1 Jean-Jacques Hublin, a paleoanthropologist specializing in human evolution, and Alain Prochiantz, a neurobiologist specializing in the morphogenesis of the brain, are two renowned researchers who have pursued surprisingly convergent paths, despite their distinct disciplinary backgrounds. For example, at the end of the 1980s, each of them devoted an introspective book to one of the founding fathers of their fields: for Hublin, Jacques Boucher de Perthes; for Prochiantz, Claude Bernard. [1] These works complemented the decisive advances for which they were themselves becoming known. In the late 1980s, Prochiantz put forward the hypothesis of messenger proteins playing a key role in morphogenesis by transferring information from one cell to another. This hypothesis of intercellular communication, which was considered impossible up to that point, was in fact confirmed by the scientific community after many years of collective research. As for Hublin, he and his Moroccan colleague Abdelouahed Ben-Ncer discovered a 300,000-year-old fossil of Homo sapiens during lengthy excavations at the Jebel Irhoud site in Morocco. This was 100,000 years earlier than the previously accepted date for the emergence of sapiens, shattering the then-prevailing hypothesis that the evolution of sapiens was confined to East Africa.

2 Their two paths have recently converged, not just because Prochiantz was the chair of morphogenetic processes at the Collège de France from 2007 to 2019 while Hublin has been the chair of paleoanthropology there since 2014, [2] but also and especially because they both devoted a series of lectures at the Collège during the second half of the 2010s to the question of the specificity, the singularity and the uniqueness of Homo sapiens. Prochiantz’s course was based on a study comparing the development and the evolution of the brain of sapiens to the brains of the other great apes (chimpanzees and bonobos in particular). Hublin’s was based on the study comparing the biological and cultural evolution of sapiens, that “orphan species,” to the evolutions of the other members of the hominin line, starting with Neanderthals and Denisovans, the two closest species to ourselves. [3] According to the two researchers, although the evolution of sapiens shows that it clearly belongs to the group of primates and hominins – and how could that even be questioned? – it is not, however, a primate or a human “like the others,” as the discoveries in neurobiology and paleoanthropology attest. We therefore wished to interview them together, for the first time, on the specificity of sapiens. [4]

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IGOR KRTOLICA: The first reason behind this interview was a circumstantial one, a kind of lucky coincidence: you both presented a series of lectures at the Collège de France at more or less the same time on the singularity – or the uniqueness – of Homo sapiens within evolution. The first question I want to ask you is: how did you both come to study this question at that same moment, within your respective disciplines?

JEAN-JACQUES HUBLIN: Personally, two aspects guided me: a purely scientific one, I would say – if science can even be pure – and a more philosophical and ideological one. On the scientific level, for a long time human evolution was seen as a kind of triumphal march, a succession of phases – in the late XIX^th century, people spoke of eras – where you drew a parallel between biological progress on the one hand and technical, social, and cognitive progress on the other. This image of human evolution, which is still visible in many museums and books for the general public, was totally dismantled in the late XX^th century, because we realized that human evolution did not unfold in that way at all. It’s a fairly complicated bush, with a lot of missing branches by the way. As for the part we know best, i.e. roughly the last million years, we find a profusion of forms. At this point we’ve discovered at least five or six. Some of these forms – which have the status of distinct species, as far as I’m concerned – existed simultaneously on the planet (generally in different regions, but sometimes on the same continent). Now around 40,000 to 50,000 years ago, something happened that meant that only one of them survived. That species – ours – experienced such a geographical expansion that it spread wherever there were others, and replaced them – occasionally by absorbing them a little, but it was a replacement for the most part – then colonized all of the dry land and adapted to all kinds of environments, in places where no hominins had lived before. It’s probably the most important event in all of human evolution and a subject that obviously interests every paleoanthropologist.
On a more philosophical level, on the level of the history of ideas, what is certain is that human societies seem in general to be seen as totally distinct from the rest of the living world. For example, in complex monotheist societies, humans describe themselves as the fruit of a separate creation. What I find interesting is that the scientific, evolutionist discourse that already undermined the distinction between the human and the non-human is still imbued with a quasi-biblical vision. In fact, people tried and still do try to find a particular point within that complex bush, which is not however a linear evolution. All sorts of criteria that would make it possible to say, “from this point on, they were humans, but before, they weren’t,” have been put forward. It’s something that has seeped into the unconscious of paleoanthropologists. The debate that has surfaced in the last few decades on the status of Neanderthals is an illustration of this. They were first placed on the other side of the divide, then brought closer to us, to a point where they jumped over it. So the ape-like image of these humans has been replaced by a radical will to identify them completely with contemporary humans. I think of the widely quoted comment by Paul Rivet, the former director of the Musée de l’Homme: “There is only one humanity, which possesses a unity in both space and time.” This is obviously the case in space today. But in time, I find that it’s an extremely debatable proposition, which revives the idea of a total separation of the human from the non-human or the pre-human. We find many examples of this tendency. And so there have been many discussions around the notion of species, a notion that is currently undergoing a crisis. When you go back in time, there is no difficulty in using a Linnaean binomial nomenclature with Latin names for the genus and the species: Homo erectus, Homo habilis, etc. But the closer we get to the present, the more denominations like “Denisovans,” “Neanderthals,” and “modern humans” readily take the place of the classical Linnaean denominations for species. In my opinion, this is a direct effect of the tendency I’m describing: that is, once one is recognized as “human,” any idea of a difference with contemporary humans becomes problematic, and therefore troublesome.

