Asia, the other cradle of humanity
When we reflect on the existence of intelligent life, we tend to think that our species is the only one out there. We try to alleviate that loneliness by scanning space, in search of some sign of a hypothetical extraterrestrial civilization, but we forget that only a few tens of thousands of years ago we shared the planet with other hominids with cognitive capacities very similar to ours.
In recent decades, there has been an increasing number of discoveries of the remains of ancient human species, and more and more findings suggest that our direct ancestors were related to some of these species. Today, scientists are less hesitant to accept this, but it was not always the case. In fact, until almost the end of the last century, our extinct evolutionary cousins had a hard time getting into the exclusive human club. Until then, our past had been painted as a chain, with each link representing a kind of hominid that had given rise to the next one. But the fossils that have come to light have made it quite clear that this is not the case. Rather, it would look like a dense tree whose branches touch each other. Its trunk would be found in Africa. If we were to go back a few million years, we would only come across our ancestors on that continent.
The first humans had the best adaptations for living in this environment, mostly dominated by a warm and dry but rather unpredictable climate. It is precisely this last aspect that may have helped us develop an ability that gives us an interesting advantage over other animals: our extraordinary ability to adapt to very different environments. This, together with the technological achievements of our species throughout its history, has allowed us to colonize the entire planet, including the desert and polar regions.
The hominid explorer
Among the various varieties of extinct hominids, one, in particular, stood out for its desire to explore the lands beyond its birthplace: Homo erectus. Homo erectus could have emerged in Africa some 2 million years ago, but only 200,000 years later it was already established in such remote places as Java in Southeast Asia and other islands which could only be reached by sea, such as Flores in Indonesia and Luzon in the Philippines. This may also mean that some would have devised a way of navigating a certain distance.
If this was indeed the case, during their journey, which took them hundreds and hundreds of generations, they entered the Middle East, a region which, according to the remains that have come to light, served them as a beachhead for entering Europe, Arabia, Russia, China, India and the most remote areas of Southeast Asia. Homo erectus continued evolving where it settled, adapting to different climates and biomes.
In this way, little by little another great cradle of humanity was developed in Asia, where, like in Africa, several human species would end up appearing. In all this, technology would play a key role. From our perspective, we might think that Homo erectus was not particularly outstanding in this respect. Today, we think it is normal for a computer to become obsolete in a few years, but at that time, centuries and centuries passed without significant advances. In fact, technology has not always advanced as fast as in our time, and if this is the case now, it is because the current achievements have been supported by the previous ones.
A very advanced technology
Well, our original hero had to plough the ground. Around 700,000 years ago, the members of this species who had settled in the caves of Zhoukoudian, some 40 kilometres from present-day Beijing, learned to design lithic tools that were considerably more advanced than those of their predecessors. A team of researchers led by archaeologist Chen Shen of the Royal Ontario Museum in Canada has discovered that these hominids not only developed specialized artefacts which they used to process the animals they captured in many different ways but also combined different materials, such as stone and wood, to do so. In the opinion of Shen and his collaborators, this shows that they had reached a level of skill and intelligence that comes close to that of modern humans.
Over the centuries, the old world became very crowded, both for Homo erectus and for other species more or less related to them. A little over 200,000 years ago, while the Neanderthals were thriving in Europe in many of the places previously occupied by Homo heidelbergensis, and the first Homo sapiens were doing the same in Africa, another human group, of which little is still known, but which was probably related like the aforementioned heidelbergensis, had already been leaving its mark in Asia: it was the Denisovans.
A melting pot of species
This area of the world would end up looking like a melting pot of species from different humanities, some of them as striking as the one which inhabited until about 54,000 years ago the island of Flores mentioned above, and whose members, which show a certain anatomical resemblance with the Homo habilis, have been nicknamed hobbits due to their small size; the Homo floresiensis were about one metre tall. The Homo luzonensis, who are also called men from Callao, because of the name of the Philippine cave where their remains have appeared, were also short. The disappearance of both more or less coincides with the arrival of the Homo sapiens to the areas where they lived, but it is not clear that one thing is related to the other.
The first specimen of Homo erectus was found on precisely another Asian island. It was on the island of Java, in 1891, although it would be almost sixty years before the scientific community agreed to give it the name it still has. That extraordinary find was followed by many more, but for the most part, they were fragmentary or could not be dated with certainty.
However, thanks to technological and scientific advances, today a single tooth can provide us with very juicy information. For example, the study of four 240,000-year-old teeth discovered between 1972 and 1983 in Yanhui Cave in Tongzi, southern China, has greatly increased our knowledge of that time in the Pleistocene.
