resources_evolution_txt_Selam. The australopithecine girl from Dikika.
Selam. The australopithecine girl from Dikika
Author: Carlos A. Marmelada
3. "Arborealists" and "Terrestrialists": The controversy over bipedalism in afarensis
1. A great finding: a 3.3 million year old infant hominid.
In the fossil record of pre-human hominids there are hardly any infant specimens. The reason is simple: it is very difficult for the bones of multi-million-year-old offspring to fossilise, due to their fragility. But if they do, they provide a wealth of information about the growth and development of the individual (ontogenesis) and the species (phylogenesis).
Understanding how the ontogenetic development of individuals of different hominin species occurs is fundamental to a full understanding of human evolution. Although this is not the first time that fossilised bones of infant Australopithecus have been found, they have always occurred in isolation; never before has a set of bones belonging to the same infant been found. The fossil record of pre-human hominids such as Australopithecus reveals a total lack of skeletons belonging to infant specimens, so it has not been possible to study the growth patterns of these species. This is why finding in Dikika, Ethiopia, is a real cause for congratulation, as an almost complete skeleton of an Australopithecus child has been recovered. Until now, the most famous child specimen found was the famous skull of the Taung Child, which, at 2.5 Ma, was discovered by chance in 1924 and belongs to the species Australopithecus africanus, named after Raimond Dart and which gave its name to this genus of hominids.
The team led by Zeresenay Alemseged (from the department of Human Evolution at high school Max Planck in Leipzig, Germany) has had the great fortune to stumble upon a fabulous finding : a large part of the skeleton of a 3.3 million year old (3.3 Ma.) Australopithecine girl *(1). This is why Bernard Wood (George Washington University) says that the finding represents: "a veritable mine of information about a crucial stage in the history of human evolution" *(2).
It really was a stroke of good luck because, as Zeresenay Alemseged himself tells us: "When I decided to go to work there, I didn't expect to find the juvenile skeleton of a hominid. I had another set of scientific questions related to palaeoanthropology that I wanted to find answers to. My idea was to investigate what had happened palaeoenvironmentally, palaeoanthropologically, geologically and stratigraphically before Hadar (the site where Lucy's famous skeleton was found)" *(3). However, it is a geological area of incalculable palaeontological value. Thousands of fossils, some of them hominid, are exposed after the rainy seasons, so scientists know that this is an area where discoveries are particularly plausible.
The skull of a presumed three-year-old female sample shows that many of the diagnostic characters of the species to which it belongs, Australopithecus afarensis, were already present in the early stages of development. But the good fortune does not end here: the finding includes some skeletal elements that were previously unknown in the Pliocene hominid record. Thus, among the postcranial remains, a hyoid bone has been found that has the typical morphology of African apes. The foot and other lower limb remains provide evidence for bipedal locomotion, but the gorilla-like scapula and the long, curved phalanges of the hands confirm the importance of arboreal behaviour in the locomotor repertoire of A. afarensis.
The findings have been made in the Sidi Hakoma Member of the training Hadar with a chronological range between 3.35 and 3.31 Ma. "Three plates coincide there, opening up the earth's crust, exposing sediments that precisely correspond to ages containing fossils of human ancestors" *(4). This is the same fossiliferous area in which the partial skeleton of Lucy, then known as the "Grandmother of Mankind", was discovered in 1974 *(5) as the oldest hominid specimen known to date. In addition, 40% of her skeleton had been recovered, which also made her the most complete hominid fossil, excluding Neanderthals and Cro-Magnons. At the time, it was undoubtedly a spectacular finding both for its age and for the number of bones recovered from the same individual. For all these reasons, Lucy may be the most famous fossil in all of human palaeontology.
Lucy and the girl now discovered belong to the same species: Australopithecus afarensis. The difference is that Lucy corresponds to the skeleton of an adult female *(6), and is 3.2 Ma. old, i.e. one hundred thousand years younger than Selam *(7), baptised by her discoverers with a name meaning: Peace.
It appears that Selam died in a flood. His body may have been washed away by a flood and deposited on the banks of a lake near the present-day Awash River, where it was quickly covered by sediment. It was precisely this that contributed greatly to its discovery three million three hundred thousand years later by members of the Dikika Research Project (DRP).