ALAIN PROCHIANTZ: My approach is different, although there are some similarities, of course. The first laboratory I directed when I was at the École Normale Supérieure was called “Development and Evolution of the Nervous System.” I’d always been interested in the question of the connection between development and evolution, given that substantial alterations of the nervous system are much easier to explain if mutations take place early on in the course of development than if they take place at a later stage, when the animal is nearly “finished.” The more you alter a gene early on in the history of an individual, the more the consequences are considerable if the individual survives that mutation. My lab is currently working on what are called the “critical periods,” the post-natal learning periods, which therefore occur late in the process, but which are particularly long in the case of sapiens. These periods of transitional plasticity enabled by “cerebral neoteny” are an essential part of humanization – Jean-Jacques will undoubtedly have more to say about this. I initially became interested in this through the most classic model, the model of binocular vision and amblyopia. These critical periods of adaptive plasticity concern all sorts of other cerebral functions, such as the cognitive functions, which are highly developed in sapiens. In our most recent work, we’ve noticed that the same mechanisms that regulate plasticity and learning in the visual system are also at work in the cognitive regions, in particular in the frontal cortex. For example, we can control the anxiety level of a mouse, increasing it or lowering it, by acting on the mechanisms that are identical to those involved in the regulation of the visual system’s plasticity. This convergence can be explained by the conservation of the cortical structure throughout its entire expanse: on the anatomical level and on a local scale, a prefrontal cortex is practically identical to a visual cortex. Obviously, the inputs and outputs are not the same, but the cortical systems that make it possible to understand or regulate the sensory or “psychological” afferences are conserved. When you deprive an animal of its binocular vision or separate it from its mother during a critical period in its development, the pathological consequences differ, of course, but their mechanisms are identical. This means – and this is very exciting – that amblyopia in mice can be a working model for psychiatric illnesses in humans, thereby paving the way to research in new therapeutic strategies.
Another important point concerns the respective sizes of the different areas of the cortex. In the course of evolution, not only did the hominin cortex grow larger, but new regions were also created, and the borders between regions shifted, changing their respective sizes. These changes, whose physiological consequences are significant, are the result of limited mutations whose effects manifest themselves at a very early stage of embryonic development. This is the whole focus of my class on the well-known genetic difference of 1.23% between sapiens and the great apes (chimpanzees and bonobos), which has been the subject of a lot of nonsense. The fact that there is a 20% genetic difference, on the basis of localized mutations, between humans and mice, and a 1.23% difference between humans and chimpanzees merely proves that sapiens is closer to the chimpanzee than to the mouse. This is no surprise; just see for yourself! The quantitative aspect of the number of mutations means something, but it doesn’t mean everything. Like the bomb made by Boris Vian’s uncle, what’s important is “the place where it falls.” [5] You can create a load of mutations that have no effect, and then have a localized one at a site that is so important that its effects become dramatic. Saying “1.23%” is mere accounting (and besides, it’s not true, because when you include the gene duplications and deletions, the figure goes up to 6%). If you take a chimpanzee and a sapiens, they both have a dry weight of forty kilos, but the human brain is four times larger than that of a chimpanzee. And then, if you just look at the forebrain, since the borders have shifted, it’s even more. So that talk of 1.23% is meaningless and needs to be demythologized. It’s like saying we only use 10% of our brains, or that 98% of our genome is made up of “junk” DNA. Ultimately, these clichés produce serious effects of non-knowledge, or anti-knowledge. I think the role of a professor at the collège de france is to try and correct that. I pretty much taught these last two courses with that in mind.