According to the paleoanthropologist José María Bermúdez de Castro, one of the experts who participated in this initiative, the analysis points out that the population of China is much more complex than it was a few years ago. "Besides Homo erectus populations, there were others, which could have reached the area between 300,000 and 200,000 years ago. Maybe they hybridised with those or maybe they replaced them. They would have arrived from the west and could be related to the groups that have been included by various authors among the Homo heidelbergensis or perhaps with the Denisovans", explains Bermúdez de Castro, who coordinates the Paleobiology Program at the National Research Center on Human Evolution, in our country.
The authors of the essay on the aforementioned teeth, published in the Journal of Human Evolution, suggest that they could belong to the lineage of the latter and not to that of Homo sapiens or Homo erectus, as was previously believed. Could a group of Homo erectus, however, have developed adaptations similar to those of modern humans and become the mysterious Denisovans? The question remains unanswered.
The discovery of the Denisovans
In 2010, the existence of this species or subspecies of human was known. That year the discovery of woman X was announced. It was actually a fragment of bone from the finger of a girl who had lived in Denisova cave in Siberia some 40,000 years ago, whose genetics did not match those of previously known populations.
In the nine years that have passed, much more has been known about these enigmatic relatives. From the study of the mitochondrial DNA - this is exclusively inherited through the mother - of a 400,000-year-old femur found in the Sima de los Huesos, near the town of Atapuerca in Burgos, it has been possible to infer that they were more closely related to the hominids who lived in this part of Spain at that time than to the Neanderthals, which, however, opens up even more questions; in this case, about the relationships and movements that archaic humans played a role in that remote past.
The truth is that the genetic legacy of the Denisovans lives on in different current Asian populations. A recent finding in the journal Nature confirmed that this was a diverse group, which extended from Siberia to China, and whose trail can still be followed in the highest region of the planet: the Tibetan plateau. The piece, just a piece of the jaw, is the most complete bone fragment so far attributed to a Denisovan. It has appeared at 3,280 meters of altitude, in a cave in Xiah (China), 2,200 kilometres from that of Denisova.
Until now, it was thought that this area had only been colonised by our species, some 40,000 years ago. But this remains evidence that the Denisovans were already there 160 millennia ago. In fact, it has been possible to determine that the gene responsible for the current Tibetans having adapted to live where they do, and which, in essence, allows them to inhabit environments with lower levels of oxygen better than other people, could actually be of Denisovan origin. And they are not alone. The genetic material of the latter is also present in Papuans, Melanesians and Australian Aborigines.
Until recently, human evolution in Asia over the past 700,000 years has been viewed in a partial way, with different groups of hominids who did not interact in space or time. But new discoveries point in the opposite direction.
An Evolutionary Continuity
It seems that not only were they connected, but that there might even have been an evolutionary continuum from the time Homo erectus spread through the area to the present day, which would confirm the appearance of a 300,000-year-old skull in Hualongdong Cave in southeastern China. In a study published in the journal Proceedings of the National Academy of Sciences (PNAS), an international team of researchers shows that these fossils are particularly important since they suggest that, although there were some regional variations in the archaic populations of East Asia, there would be a continuity of human biology.
One of the co-authors of this essay is Erik Trinkaus, a renowned paleoanthropologist from Washington University in St. Louis (USA). "Hualongdong's remains have provided key information about the shape of the teeth and the skull in general. His analysis, along with what we know about other previously found remains, indicates that, although there were differences in the way anatomical features combined in individuals in different regions, there was a continuity in human evolution," says Trinkaus.
What these researchers come to show is that, ultimately, this phenomenon, which is seen, for example, in the progressive development of more graceful craniofacial structures, could be traced back to ancient Homo erectus to modern human populations, and not just in Asia, but also in Europe and Africa.
The Hualongdong skull and the possible Denisovan jaw of the Tieba are the last pieces of a puzzle which reveals--if it wasn't clear before now--the complexity of our evolutionary family tree. "In a way, this corresponds to the marked diversity in body shapes that emerges from the study of the fossil record. In a way, we are at the end of simple models," says Spanish biologist Carles Lalueza Fox, a world authority on the study of ancient DNA.
For Trinkaus and the other co-signatories of the study published in PNAS, it is possible that the remains of the remote Denisova cave or those that appeared on the island of Flores are nothing more than evolutionary, peripheral experiments, and that they do not represent authentic human
evolution during the Pleistocene in that part of the world. The keys to understanding it and, with it, the background against which our species emerged, should be sought in the heart of the continental regions. Something similar would have happened in Africa.
In fact, although it has been accepted that our most remote ancestors came from the eastern part of Africa, where they would have lived some 200,000 years ago, the discovery of fossils 100,000 years older in Morocco suggests that by then they had spread throughout the continent.
It is even possible that today's humans have evolved from various populations that began to interbreed about 500,000 years ago. However, this continuity referred to by scientists has been the subject of debate for a long time, and is likely to remain so, especially in the case of Asia. Most of the fossils found there are either too fragmented, in poor condition or cannot be dated with certainty, which does not help to clarify the issue.
