Efectivamente, con los datos genéticos conocidos en la actualidad del Neandertal no es de extrañar que relativamente pronto (dependiendo del interés que tenga lo buscado y el dinero disponible) tengamos datos concretos. Sin embargo, esto nada nos indica sobre la existencia de este carácter en las especies humanas más antiguas.

Sin duda, el hecho de la existencia de una esclerótica blanca favorecería la comunicación social entre los miembros de las sociedades de homínidos, pero no creo que su existencia fuera imprescindible para el desarrollo del lenguaje. En mi opinión el lenguaje es un proceso multifuncional de gran complejidad, que en su evolución (con independencia de cuando se inició) tuvo que desarrollar una serie de conceptos abstractos de muy variada índole (temporalidad, territorialidad, individualidad social y personal, inhibición conceptual, mentira, desarrollo de la teoría de la mente, etc.). Todo ello partiendo de las primeras manifestaciones sonoras humanas y de los primates, como serían las expresiones emocionales, señalizadoras, etc. Cuando se habla de lenguaje humano en qué intermedio evolutivo nos encontramos dentro de este complejo desarrollo. El lenguaje es la pieza clave del desarrollo cognitivo humano, y su desarrollo puede analizarse por medio de las características conductuales que podemos observar en el registro arqueológico del género Homo. Sin embargo, en este heterogéneo (temporal y espacial) desarrollo cognitivo, conductual y lingüístico se aprecia un importante desarrollo (en los conceptos abstractos antes mencionados) al final del Paleolítico medio y comienzo del superior con las especies de Neandertales y Humanos anatómicamente modernos. Si estos grupos humanos tuvieron una esclerótica blanca, sin duda favorecería su relación social, pero no creo que fuera totalmente determinante.

Thanks for raising this! It is a crucial question. I always show a slide in my undergraduate teaching on the Stone Age with an Australopithecus recnstruction portrait, once with white sclera, once with animal eyes, to show that our gut feeling regarding "ape" or "human" impression solely depends on this feature.

Angel,

Thank you very much for your commentaries.

1) “Sin duda, el hecho de la existencia de una esclerótica blanca favorecería la comunicación social entre los miembros de las sociedades de homínidos, pero no creo que su existencia fuera imprescindible para el desarrollo del lenguaje.” (English: “Undoubtedly the white sclera would favour the cooperative way of life. However I don’t think that this type of eye was necessary for the evolution of language.”) It is true that the relation between the white sclera and a cooperative lifestyle is less controversial than the other two relations which I focus on. The latter (the universal white sclera would have been selected little after the appearance of pointing gestures; and pointing gestures preceded human-like language) are indeed very likely, but they are (your criticism is right) far from being uncontroversial. However, I choose to pay attention to them.

2) A second very interesting commentary of yours. “Una posible comprobación por parte de la Paleogenética de que los Neanderthales poseían la white sclera no nos indicaría nada sobre la existencia de este carácter en las especies humanas más antiguas.” (“A potential checking on the part of Paleogenetics stating that Neanderthals possessed white sclera would not indicate anything regarding this feature in more ancient species.") Of course, I agree. But let's go step by step. Firstly, let's check whether Neanderthals did possess it. If the answer is “yes”, science might be able to explore other ways towards a more remote past. (The future of science has always been difficult to predict). But let's move on to a much more important issue which is implicit in your commentary. Is that checking necessary? Couldn't it be taken for granted that the white of eye existed at that point in evolution? We certainly know that Neanderthals possessed a great brain, and also showed (mainly in the Châtelperronian) a previously non-existent level of innovation. In addition, if the Last Common Ancestor of Neanderthals and our own species would have lacked great capacities, how could we be here? However, we may think that Neanderthals' brains and innovations would have followed a path different from ours, a path which turned out to be less efficient. Indeed, it is intrinsically very difficult to imagine how it could be a cognition relatively advanced which does not follow our patterns. But, if it were necessary, it would have to be imagined. However, dealing with this now, in these lines, does not make sense. First, Paleogenetics should give us the answer.

3) “No es de extrañar que relativamente pronto (dependiendo del interés que tenga lo buscado) tengamos datos concretos” (“It is not surprising that relatively early (depending on the interest of what is looked for) Paleogenetics offer specific data”). Specifically with this Question, I am trying to underline the interest of the white of eye, in order to ask Paleogenetics for help from the field of language evolution.

Eva,

Thank you for pointing out a very important aspect which I had left aside.

The possession of the white of eye is vital in order to resemble human (and even for animals to resemble human: let us remember Walt Disney). If we came across, on the bus, someone lacking white sclera, we would very likely be surprised which would border discomfort and almost fear. In short, as you have said, “our gut feeling” perceives an intrinsic connection between the white of eye and our human condition. Thank you, I repeat.