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I. KRTOLICA: Apart from the circumstantial reason I mentioned, there was another, more theoretical reason for this interview. In your respective projects, the idea of the singularity or uniqueness of the human race among all living beings is not a starting point, as it may be in the social sciences or philosophy when we presuppose a form of human exceptionality. Instead, it’s a point of arrival, considering not only the fact that your fields are part of the natural sciences, but also and perhaps especially the central role that the theory of evolution plays in your fields in the broad sense, given that sapiens is the product or the result of a completely natural evolution. So in the reconstruction that each of you propose of the evolution that led to Homo sapiens, you make a point of employing a whole vocabulary of discontinuity, by speaking of “continuity” vs.“disruption,” of “turning points,” of “jumps,” of “sudden and irreversible phase changes,” of “decisive leaps,” etc. How, then, do you understand the appearance, against the backdrop of evolutionary continuity, of a form of discontinuity, or a series of discontinuities – one(s) that mark(s) the singularity of sapiens compared with other animal species, both human and non-human? In your opinion, do these discontinuities that mark the emergence of sapiens have another status than the other discontinuities (mutations, speciations) that punctuate the course of evolution? Or instead of “discontinuity,” do you prefer speaking of “distance” or of genetic “difference,” as you sometimes do, although – as you have just pointed out – we shouldn’t confuse quantitative differences with qualitative ones and we don’t have “a genetic yardstick” at our disposal “to tell us when we are dealing with one or several species”? [6]

J.-J. HUBLIN: Alain explained it quite well. You can’t just count mutations to measure the degree of difference or resemblance. It’s more complicated. You can use that to create family trees, things like that, but when you get interested in characteristics, capacities, behaviors, etc., that’s another story. Speaking for myself, I’m a little bothered by discontinuity. We come back to the problem I mentioned earlier, on the search for the magic moment, the moment when God placed his finger on us... It’s a recurring question, which arose with sapiens: people talked about the “African Eve,” the “Garden of Eden,” a whole vocabulary that is not neutral. But this also applies to other stages of evolution. We see this in retrospect, when the dust settles, that we were a bit too enthusiastic in seeing a difference between before and after. Personally, I’m a partisan of continuity and I think it’s a little vain to keep looking for that discontinuity. That doesn’t mean that there are no accelerations, or that uniqueness is in contradiction with continuity. I’ve been working on the evolution of Homo sapiens these last few years, and I had arguments with colleagues on what should go under this term. Are the oldest sapiens really sapiens or not? It’s interesting because there’s a constant hesitation in the vocabulary between “moderns” and “sapiens.” I hate the expression “modern,” which everyone uses, including myself, because when you say “modern,” people think “humans like us,” whereas what we call “modern humans” in paleontological jargon, if you go back further than 100,000 years, are humans that aren’t modern either on an anatomical or a behavioral level, but only on a cladistic one, since they’re on the line that leads to all contemporary humans.

A. PROCHIANTZ: You’d have to differentiate modern from contemporary, as in art history.

J.-J. HUBLIN: Those who fought so that evolutionist ideas would win out were always a little uncomfortable with evolution, actually. People liked evolution, but sometimes they wanted to get rid of it, because it doesn’t allow for clear-cut discontinuities. Having said that, even though humans are indeed primates, mammals, who fit into the stream of vertebrate evolution, and so on, they have a whole set of characteristics that completely differentiate them from the rest of the living world. The reproductive success of species participates in an adaptive process that allowed them to exploit a particular ecological niche in nature. The sperm whale can dive 1000 meters to catch squid. Our thing is modifying our environment, what’s called niche construction. We didn’t just adapt to a natural niche: we did what other species do, but to an even higher degree: we built our own environment, at increasingly larger scales. It started with the use of shelters, or the domestication of fire, which are already alterations of the environment; it ended with the alteration of the landscapes and the climate of the whole planet. During these alterations of the environment, we observe a feedback effect on our biological evolution, in the same way that beavers have anatomical characteristics that allow them to live in artificial ponds and lakes that they create themselves. We too have a lot of biological characteristics that are the result of this artificial niche we’ve constructed for ourselves, and human evolution as a whole is an interaction between biology and culture, like I said in my inaugural lecture. In order to occupy their particular niche, sperm whales have to resolve a set of problems: problems of anatomy, of the respiratory system, of oxygenating the blood, of keeping it warm, etc. As for the hominins, the brain is what’s most important. We have set off on the path to its constant complexification, which is a costly evolutionary path, but which has provided us with all of the changes that made us truly human. The brain is a fragile, energy-intensive organ, that we’ve developed “at all cost,” because it was what made us successful. And among all of the species of hominins, ours apparently did even better than the others in that direction, with the development of absolutely unparalleled cognitive capacities. That’s where the real uniqueness of our species lies. And its reproductive success has been stunning. There are those who may deplore it, but 95% of the mammals living on Earth are either humans or domesticated animals, whereas 10,000 years ago, humans represented practically nothing in terms of biomass. That’s something unique, in mammalian history in any case.

