Jean-Jacques Hublin is a paleoanthropologist, author of numerous works on the evolution of Neanderthals and the African origins of modern man.
He has played a pioneering role in the development of virtual Paleoanthropology, which makes extensive use of medical and industrial imaging techniques, and computer technology to reconstruct and analyze fossil remains. He is also interested in the evolution of growth rhythms and brain development in hominids, and in the history of his discipline.
Visiting Professor at the International Chair since 2014, he will become Paleoanthropology Chair in 2021.
Paleoanthropology has come a long way since its inception, particularly in the last fifty years. How have these advances transformed our perception of human evolutionary history?
Jean-Jacques Hublin: To understand this, we need to go back to the vision that most scientists had of the "human tree" up until the middle of the 20th century. In the 1880s, Gabriel de Mortillet, one of the founding fathers of prehistory, saw human evolution as progressive and linear, and drew a parallel between technical progress and biological evolution. His influence persists to this day; museums and popular science books feature friezes of fossilized man following one another in a continuous line. Until the 1970s, the "single specieshypothesis " held that man was such a unique adaptation that it would be impossible for several species of the genus to have coexisted in the past. Yet, even then, it became clear that the history of mankind was not so simple. In the Near East, remains ofHomo sapiens older than those of Neanderthals known in Europe have been discovered, sweeping aside the idea that the latter were our ancestors. In East Africa, fossils of both Homo and Australopithecines have been found in the same geological layers. The linear vision of our evolution no longer worked, and the proponents of a more complex tree took over; rather than succeeding one another, several human species have cohabited through the ages. The discovery of new specimens in recent decades has only reinforced this "bushy" model.
How can we explain this degree of speciation [1] in the human genus?
In African apes such as chimpanzees and gorillas, there are regional subspecies, but certainly not as much diversity as in man's ancestors. This is largely due to the fact that, over the last 2 million years, humans have spread over a large part of the planet and encountered very different environments. This wide geographic distribution has added the phenomena of isolation by distance to adaptation to local conditions. For example, Neanderthals and Denisovans, two sister groups separated around 450,000 years ago, evolved in the wide open spaces of Eurasia; the size of the continent was a determining factor in the differentiation of these groups. The case ofHomo floresiensis, characterized by its very small size, is even more evocative, since it lived isolated on an Indonesian island for hundreds of thousands of years. Dispersal in mid-latitudes explains Neanderthal's rapid divergence. In these regions, extreme climatic fluctuations and ice ages caused periodic demographic collapses. Certain areas were emptied of their populations, while small groups survived in more clement regions. In such a situation, a phenomenon known as "gene drift" occurs: by chance, characters become fixed within a population simply because they are present in a small group, which will itself be the source of future, larger groups. When this phenomenon is repeated, the phenotype - i.e. the set of biological and anatomical characteristics - evolves rapidly. This explains why there are few anatomical differences between a common chimpanzee and a dwarf chimpanzee, even though they diverged 2 million years ago. Conversely, the differences between Neanderthal man and our species are obvious, even though the divergence is much more recent.
Despite these major differences between human species, there is evidence of inter-fertility, particularly between Neanderthals and Sapiens. Is the notion of different species still appropriate?
The problem of defining species is not limited to fossil man, but now affects the entire kingdom of living organisms. Since the work of Ernst Mayr in 1942, the idea that species are separated by a barrier of non-interfecundity has taken hold. However, if we were to adhere strictly to this principle, almost a quarter of the species recognized within the Mammals would have to be eliminated. The fact is, some primate species that have been separated for over 5 million years and assigned to different genera (Papio and Theropithecus) are still able to reproduce with each other, and they produce fertile hybrids! It's important to understand that speciation is not a one-off event: you don't just wake up one morning as a new species. It's a long process, and total non-infecundity is a kind of end result of this process, usually manifesting itself after several million years of separation. Yet along this road, beyond a certain point, there is no turning back. So we need to rethink our definition of species.
In what way?
We need to turn to the notion of "metapopulation lineages", lineages that separate but remain inter-fertile for some time, even though certain mechanisms are already in place to maintain their integrity; for example, because the hybrids generated are less fertile. This is why these lineages can be considered as species. Thus, we often find a bit of the genome of one species in that of another, without the two species having merged into a single one. I'm tempted to think of Homo sapiens and Neanderthals as two species in formation. They have been in contact on several occasions, and there is evidence of introgressions of mitochondrial DNA [2] and Y chromosomes of African origin into the Neanderthal genome over 250,000 years ago. This explains why Denisovans have mitochondrial DNA and Y chromosomes similar to those of ancient Neanderthals, whereas recent Neanderthals are closer to our species in these respects. Yet all this has not prevented these lineages from continuing to diverge. Another age-old problem, often discussed but never resolved, is that of species boundaries in time. All living beings are linked by a chain of inter-fertilization with creatures that preceded them long ago. However, if you were to meet a primate from the Oligocene (between 33.9 and 23 million years ago) belonging to your ancestry, I doubt you would recognize it as a kindred spirit or be able to reproduce with it. How, in a vertical living tree, can we define horizontal cuts other than points of divergence between lineages? The problem remains.