The skull was found by Tilahun Gebreselassie on the evening of 10 December 2000. In an interview with Kate Wong, Alemseged describes the exact moment of the discovery at finding with great emotion: "I was just a few metres away. We immediately realised that it was a hominid. The absence of brow prominences, the lack of a postorbital groove, the small canines, the vertical symphysis of the jaw - all these features indicated it. I looked around (observing the surrounding ground) and immediately saw the forehead. But with only four members of our research team there, it was not possible to do the excavation work necessary to recover the remains. We had to return to Addis Ababa to get the necessary supplies" *(8).
In the same campaign and in those of 2002 and 2003, the postcranial remains that are now associated with it were found at the back of the head embedded in a hard ball of sandstone. The skull was embedded in a sandstone matrix and it took five years and more than a thousand hours of patient work to extract it without damaging it, even using dentists' tools. But the effort was worth it, because, for the first time, it has been possible to recover the face of a specimen of Australopithecus afarensis.
On the other hand, the skull is almost intact except for parts of both parietals. This absence makes it possible to observe the marks left by the brain in the endocranium, which will make it possible to study the brain structure of these australopithecines in order to compare it with humans. There is also a slight deformation at the back of the calvaria. The lower jaw was found still attached to the upper jaw and all the teeth are in very good condition, except for the crowns of the lower left incisor.
The hyoid bone has been recovered well preserved just below the palate. This bone is very important, as it provides information about the hominid phonatory apparatus. However, it is a miracle that it has been found, as it is cartilage and it is very difficult for it to fossilise. In fact, only one has been described in the scientific literature and it corresponds to a Neanderthal from Kebara (on Mount Carmel, Israel), discovered in 1989. And in the entire palaeoanthropological record only three can be counted: the one mentioned above and two more found in the Sima de los Huesos in Atapuerca *(9) and corresponding to Homo heidelbergensis, with an age of 400,000 years. This scarcity makes the Dikika hyoid even more important. Which, pending future studies programs of study, sample is more similar to that of panids than to that of humans. This confirms the assumption that Afarensis could not speak, but would have made guttural sounds like chimpanzees.
Postcranial skeletal bones that have been found include both scapulae and the clavicles. The cervical and dorsal vertebrae, as well as the first two lumbar vertebrae and several ribs. Some limbs have been found at a small distance from the rest of the bone assemblage; they include a distal fragment of the right humerus including the epicondyles (the two bony lumps or protuberances at the end of the humerus), which were separated from the rest of the bone. The bones of the hand include the proximal, middle and distal phalanges. A good part of the distal femur and the proximal tibia have been recovered. The distal part of the tibia and fibula includes an epiphysis that was separated from the bones. What has not yet been found is the pelvis, the lumbar region and part of the extremities, mainly the upper limbs.
It appears that Selam (whose technical name is DIK-1-1) is a female. This is what the study carried out on the dental crowns indicates. This was done using the clinical technique of computerised axial tomography (CAT), which is increasingly used for anatomical programs of study of fossils. The scanner revealed that the crowns of the first molar, although not yet erupted, were already fully formed, as well as part of the crowns of the second molar. The crowns of the premolars, canines and incisors are also present. Comparing the dental development patterns of Afarensis with those of present-day chimpanzees, it is concluded that Selam must have died at the age of three.
The skull (DIK-1-1a) of Selam has been compared with the skull of afarensis AL 333-105 *(10) and with that of the Taung Child. The directly calculated brain capacity is estimated at 235 cc. But it must be taken into account that the rear part of the calotte is somewhat deformed, so the researchers believe that the cranial volume size of DIK-1-1st is between 275 and 330 cc. More or less the same as that of a chimpanzee of a similar age. The problem is that Selam was only three-quarters developed, whereas a panid is 90% developed by that age. This means that Zeresenay's team calculates an endocranial volume of 425 cc as an adult. Little more than the 380 cc. of a current adult chimpanzee and slightly higher than the female of Flores (417 cc.) found by the team of Morwood and Brown; and very far from the 1350 cc. of average of the current humans.
Morphological details of Selam's face are reminiscent of afarensis features and differ from africanus. The nasal aperture is narrow, as in other juvenile specimens of afarensis, such as specimen AL. 333-86. The canines and molars do not reveal ape-like characteristics as in Australopithecus anamensis and Ardipithecus ramidus. In fact, it is the dental similarities between this juvenile specimen and the adult individuals of the species afarensis that make it advisable to include it in this taxon.
3. "Arborealists" and "Terrestrialists": The controversy over the bipedalism of afarensis.