A. PROCHIANTZ: On the question of time, we shouldn’t confuse physical and biological time. We can calculate biological time as a function of the number of mutations – even neutral ones – that accumulate in a genome. And there can be extremely rapid biological evolutions within a relatively short space of physical time. Another interesting question in this debate on continuity and discontinuity is the epistemological battle between gradualism and saltation in the theory of evolution. Darwin was interested in the survival of the fittest, [7] but he didn’t know about the genetic mechanism. Being a gradualist, evolution for him was an accumulation of small alterations. He said “natura non facit saltum” (nature does not make jumps). This idea prevailed for a long time among evolutionists, in particular by virtue of the modern synthesis in evolution put forward by the school of Ernst Mayr, which is not unrelated to the power of mathematical models in population genetics. When Richard Goldschmidt presented his theory of the “hopeful monster” [8] and revived the theory of saltation, proposing the idea of rapid evolutionary jumps – for example if the mutation occurs on a developmental gene – he was lambasted by the gradualists, including Mayr. In the last thirty to thirty-five years, in particular through the work of Stephen Jay Gould and Niles Eldredge, people have realized that the discontinuities in species between geological layers could be a reality, and that they didn’t just result from the disappearance of the intermediate layers. These discontinuities in evolution don’t mean that there are other evolutionary mechanisms besides those of mutation and selection: instead, they reflect the existence of mutations whose effects are more or less pronounced. The question you ask therefore alludes to this history of the sciences. There was a real scientific battle within evolutionism between those who refused the idea of a possible acceleration in the evolution of species and those who thought that there could be, from time to time, one or several mutations that produced a hopeful monster. Most of the time, these mutations are catastrophic, but occasionally a set of mutations, in a very specific niche, will encourage the emergence of the “beast of the future,” if I may put it that way. These mutations affect coding regions, but also regulatory regions. Among the mutations, some permit epigenetic modifications, for example by causing differential methylation within certain regions of the genome. A few mutations are enough to modify the gene expression above and below these regions. So some of Jean-Jacques’s colleagues from Leipzig, from Svante Pääbo’s group, have reported that a few regions of the Neanderthal genome, as far as their methylation was concerned, were closer to those of a chimpanzee than those of a sapiens, even though these two hominins, who are very close in chronological terms (separated by less than 400,000 years), branched off from chimpanzees seven million years ago. We need to examine these questions of time, continuity and disruption together.

J.-J. HUBLIN: Along the same lines, since we’re talking about time, there’s also the question of scale: when is an evolution “rapid”? The evolution of hominins is spread out over seven to eight million years. Earlier I talked about the changes that took place in sapiens between 150,000 years ago – at the end of the Middle Stone Age [9] in Africa – and 40,000 years ago, when people came to the Chauvet Cave to paint lions or when they painted pigs pursued by bird-headed men on the Indonesian island of Sulawesi. It’s an enormous change, a kind of explosion that meant that our species spread out everywhere. But compared with seven million years, 100,000 years is practically a snapshot. If you talk about a change that happened four million years ago but that lasted 100,000 years, we wouldn’t have a chronological resolution that was fine enough to grasp it, even with current dating methods. You’d probably think it was a major, nearly instantaneous accident.

A. PROCHIANTZ: For various reasons, the rate of mutation per unit of time can accelerate dramatically in some lines. For example, the coding region foxp2, a gene that has had its moment of glory, clearly underwent a more rapid evolution in hominins, in sapiens, than in chimpanzees. It’s not something that some god decides; it’s the history of an evolution with no end or finality. In this matter, sapiens thus acquired capacities for language that were somewhat unusual compared with the other hominins and that, in conjunction with other changes, were cognitively advantageous. Advantageous until now, at least, as our cognitive hypertrophy could constitute a case of hypertely, or evolution of a previously favorable trait that becomes unfavorable if pushed too far, leading our species to its doom, with the prospect of a world without humans. But for now – despite the energy requirements of a brain, which consumes 20% of the daily calorie intake while only making up 2% of human body weight – sapiens remains an extraordinary evolutionary success. We occupy 70% of the dry land on Earth and we’ve put humans on the Moon. No other animal has done that or is in a position to do so.

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I. KRTOLICA: in your work, both of you attach decisive importance to the growth of the brain, i.e. to encephalization in the history of the human line since the hominins branched off from the panins (chimpanzees, bonobos) around seven million years ago. However, you emphasize the correlation between encephalization and other aspects: the anatomical aspect, with bipedalism, the freeing of the hands and the transformation of the skull; the metabolic aspect, with the modification of the diet made necessary by a human brain that is very energy-intensive; the genetic and developmental aspect, with neoteny, social learning, and the technical externalization that follow from that aspect; and so on. considering the correlations between these various phenomena, why privilege the analysis of the evolution of the brain? Do you agree with the philosopher Raymond Ruyer when he says that this privilege stems from the fact that human evolution demonstrates a “reversal in the role of the brain,” in the sense that the brain is no longer in the service of the organism, as it is with all the other animals with brains, but rather the organism that is in the service of the brain? [10]

