July 8^th, 2015 was the 21_st anniversary of a revolutionary finding for prehistory in Europe. At noon, Aurora Martín recovered a pair of human teeth at level 6 (TD6) of the archaeological site, which for thousands of years had finally silted the Gran Dolina cave. Those two teeth, archaic-looking and very different from ours, certified the presence of hominids in our continent at a time before the 780,000-year limit. Working intensely on an archaeological survey, Aurora and her excavation colleagues had gone over that time limit one meter further up, where the iron ore in the sediments abruptly changed its magnetic orientation. At the same time, the data provided by the fossils of animals and the provisional geochronological dating revealed that the fossil remains at TD6 could have been deposited in a warm period, very likely about 850,000 years ago. Since the oldest known human remains in Europe were no more than 600,000 years old, this instant had deepened an eternity in the roots of the first population of our continent. But this was not the only news.

 

The days that followed and the 2005 and 2006 campaigns completed a collection of nearly a hundred human fossils, with 260 stone tools of very primitive manufacture and several thousand remains of micro- and macro-vertebrates. By then, there were no doubts that the Matuyama-Bruhnes palaeomagnetic limit was located at the top of TD7 while dating confirmed the great antiquity of the human remains. The detailed study of fossils revealed the oldest case of cannibalism known to date in human evolution. This is a relevant scientific finding, but it left us with the uncertainty of not being able to investigate complete fossils. The fragmentation of the human remains was a handicap for the study of important anatomical parts from a taxonomic point of view. Spectacular fossils were missing and many of them had belonged to children. However, the whole revealed a unique combination of primitive and derived features regarding the first populations of the Homo genus. In particular, modern facial features were visible both in adult individuals and in children and they contrasted with the primitive features of the teeth. This fact and the age of the fossils encouraged us to write a paper to try to have it published in the Science journal. The manuscript was submitted on February 6^th, 1997. It proposed a new species of the Homo genus, H. antecessor, with the hypothesis that this species could represent the last common ancestor of H. sapiens y H. neanderthalensis. The article was accepted on April 15^th that year and published in late May.

 

It took the H. antecessor species over a decade to be accepted by most of our colleagues. However, the second hypothesis was never taken into consideration. The origin of our species in Africa is too powerful an argument against the hypothesis proposed in the Science journal. Moreover, most of the data provided in later years by geneticists indicated that the separation of the two species occurred just over 400,000 years ago. Although the error range of this date is very wide, no one considered the possibility of reaching (and even less exceeding) the age of H. antecessor to accept this species as the last common ancestor of modern humans and Neanderthals. Our team fully understood the reasons our colleagues exposed and we have not insisted on the defence of the hypothesis.

In the late twentieth century, we began a regularisation of the sedimentary sequence of the Gran Dolina site in 2003 and new and important findings about H. antecessor arrived. Gradually, we reached the figure of 160 human remains, while we refined our investigations. The doctoral theses of the next generation were very important because they helped clean up the scene of old prejudices. 

 

Soon, we began to identify some features in H. antecessor which until then were considered of Neanderthals. It was very evident that these features had emerged in the Lower Pleistocene, maybe a million years ago. Did this finding mean that, indeed, H. antecessor was related to Neanderthals and modern humans? Did this confirm our first hypothesis then? Not necessarily. In 2003, we published the first article in which we left behind us the idea that the humans at TD6 belonged to the species that gave rise both to Neanderthals and to populations of our species. We had to propose an alternative to explain such a particular combination of anatomical features in the bones. Some reminded of Middle Pleistocene fossils from China, although most of the derived features made the European imprint of the H. antecessor clear. Leaving the primitive characters aside, as they did not help clarify the origin and descent of this species, everything pointed towards a certain relationship of H. antecessor with European populations of the Middle Pleistocene.

 

Many colleagues have advocated (and still do) that these latter populations reached Europe in the early Middle Pleistocene, after two very cold episodes in the northern hemisphere. Thus, members of H. antecessor would have arrived to Europe before and perhaps were not the first to arrive in these lands (remains of the site of Sima del Elefante: 1.2 million years). In short, we can postulate that Europe was colonised in several waves of population at different times of the Pleistocene. If this hypothesis is correct, explaining the similarities of all the populations that were entering our continent would still be pending. There are two possibilities: one, that each new population hybridised with the previous one and retained part of its genome; and two, that all populations came from a common "mother" and received a part of its genome from it. The two hypotheses are not mutually exclusive. 

 

If we consider the second hypothesis, it is conceivable that that common mother could have evolved in Southwest Asia, perhaps hybridising with other people from different places (Africa, for example). The designated place is a real "hot spot" for biodiversity because it is a crossroads between Africa, Asia and Europe. If this is so, the scenario would be very complex and difficult to understand. From that crucible, and perhaps at a much earlier moment than what data provided by geneticists propose, the separation of populations that led to Neanderthals in Europe and modern humans in Africa could have happened. H. antecessor would then be perhaps a not too distant cousin of that common ancestor. 

 

The study of human evolution has taught us repeatedly that simple evolutionary scenarios do not work. We tend to simplify models, because the fossil record is limited and because we are incapable of understanding a story that lasted thousands of years in a vast territory. The H. antecessor collection is the only window that allows us to look to the most distant past of our continent. The excavations expected at TD6 in the coming years will be exciting. The H. antecessor species has a very promising future and can be key in understanding human evolution in Europe over the coming decades.

 

José María Bermúdez de Castro Risueño. Co-director of excavations and research in Atapuerca