated the brightness of the sclera among apes with three characteristics: prosocial behavior (e.g., food sharing), social tolerance, and aggression – says prof. Wacewicz. – The brighter the sclera, the more social behavior and tolerance towards others a species of monkeys displays. What is more, the more seldom do fatal attacks occur.
There exists a hypothesis that especially animals which got self domesticated or were domesticated by a human display a whole range of common traits. These are, most of all, a more globular skull which looks like in an new born baby, decreased reactive aggression and lesser pigmentation. These are more common among domestic animals with light or white fur.
Whereas this is connected with skin pigmenting, we have doubts if it is also connected with the pigmenting of the eye – says prof. Wacewicz. – This, however is a different type of a tissue. The research on the appearance of the external eye is a new, fast developing area, and there is still plenty for us to check. What interests us is comparison between humans and other primates, an attempt to set the selective pressure which would make it possible for the human eye to look untypical, different from other primates.
The scientists take into consideration at least five evolutionary pressures, which could have affected the discoloring of the sclera in humans: individuals with brighter shade of this part of the eye could have been selected as healthier, younger, more attractive, less aggressive, and more trustworthy. Our research has confirmed all these hypotheses, therefore there is still a long way to go from the results to conclusions. The results of the experiment were published in Scientific Reports, in article "The adaptive significance of human scleral brightness: an experimental study".
Apart from the color, a characteristic feature in people is also the width of the eye. This may be connected, for example, with the fact that we live on the ground, do not climb trees. There exists correlation: animals living on trees have more lengthened eyes vertically, whereas for people it is the horizontal surface which matters more, especially for those living in flat landscapes such as savannas. This explains a lot though it does not clarify why our eye is constructed in the way it is – explains professor Wacewicz.
The scientists from CLES NCU work in the Zoo Botanical Garden in Toruń with pygmy marmosets, which are extremely cooperative and have a fairly bright, but very small and hardly visible sclera. The researchers want to check how the pygmy marmosets will react to faces with brighter and darker scleras. Will they be afraid of those latter more? – We, which means our research group, are untypical linguists, interested in language evolution in the sense of its origin – explains professor Wacewicz. – In general, we have an evolutionary perspective of looking on various issues, also language and, what is more, communicative phenomena. This allows to set interesting hypotheses which connect multiple areas of knowledge of various disciplines.
It is said that monkeys do not speak as their larynx is of a different anatomic structure or, colloquially speaking, they are "too stupid" to speak. – I think that this is a completely wrong way of thinking – point the linguist from Toruń. – In my opinion, monkeys do not use a language as they do have social fundaments, they do not have trust to one another in a group. We may not notice it on a daily basis, but people are an extremely pro-social kind. We, for example, share food with those that we are related to, which in the animal world is a rarity. If animals do not share food with one another, why should they share any information? This simply makes no sense to them.
Therefore, it is more and more accepted to think that the emergence of a language, or an advanced system of communication (for example through gestures) requires a high level of trust in a group. This is a complicated argument, but it can be explained in bits and pieces though the analogy to a poker game. Poker players at the game table say nothing, even though there are no limitations in their larynxes. They simply do not trust one another, and thus they are not interested in what the others have to say. It is utterly known that all players play the game in their own interest, not in the interest of another person, so any conscious transmission of data is not credible, and they are nothing, not even worth considering.
In the shortest, we believe that if any exchange of information on a large scale made any sense, there would need to exist and be maintained a certain level of trust. – explains professor Wacewicz. – We suppose, therefore, that our predecessors first developed prosocial behavior, cooperation and trust, then the language. The discoloring of the white of the eye could have been linked with that very first stage.
If people lack trust to one another because of a conflict of interests, the way they glance gives some advantage. The way we look well displays our intentions as it is generally difficult to take any action in the direction you are not looking into. This is well familiar to sports people who predict the actions of the opponents on the basis of the direction of looking. When looking for a partner in cooperation, and at the same time putting trust in him/her, it is better to choose people who are more predictable and does not hide their sight.
The earlier mentioned Kobayashi and Kohshima put forward hypothesis that the function of dark sclera in monkeys is simply masking their look. If an animal's sclera, iris and the skin around are dark, no one will know where it is looking until it turns its head. – This is very useful when we complete with one another like in the mentioned above game of poker. – claims professor Wacewicz. – Please note that poker players very often sit at the table in dark glasses so that their eyes betray the direction of their look. What is interesting, in the evaluation of a person's honesty and credibility all over the world it is the eyes that focus attention ("you've got such kind eyes", or "Aren't you lying? Show your eyes."), which is confirmed in intercultural research.
The hypothesis about dark sclera which helps to hide the look is logical, yet the results of the research are not clear. Strongly competing chimpanzees, for example, do have dark sclera, but at the same time they have light-yellow (which means bright) iris, therefore there is a contrast in them like in people, bit in the reverse way. This was described for the first time by Juan Olvido Perea-Garcia, an old partner of CLES in Toruń, a coauthor of "Scientific reports", now working at Leiden University. At the same time, Fumihiro Kano, working at the university in Konstanz conducted very interesting research on people and chimpanzees trying to establish whether the structure of the eye really facilitates the tracing of the look. The results are such that our uniformly white sclera allows to determine the direction of looking more effectively, especially in poor conditions such as dim light or from a long distance.
Professor Wacewicz aims to explain what has made people become a cooperative and prosocial kind, and on this "platform of trust", as he calls it, they first created a protolanguage, and then a language. Does it have anything to do with the structure and color of the eye? If yes, what is it?
Wacewicz, S., et al. (2022) The adaptive significance of human scleral brightness: an experimental study. Scientific Reports. doi.org/10.1038/s41598-022-24403-2.