J.-J. HUBLIN: Here I return to the idea that human evolution is an example of niche construction. We didn’t come out of nowhere. We already belonged to a group of social primates who tended to be pretty clever, with a big brain, few children and a fairly slow development. But we could say we followed that lead to an absolutely extravagant degree. And in fact, we progressively “locked” ourselves into that evolutionary path. As for me, I’m not just interested in the evolution of the brain, but since the adaptive success of humans is essentially linked to an externalization of different functions within a technical and then a social sphere, it’s clear that this presupposes a complicated, efficient brain. This means that once we go in that direction, there’s no turning back. Actually, that’s not completely true: among the hominins, we have one example of reduced brain size and another example where it stagnated. These rare phenomena occurred because the brain has a high energy cost that has to be met without turning back or going extinct. This extremely costly engine led to a number of evolutionary changes. You mentioned the diet: we could also talk about locomotion. When you’re a biped, you use less energy than when you’re a quadruped. Lucy used less energy than a monkey of the same weight that walked on all fours. And with the same weight, we use even less energy for locomotion than Lucy did. So there’s a tendency to nibble away at different systems in order to collect energy for the brain. What’s more, the absolutely amazing part of this whole process is that some adjustments will take place, of a biological or behavioral nature – sometimes the boundary between the two is hard to define. These adjustments have secondary consequences that are beneficial to our behavioral complexity: consequently, they too become subject to positive selection pressures. In particular, one of the problems posed by a big brain is its initial development: how is it possible to bring a child into the world with a brain that’s too energy-intensive for the mother bearing it? By bringing that child into the world with a brain that’s still relatively small in proportion to its final adult size and by stretching its development out over several years. This ends up resolving an anatomical problem at the same time. A biped can’t have its pelvis growing indefinitely wider. A recent study on the subject shows that the constraints on the pelvis and the birth canal are tied not so much to bipedal movement as to the need to resist gravity in order to contain the fetus, as well as all of the abdominal organs! A major consequence of these adjustments is that the brain develops for the most part after birth while interacting with the outside world, with all of the neurodevelopmental consequences that follow, all of the plasticity that Alain mentioned, which contributes to the complexity of our capacities. Language is something we acquire while our brain is still developing. This delayed and extrauterine development is first of all a response to a challenge involving energy and anatomy, but it is then subject to a positive selection process, for it contributes to the growth of our cognitive capacities.
Another quite remarkable example is weaning. Humans have a very long growth period, but oddly enough their weaning takes place earlier than for the other great apes. For orangutans, it takes place at the age of five to six, whereas for humans it takes place much earlier. Why? Precisely because the brain is so costly to develop that mothers are in a hurry, so to speak, to share that energy-intensive burden with the adults of their group. The children, while they’re still totally dependent, only survive through the contributions of other adults besides their mother. Beyond breastfeeding and thanks to an early weaning, the whole group may contribute to their development. As a result, the small child quickly learns how to interact both with other adults to maintain their interest, and with other children. To a large degree, our prosocial behavior derives from that. You could say that social complexity exists because we have big brains. But actually, it’s more the case that in order to have a big brain, you need social complexity, without which you don’t manage to pay the necessary price for the development of that organ. We see this permanent interaction at work throughout human evolution.

A. PROCHIANTZ: Your question clearly shows the persistent influence of André Leroi-Gourhan. Our physical weakness at birth compels us to develop a strong social and cultural organization, in order to protect and educate the young over several years. Sapiens is an animal whose brain, on the basis of a genetic potential dictated by evolution, develops considerably after birth, while interacting with the world. This construction, which continues for an extended period – at least until the end of adolescence – is the fruit of genetic strategies of development that make room for a great deal of epigenetic liberty in the broadest sense, i.e. for a large capacity for post-natal adaptation, during the critical periods and even beyond, since some amount of plasticity is present in the adult nervous system. Because of this, sapiens invented rules for living together and, most importantly, it invented techniques and tools. We are technical animals. It’s true that animals have a sense of culture. We see species of crows that can take a twig and dig a larva out of a stump. In some places, groups of chimpanzees of the same species have practices that they don’t have elsewhere. You’d be wrong to say it’s not culture, but still, it’s not the same thing that it is with sapiens, for one extraordinarily simple reason. As a result of this post-natal learning period, a child grows up in an environment that’s different from the one in which a child from the previous generation grew up. You therefore have an irreversible, cumulative ratchet effect. And the invention of writing heightened this cumulative aspect even more, because we were able to leave traces for the next generation, moving beyond a mere oral tradition. Better still: today, while the bicycle extends my legs, the brain created the computer, which extends itself. This is all tied to an exceptional nervous system. As a neurobiologist, I’m obviously interested first of all in the development of the brain, but of course, I don’t separate that from the body’s development, as Jean-Jacques put it so well. Because there is a kind of debility in the human body and a delayed development, the brain has become what it is; it has invented these functions, but the functions it has invented have reinforced the brain’s importance in the adaptation and survival of the species. Having said that, the species is not old: its age is currently estimated at 300,000 years, thanks to Jean-Jacques. That’s nothing in relation to the history of living systems and the future is unwritten.