When did this divergence occur between the Hominines (human lineages) and the Panines (chimpanzees)?
There is no clear paleontological evidence of the divergence between Hominins and Panines. The clues we do have come mainly from genetics and molecular biology. We can calculate a "coalescence time", i.e. the moment when two lineages meet in the past at a time when a common ancestor existed. The problem with this method is that it's based on a range of assumptions about mutation rate and generation length - parameters that vary widely from lineage to lineage. A few decades ago, it was thought that divergence could be no older than 6 million years, and geneticists scoffed at paleontologists who presented Hominin fossils 7 million years old. Today, a range of between 5 and 10 million years is proposed, and most scientists agree that the divergence occurred around 8 million years ago. The discovery of new fossils is crucial to answering this type of question, and so is maintaining the dialogue between paleogeneticists and paleoanthropologists.
What role did extinction play in the evolutionary process of human lineages?
Evolution is above all a great story of extinctions. Without them, dinosaurs would still be walking the Earth and, who knows, Homo sapiens might not even exist. There are extinctions of species, but also of populations and small groups. In the case of Neanderthals, we often imagine that, during the Ice Ages, they found refuge in southern Europe, then returned north when the climate became milder. This is a false view of reality. In fact, groups living in areas where the climate deteriorated eventually disappeared, while those already occupying more southerly regions survived. Then, when the emptied territories became habitable again, the surviving populations spread out. But this was a gradual movement, spread out over generations, and not a rapid, deliberate exodus as is sometimes imagined of prehistoric "migrations". Environmental change and competition with invasive species, such as our own, are two of the mechanisms often involved in the extinction phenomenon, which itself catalyzes evolution.
Arethe invasiveness and adaptability ofHomo sapiens the key to its success?
In terms of adaptability, our species has long been comparable to other hominins. Neanderthals, Denisovans and other earlier human species also embarked on the same path of behavioral complexification and niche construction [3]. Humans strive to modify their immediate environment to suit their needs. And over the last few millennia, he has done so on a massive scale; just look at the landscapes that surround us today, most of which no longer have much in the way of natural features. But this alteration of the environment goes back further than the emergence of agriculture, and we suspect that Paleolithic hunter-gatherers were already engaged in it, notably through the use of fire. The changes leading to modern humanity also have a social dimension. Like other social mammals, humans form coalitions. A pack of wolves will always be more effective than a lone wolf. In the case of modern man, these networks of solidarity are not limited to the scale of a small group; they extend considerably and can link individuals over great distances who don't even know each other. There is no simple answer to the question of what defines man and his success. Many of the behaviours and aptitudes evoked to define the human can be found elsewhere in the animal kingdom. So it's pointless to talk about their presence or absence. What counts, rather, is the frequency and complexity of these behaviors; and this is certainly where the differences between Homo sapiens and other Hominins lie. Homo sapiens has done what other hominins have done, but over the last 100,000 years, he has probably done it faster and harder. Our species thus reached a decisive threshold more quickly, precipitating the replacement of other groups.
How has the bias inherent in the study of man by man evolved since the early days of paleontology?
We like to think that our methodological tools are formidable, whereas our predecessors of the 19thand early 20th centuries were quite helpless. In fact, ever since we first started looking at these issues, every era has based its vision of the past on archaeological and paleontological knowledge, but also on a healthy dose of ideology. Man has never been a neutral subject. Paradigms change, but there are always paradigms, and our time is no exception. For a long time, religion and creationism set man apart. Today, thanks to science, we know ourselves to be mammals and primates... However, we still have a strong desire to define ourselves, and, more recently, to be as inclusive as possible in this definition of the human. This translates, for example, into a certain desire to make Neanderthal too similar to ourselves, when we should be celebrating both our similarities and our differences with this extinct cousin. In our modern societies, Paleoanthropology has taken the place of a narrative of origins once rooted in mythology and religion. 50,000 years ago, we liked to sit around the fire and listen to our ancestors tell stories about the human past. Today, we open our newspapers or turn on our televisions, and enjoy listening to the story of our origins, which becomes clearer with each new discovery.
Interview by William Rowe-Pirra