The CT scan scanner reveals that the foramen magnum (the hole where the vertebral column inserts into the skull) is located much more anteriorly than in apes of the same dental age. A fact in favour of bipedal locomotion as a means of terrestrial displacement. From the waist down, the skeleton of Selam indicates that its locomotion was bipedal. But from the waist up it retains primitive or ancestral features suggesting that it still retained the ability to climb trees and move through them by swinging on branches, as indicated by the length and curvature of the phalanges of its hands. In Australopithecus afarensis: "arms and body proportions indicate considerable muscular power. Arms relatively long relative to legs. Lucy's humerus-femoral index is 83.9. In present-day humans the index is 74.2; and in present-day pongids it is greater than 100 (as they are suspensory brachiation), but in Pan paniscus it is 97.8 (...) In conclusion, they had great facility for jumping, equivalent to that of a present-day chimpanzee, and for climbing trees by holding tightly to the trunk with the mesial side of the feet while hoisting with the arms as deduced from the anteoposterior position of the lesser trochanter (according to M. Pickford). Their legs were short, as was their stride. They would use bipedalism to move overland in an open forest environment and to exploit terrestrial econiches (according to R. Susman). O. Lovejoy, on the other hand, interprets it as terrestrial locomotor bipedalism" *(11).
Afarensis are known to be bipedal, but it is also known that their upper limbs are longer than their lower limbs, a clear primitive trait inherited from an ancestor that retained this ape-like feature and passed it on to the architecture of the Afarensis skeleton. However, there is a debate among scientists about the evolutionary role of these long, ape-like arms. discussion . For some, it would be a useless inheritance that has fallen into disuse because they were fully efficient bipeds, as Donald Johanson, one of the famous discoverers of Lucy, traditionally believed; but the famous scientist from Arizona State University now believes that there is accumulating evidence to suggest that it must also have developed a certain arboreal life *(12). It is quite possible that at night they climbed trees to sleep, seeking shelter from terrestrial predators such as lions, hyenas and sabre-toothed tigers. It is also likely that they moved through the branches of trees in search of ripe fruit, which formed an important part of their per diem expenses. The puzzling thing is that, in a scenario like this subject, a chimpanzee-style scapula might be found; however, the scapula found at Dikika is more similar to that of gorillas, the least arboreal (given their large size) of all the apes known today. However, there is no consensus here either plenary session of the Executive Council ; in the opinion of anthropologist Owen Lovejoy (Kent State University, USA) the scapula "is primitive but, in reality, it is more similar to that of humans than to that of gorillas" *(13).
topic The controversy surrounding the subject locomotion of afarensis, as well as its way of life, has been a recurrent issue for almost three decades. This controversy has not always been peaceful, but has at times been the subject of heated debates between specialists. Lee R. Berger (of the University of the Witwatersrand in Johannesburg, South Africa) tells us how: "In the 1980s and early 1990s the discussion about arboreal or terrestrial life centred around afarensis (...) This discussion was surprisingly strident, to the extent that scientists divided into camps of "arborealists" on the one hand and "terrestrialists" on the other (...) The vehemence that accompanied any discussion about human origins was not absent from this tree-climbing controversy. Indeed, the 1980s and early 1990s were a period of profound disagreement between the two camps, so sharp that interpretation of the function of a single muscle could lead to fierce public clashes between scientists. At conferences, some were excluded from their group for simply addressing a member of the opposing camp" *(14).
In turn, programs of study on the environment in which Selam *(15) lived indicates that it was a palaeoenvironment in which woodlands were combined with clearings and where grassy surfaces were abundant with plenty of water from a lake or delta. The fauna associated with this juvenile specimen of afarensis includes hippopotamuses, crocodiles and snakes.
Afarensis are australopithecines that lived between 3.9 and 3 Ma. and lived in the same geographic areas where the remains of Australopithecus anamensis have been found (both in Ethiopia and Kenya), which are older (4.2 million years). Thanks to the recent discovery of anamensis in the Afar region of Ethiopia, it has been possible for the first time to link these three ancestral hominin species: Ardipithecus ramidus, Australopithecus anamensis and Australopithecus afarensis in chronological succession in the same area. The proximity of the new discoveries raises the possibility that anamensis is the ancestor of afarensis. This August, in the Journal of Human Evolution, Meave G. Leakey (the discoverer of A. anamensis), Donald C. Johanson (the discoverer of Lucy), Yoel Rak and others published a article in which they raised the question of whether or not anamensis was the direct ancestor of afarensis *(16), betting on it. However, Lee R. Berger, for example, does not see things so clearly and doubts that anamensis can be, without more, the ancestor of afarensis because, he claims, the skull of the former is notoriously more primitive than that of the species to which Lucy belongs, the remains of the postcranial skeleton seem to be of a more modern morphological subject , and yet the specimens of anamensis are almost half a million years older than those of afarensis. Thus Berger asks: "How was it possible that a creature supposedly more ape-like in its cranial morphology than africanus and afarensis, and which had lived at least half a million years earlier, was more advanced in terms of anatomical evolution from the neck down?" *(17).