J.-J. HUBLIN: That’s what’s incredible. Individuals are born with the technical heritage of the humanity that preceded them: no one has to reinvent the wheel, the steam engine, etc. They’ve been invented already, and you can move on to something else. This human trait has become considerably more pronounced over the course of our recent evolution...

A. PROCHIANTZ: And is accelerating at a rapid pace! In the early XIX^th century, a boy born in his village could see the world only if he had the “chance” to go to war, to enlist in the army, Napoleon’s army in our case. He might end up dying there, but at least he saw something. Today, with the internet, a child is quickly able to obtain knowledge that, as flawed as it is, surpasses the village framework and allows them to travel in time and space. So there has been a marked expansion of the human environment. This has been true throughout human history, but in the last 100 to 200 years, it has taken on dizzying proportions.

J.-J. HUBLIN: We could criticize the study of ancient, Paleolithic cultures in this area. For lack of anything better, it emphasizes the technical aspect of things, because it’s obviously the most accessible aspect for us, notably the technology of knapped stone tools. But we shouldn’t underestimate the importance of this communal society that I was describing earlier when I brought up weaning: the survival and development of individuals are totally dependent on the effectiveness of a social network, which is first a family network, then it grows larger. What our species does, and what the species that preceded us or were provisionally contemporary to us (like the Neanderthals) did much less well – which could then, in addition, be a sign of our uniqueness – is precisely this construction of extended networks. So we start to mark out the contours of ancient ethnolinguistic entities. During the recent history of humanity, these networks obviously experienced an unparalleled level of development to the point where they became planetary. No one can build a smartphone themselves, but we manage to produce them because there are one or more human networks that bring the components within them together. The social network in which technology is used is an absolutely decisive factor. When we talk about Paleolithic societies, we probably underestimate this phenomenon because we have difficulty measuring it within extinct groups. And we’re able to construct networks that are not just collaborative networks: they also ensure group cohesion. They’re based on sharing beliefs and languages, which are ultimately a kind of insurance for those who are members of the network.

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I. KRTOLICA: would you say, then, that what people have called the “mental,” “cognitive,” “symbolic,” or “linguistic” revolution played a more decisive role in this evolution than the transformation of the lithic industries, which have been used since the time of John Lubbock to divide human prehistory into eras?

J.-J. HUBLIN: Even though I and most of my colleagues don’t really believe in the idea of a sudden transition, a cultural revolution that changed everything, we have to recognize that in the space of 100,000 years, the acceleration was so pronounced in Africa that there was nevertheless a quick shift from one world to another. And I think that this acceleration took place more in the social sphere than in the technical one. I deliberately alluded to the sharing of beliefs earlier. We do in fact see an increase in the number of non-utilitarian objects, which are used to identify the social status of individuals, to carry out exchanges, and to represent things. When the rock art of the Upper Paleolithic emerged somewhat later, we clearly see that it’s the expression of complex beliefs that remain unknown to us: a mythology, a narrative of origins that probably ensured the cohesion of these groups and probably has no equivalent in earlier hominin groups. The history of our species is probably not one of a single, instantaneous mutation, but rather one of an acceleration in the direction of complexity for more than 100,000 years.

A. PROCHIANTZ: Yes, but the brain of a modern sapiens from 40,000 years ago shouldn’t be that different from the brain of a sapiens living today...

J.-J. HUBLIN: Indeed, two years ago, we carried out a morphometric study to see what happened to the hominin brain over the past million years. With Homo erectus, the Neanderthals, and certainly the Denisovans, increasingly larger brains appear. All of these groups head in the direction of complexity. But the remarkable thing about our species is that around 300,000 years ago, when our brain got to the size it has today, its form and organization went in a new direction: it stopped growing bigger and underwent a reorganization. We see parts of the encephalon, like the cerebellum, becoming much larger. We could split our species up into chronological groups, and create “distribution clouds” for each of them – clouds that move, overlay each other, and overlap each other – up to today. Like Alain said, if we take a human from 40,000 years ago, there’s a good chance they will fall within the distribution of modern-day brain sizes. But we also see that evolution has never stopped.

A. PROCHIANTZ: This is because major changes have taken place in the human race, its organization, and its culture over the last 40,000 years, in particular with the invention of modern science, which is relatively recent: it has been in existence in Europe for around 600 years. And yet its consequences have been enormous, comparable to the invention of writing. Calculation, like writing, has made it possible to understand reality, to manipulate it, solely by thought as it were, without having to move mountains. With no changes in the brain, the invention of modern physics over the last 600 years – Copernicus, Kepler, Galileo – has thus produced a dazzling acceleration in culture, first in Europe and then in the rest of the world. Let’s meditate on the fact that in 1900, people were building airplanes that rose one meter off the ground and landed after a flight of thirty meters and that sixty years later, two men walked on the Moon. Such are the stunning effects of cultural and technical acceleration associated with language, writing, our capacity for calculation, and especially that generational increment that means that, in a way, we are all the next generation of apes.