“We alone among mammals have eyes with enlarged white sclera”… I am not a primatologist but there is research demonstrating that many gorillas exhibit this trait too, even though not to the extent that humans do.

https://www.ncbi.nlm.nih.gov/pubmed/25846121

I am not aware of the particular genetic variants underlying the “white sclera” phenotype, if we assume that the gorilla phenotype is not a homoplasy (which it could be given that e.g. in different non-African pops some of the alleles causing a similar light pigmentation phenotype are at different loci and this is on a much much shorter timescale) this would suggest that this trait has been “around” on the hominid lineage as a neutral variant for a long time.

As for Neanderthals, my gut feeling would be that they possessed this trait, mainly because the fixed amino-acid differences between humans and Neanderthals are so marginal and most of the phenotypic variation seems be explicable by regulatory polymorphisms and resulting differences in gene expression. Of course the “white sclera” phenotype might also be just due to the developmental suppression of melanocytes which could in fact be regulatory.

Alexander

Mayhew and Gomez, 2015 found "a partial or total, binocular or asymmetric, sclera depigmentation in 72% of gorilla eyes". That is, there is sufficient variation in gorillas. But the white sclera has not become universal in them. Thus, this absence of universalisation would have been caused by the absence of selection, that is, by the absence of adaptive advantages. In your words, “this trait would have been ‘around’ on the hominid lineage as a neutral variant for a long time”. But then (as suggested in my Question) the human ‘universal all-white sclera’ would have been selected by the adaptive advantages that this type of eye provides.

Please, I ask to genetists and Paleogenetists the effort of checking whether Neanderthals did possess the white of eye. As the old rule says, if something can be checked, it must be checked. Since you are a genetist, I particularly appreciate your interest. Certainly I do not know how to evaluate your concrete suggestions (for example, that the ‘white sclera’ phenotype might also be just due to the developmental suppression of melanocytes which could in fact be regulatory”). However those suggestions feed my hope. The answer will come in relatively few years.

.

Teresa, I agree with you regarding the importance of this trait. The first and much harder step has to be to properly understand its genetic basis in modern humans . Once this is done checking for the respective variants in the available Neanderthal genomes (where the amount of data has grown tremendously in the last few years) should be (almost) "simple". I understand that there are a range of hereditary (often quite severe) diseases, where a dark sclera is one symptom among many, these could provide a starting point.

Luca

Your answer has given me a lot of confidence and joy. Thank you.

Still a question of a vital importance for a university teacher,by the way

Some gorillas certainly possess this trait, as Alexander Moseburg writes, but as Neanderthals were human surely they must have done?

Shipman, 2015 also says: “One of my speculations is that Neanderthals didn't have whites to their eyes”. This represents such an enhancement for this Question, that I must add this new commentary. But the two proposals –hers and mine– are different. Shipman (in her very attractive hypothesis about the role that domesticated wolves would have played in the process that led to the dominance of Homo Sapiens) says: “Modern humans had adaptations that fostered better communication with wolf-dogs and possibly (we don’t know) Neanderthals did not. For example, humans are the only primates with white of eye, which makes communicating the direction of gaze —that is, where you are looking— very obvious.”

But my interest in the origin of the universal white of eye has a different motivation. My proposal (Bejarano, 2011, chapter 3, and with more detail in Bejarano, 2018, or Bejarano, in preparation, 2.3.2) begins underlining that the white of eye (in our horizontally elongated eyes) makes the travelling of iris conspicuous, and in this way facilitates the human reception of pointing gestures. (We must keep in mind that the crucial element for children to first understand those gestures is not the hand's movement, but the alternance of gaze between object and recipient.) More concretely, the travelling of iris facilitates the recipient’s understanding of the connection between instant R (producer looks at the Recipient) and instant O (producer looks at the Object). Please note that, if that connection is not grasped, instant R is, of course, received as a communicative gaze, but instant O is not: At instant O, the recipient will feel like a mere spectator, not an addressee. Thus, in that case, which is unlikely in our species, the recipient will not be able to understand that the producer is paying attention to the object to point at it for him, for the recipient. In other words, that movement of the iris seems –or, perhaps more exactly, it seems for the human recipient– like if it was injecting in the communicative gaze the result of the other gaze (the gaze towards the object). This truly efficient type of reception of pointing gestures, which meant a strong adaptive advantage, would have (as I have argued in above cited works) required a new resource, the intrinsically ‘foreign’ mental content, which is very different from the expectations (the evolutionarily previous resource to attribute states to someone else). Those expectations, although they are attributed to another individual, continue to always be the subject's own expectations and could be described as ‘what I (the subject) would see if I were in that individual's location, posture and orientation’.