J.-J. HUBLIN: We don’t really know if it was the same brain 40,000 years ago. We talk in terms of overall anatomy, but we don’t know what’s happening inside the brain, on the level of connectivity. We don’t have a cortex at our disposal from a hominin who lived 100,000-300,000 years ago, and we never will. But it seems as though there was first a general tendency toward increased brain size, then about 300,000 years ago, evolution went in another direction: it stopped wasting time making the brain bigger, probably because of what it cost in energy, and it worked on reorganization. In the last few millennia, there has even been a new tendency toward reduced brain size! This is probably why the evolution of the brain’s connectivity has taken on such importance...

A. PROCHIANTZ: The increase in cerebral connectivity, particularly on the local level, has been something extraordinary for humans. But when did it begin? We may never know, unless innovative technologies like the in vitro culture of “Neanderthal” cerebral organoids open new possibilities. I’ll say it again: the essential point seems to be the extension of the learning period until adolescence, since our brain – some parts of it, the cognitive parts in particular – learns intensely until the age of twenty to twenty-five, and continues to learn through adulthood, unless an accident occurs. This prolonged modification of connectivity and this ability to learn are responsible for the metabolic cost that Jean-Jacques mentioned. This metabolic cost is also one involving pathologies. The price for producing energy is free radicals and the oxidation of biological molecules: lipids, proteins, and nucleic acids, with the molecular lesions that follow and that become harder and harder to repair as one grows older. Many neurological illnesses that appear with age like Alzheimer’s and Parkinson’s are probably metabolic illnesses of an “excessively large” brain. Chimpanzees and other animals rarely experience illnesses of this kind, which are another human trait. Finally, let’s not forget the parameter of coevolution. For example, with the current interest in microbiota, we don’t just look at the human genome anymore: we look at the hologenome, which also includes the genomes of billions of bacteria that inhabit our mucous membranes and participate in human physiology. These coevolutions play an active role in human evolution itself. Today we discover that a number of psychiatric illnesses may be tied to imbalances in the microbiota, which therefore have an effect on the “noble” cerebral functions of sapiens. This coevolution, which introduces microbes into our physiology, leads to an increased level of complexity in research work. When discussing the evolution of sapiens, we have to admit that we’re not just dealing with the evolution of the human line: we’re also dealing with a symbiotic coevolution in which microbes, bacteria and viruses play a full role. Human and non-human microbiota differ, but microbiota also differ according to geography and nutrition. This coevolution is an additional vexation, similar to the Galilean and Darwinian vexations. The earth revolves around the sun, the apes are our cousins and bacteria influence our cognitive faculties!

J.-J. HUBLIN: There’s been a lot of discussion about externalization during human evolution. We’ve externalized chewing, digestion, locomotion, and so on through the use of utensils, weapons, and machines: we’ve also transferred our memory through writing and today through the use of digital media. This whole process is on the boundary between the cultural and the biological. But there’s now a form of internalization as well: a sizable percentage of the people over seventy that we come across in the street have undergone hip replacements. These are metal prostheses, but we can easily imagine many other future assemblages interacting with the human body on the frontiers of biology and technology. In fact, one of the directions medicine is taking now involves a process of reinternalizing technology within the body. For two million years, humans externalized a whole set of biological functions: today, they internalize technology within the body itself.

A. PROCHIANTZ: This could even involve cells derived from the transformation of ordinary adult cells into other cells. For example, we can take some fibroblasts by scraping the surface of a mucous membrane and transform them into neurons for transplantation. This regenerative medicine could extend human life, and I’m optimistic that the next generations may benefit from these regenerative therapies, including brain therapies, that are being developed today.

J.-J. HUBLIN: What’s more, when we examine the human genome to see what’s been subject to a positive selection over evolution, we notice surprising things. In the list of the parts of the genome that have been positively selected, we find genes that seem to favor neurodegenerative disorders. How is this possible? Probably because they offer some kind of advantage in early development, by encouraging cerebral connectivity, for example. When humans died young, these genes were undoubtedly advantageous from the perspective of natural selection, although we pay the price today because we live so much longer.

A. PROCHIANTZ:The reverse is true as well: genes that are bad for the young but that favor longevity. But since the sapiens species is social, it needs old people, even after one’s reproductive years, to take care of everyone. So it’s a net positive.

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I. KRTOLICA: The theory of evolution often turns to the idea of evolutionary failure or success. The ethologist Pierre Jouventin recently published a book titled L’Homme, cet animal raté [Man, That Failed Animal] (Paris: Libre & Solidaire, 2019), where he defends the idea that humans are failed animals because they’ve become maladapted to the socioecological conditions of existence that they themselves helped set in motion. Similarly, the paleoanthropologist Pascal Picq has proposed the concept of “malevolution” in his book Sapiens face à Sapiens [Sapiens vs. Sapiens] (Paris: flammarion, 2019) to describe sapiens’s recent disadaptation to its living conditions. Both authors stress the importance of the Neolithic in this respect. But we know that the evolution of life on Earth has experienced a few catastrophes (five mass extinctions of biodiversity, as we await the sixth), that there were also many evolutionary dead ends, and that many successes barely survived, through an extraordinary set of contingencies. Therefore, if we work on the two-part assumption that all species necessarily change through the effects of phylogenetic drift and that they are also all doomed to extinction – in the long term if not the short or medium one, until the programmed extinction of all life on Earth – how can we really gauge an evolutionary success in the living world? is reproductive success a sufficient criterion? Or does the very distinction between success and failure seem questionable to you from an evolutionary point of view?

J.-J. HUBLIN: Something important must be said first of all: evolution is a long history of extinctions. When we look at what happened with hominins for example, around thirty species have been identified. While the validity of some of them is still debatable, one thing is sure: they all went extinct at one point or another, or were absorbed by a neighboring species, who were better adapted. So once we say that extinction is a significant mass phenomenon, can we call a species an evolutionary failure if it existed for several million years before going extinct? Two main elements need to be recalled. In general, without sex or death, the evolution of living organisms would have been much slower. Next, the extinction of some species is precisely why others can take their place while occupying the same ecological niche. As for the human race, it doesn’t seem as though we are on the verge of extinction, for now. Animal life on earth is still mainly represented by arthropods, worms or even fishes, which make up a much larger part of the total biomass than humans do. On the other hand, if we only look at mammals, it’s fairly staggering, because humans alone represent 8.6 times the biomass of all wild mammals. From that perspective, it’s hard not to call that a success. Even so, there is something that’s quite specific to humans: as a result of their cognitive complexity, they’re aware of this issue of extinction. They’re conscious of their impact on the environment: they can discuss it and even make decisions to moderate their way of altering their environment in a way that benefits them. It’s really something unprecedented, because for now, all of the species that have existed on Earth for hundreds of millions of years have done what they could and then went extinct without wondering if they were heading that way. I don’t think we can avoid this issue, which is absolutely critical. We send rockets into space to understand how we can deflect asteroids, and just a few generations after the discovery of the effects of human activity on the world’s climate, we’re already trying to take measures to correct that. Since I was trained as a geologist, I’m used to speaking in terms of millions of years. So the idea that we’re trying to have an influence on the climate – in a good way, if I may say so – in the space of two or three generations seems quite remarkable to me.

A. PROCHIANTZ: The human race is an extraordinary species and sapiens does seem to be the only animal that is aware of its possible, and even inevitable, extinction, as an individual and as a species. This existential yet creative anxiety, a product of the human brain, is the source of culture. And if the success of a species is not just measured by its longevity, but also and especially by its ability to produce scientific, aesthetic, and literary artifacts, then sapiens is truly the king of animals. After all, Notre-Dame Cathedral, Kant’s critique of Pure Reason, Bernini’s Ecstasy of Saint Teresa or the Apollo rocket is not the same thing as a chimpanzee using a piece of straw to catch termites. So there’s a difference, which we owe to our brain, although the existential tragedy is the price we have to pay.

J.-J. HUBLIN: This brings us back to the brain, and the cognitive capacities that endow humans with consciousness in the broad sense, i.e. consciousness of the past and the future, of their role on the planet, of the end, of their own end, etc. We’ve been talking about uniqueness: that’s where it lies.

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I. KRTOLICA: And paradoxically, doesn’t the uniqueness of sapiens also lie in the consciousness that it has of its dependence on all of these phenomena with which it has coevolved?

A. PROCHIANTZ: Yes, and I’m pretty confident in the technical capacities of sapiens. As Jean-Jacques was saying, we’ve witnessed the growing awareness of global heating. So some colleagues have started preparing technical solutions for storing carbon dioxide, or thinking about ways to prevent the sun from heating the earth. I mean, that’s fabulous. That’s another example of imagination. The same intrinsically human imagination that’s at work among poets is at work among scientists as well. Sapiens is a technical animal, whose survival has depended from the start on technique, which was necessary precisely in order to compensate for its physical weakness. You only have to observe the speed with which we’ve produced vaccines to fight the Covid epidemic, even though we may be partly responsible for it through the removal of natural barriers between species.

J.-J. HUBLIN: Here I return to niche construction and that control of the environment that has reached a planetary scale today. That’s the key to the adaptive success of our species. And I don’t see how it can stray from that path. The only way to find the solutions to the problems encountered by humanity is by continuing in the same direction.

A. PROCHIANTZ: Up to the point when we may not be able to resolve them anymore. Ultimately, we may end up becoming victims of our own cortex, through hypertely. But there would be some dignity to such an end.