Since my proposal so strongly emphasizes the importance of that efficient type of reception, I am very interested (as the reader already knows) in the origin of the universal white of eye being discovered. I am persuaded that Genetics and Paleogenetics will eventually discover it. But will I be alive by then? This is why I have added now the allusion to my proposal and the corresponding self-citations, in short, all what I planned to add in the future, or, more concretely, when that origin was finally located. But let's be optimistic: Maybe soon we will all be talking about that answer.

The white color of the sclera facilitates the non-verbal social communication of human populations. We know that the great primates (only known evolutionary ancestors) do not have this white coloration of the sclera. We also know that their social development is not as developed as in human populations. This makes us assume that the change of color must have occurred in the evolution of the Homo genre. This change favored social communication and took place within the cognitive-cultural niche that frames human evolution. The problem lies in knowing when it occurred, that is, in which species the genetic mutation took place. If it was before the separation of Homo sapiens from the Neanderthal, the two human groups would have the white color of the sclera. If it was after and within the anatomically modern Humans lineage, the Neanderthals would lack that anatomical feature. Is there any cultural or behavioral indication that indicates any of these two possibilities? There has been much discussion between the equality or behavioral inequality of these two human groups. My opinion is that both populations had very similar behavior, but not the same. Inequality lies not in the production of objects or concrete situations (ornaments, tools, distribution of space, hunting, etc.) but in their development (universality of their production and speed of expansion, social and commercial communications in their own degree). of expansion, and in general a more active, dynamic and social social life). These differences would be of degree and due to cognitive processes that we still do not know well (different emotional cognition?). In this context, the change of color of the sclera could indicate or favor these differences, or at least be an important coadjuvant. Apart from these considerations, only Paleogenetics can clarify when this change occurred. Due to the interesting thing, I do not think it takes a long time to be produced, because we are in the height of genetic studies in prehistory.

I am pulling this up again as there is a new publication: https://www.nature.com/articles/s41598-021-92348-z

Thank you very much, Eva. Yes, there are recent studies that argue against the human uniqueness of the iris / sclera contrast. I can add García-Perea et al., 2019 (but their method of quantifying that contrast has been criticised as “very limitedly applicable to naturalistic perception”: see Mearing & Koops, 2021). Kano et al., 2021 and Kano et al., 2022, favourable to the very enhanced visibility of gaze-direction the human eye, have replied in various ways.

But I am going to underline only a point that stays out of the attacks. Human eyes (with their horizontal elongation and totally white sclera) make the location of the iris conspicuous both in averted gaze (as in some gorillas, e.g.) and in direct gaze. Thus, human eyes can make the successive locations of the iris very conspicuous. Therefore, the visibility of the alternation between gazes is really enhanced. And this alternation is developmentallly crucial. (Paulus & Fikkert 2013 show that the necessary and sufficient element for children to first understand pointing gestures is not the hand movement, but the alternation between ‘the gaze towards the object’ and ‘the gaze towards the recipient’).

This is the point that, according to my proposal on the adequate reception of pointing gestures, is the important one. I hope to announce soon more works.

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See "The origin of Human Theory-of-Mind", Preprint The Origin of Human Theory-of-Mind

It is a long article that in sections 7 and 8 focuses on the human white of eye.

Years after having written this question, I would like to emphasize the following points:

1) What matters is not the whiteness of the sclera per se, but rather the contrast between the sclera and the iris.

2) However, this contrast is only useful when the sclera is markedly enlarged, that is, elongated along the horizontal axis. Admittedly, the width-to-height ratio of the eye varies across primate species, but it is possible that humans exhibit the highest ratio. This question might be easier to address than the one concerning the genetic basis of the trait.

3) As for the usefulness of the combination of these two traits, I have suggested that it may lie in making the horizontal traveling of the iris conspicuous. The perception of this traveling does not serve to facilitate gaze-following, but rather enables the integrated reception of pointing gestures—i.e., a reception that unifies the two gazes, or, in other words, extends the communicative function of 'gaze directed at the addressee' to 'gaze directed at the object'. Such reception would also take into account the emotional expressions of the producer. (Emotionally neutral pointing gestures are relatively rare, as their most probable use is in response to a question or a gesture of uncertainty.)

4) In the first version of my request to researchers in Paleogenetics (and Genetics), I referred only to Neanderthals, but should genomes of other fossil Homo species become available in the future, I would extend my request accordingly.

In short, my curiosity remains, as does my desire to convey it in the context of this question. Thank you very much.