On the contradictory nature of the combination of archaic and modern characters in anamensis, Daniel Turbón makes the following comment: "The primitivism of the skull contrasts with the surprising modernity of the tibia: both tibial platforms are well excavated, that is to say, its knee did not have the lateral movement capacity of A. afarensis. For this reason, some authors consider that bipedalism in the Homo lineage would have already occurred at that time, 0.5 chron before the Laetoli footprints. Meave Leakey, on the other hand, believes that it is a very primitive hominid ancestor of A. afarensis. Both views are not incompatible since the remains could belong to several individuals - a constant in the Rift, for reasons already described - and it is possible that the remains of A. anamensis belonged to two sympatric species" *(18).
In fact, there is still an ongoing controversy as to who the hominid author of the Laetoli footprints was. It is a collection made up of hundreds of footprints belonging to different species, including 69 footprints of clearly bipedal hominids; among them, curiously, some are intentionally superimposed, a fact that has been interpreted in different ways. For some specialists, afarensis must have been the author of the masses; for others, it is more likely to have been A. anamensis. The latter argue that: "A. anamensis has well-excavated tibial platforms, in which the femoral condyles fit snugly, as is the case with the present-day human knee. In contrast, the tibia of A. afarensis, represented by "Lucy", is different. It has an external tibial platform with a convex edge, which, in biomechanical terms, means that its knee would have some rotational capacity, very useful in arboreal life" *(19).
- Cf. Zeresenay Alemseged, et al.: A juvenile early hominin skeleton from Dikika, Ethiopia; Nature, Vol. 443, 21 September 2006, pp. 296-300.
- Bernard Wood: A precious little bundle; Nature, Vol. 443, 21 September 2006, p. 278.
- The text is taken from an interview Alemseged gave to Kate Wong for the online magazine Scientific American.com:http://www.sciam.com/article.cfm?articleID=0004A45D-44EC-1510-840D83414B7F00FF.
- Rex Dalton: The history man; Nature, Vol. 443, 21 September 2006, p. 269. This is a small article where Dalton pays tribute to Maurice Taieb, one of the leaders of the team that discovered Lucy in 1974, a few kilometres away from where Selam has now been discovered.
- A year earlier, the French-American research team, co-led by Maurice Taieb and Donald Johanson (a team that also included Yves Coppens and Tim D. White), had found the bones of a knee joint three million years old. What was surprising was that they showed a striking similarity to the knee of modern humans.
- For years it was debated whether Lucy was in fact a female or a male. For P. Schmidt and M Hanser Lucy is not a female but a male; something that Don Johanson, and many others, reject outright, as it would imply that afarensis females, due to sexual dimorphism, would have to be about half a metre long, which is highly unlikely.
- Although Selam is older than Lucy, it seems inevitable to succumb to the temptation to call her: "Lucy's Daughter", as Kate Wong suggests in her article: Lucy's Baby; Scientific American.com: http://www.sciam.com/article.cfm?chanID=sa003&articleID=00076C1D-62D1-1511-A2D183414B7F0000. Although, perhaps, it would be more appropriate to call her: Dikika's Child, according to the team's own expression researcher.
- Cf. Kate Wong's interview with Zeresenay Alemseged; op. cit.
- "At the Sima de los Huesos site in Atapuerca we have found a skull with its base practically complete, Skull 5, as well as most of two hyoid bones. We will have to wait for the culmination of the ongoing research on this extraordinary fossil material to learn more data about the origin of human speech" (Juan Luis Arsuaga and Ignacio Martínez: La especie elegida; Ediciones Temas de Hoy, Madrid, 1998, p. 314.
- A cranial reconstruction made from the remains of several individuals.
- Daniel Turbón: La evolución humana; Ariel, Barcelona, 2006, p. 118.
- "From the beginning I was a staunch supporter of thesis that afarensis was a strict terrestrial biped. But now it has to be recognised that it is not impossible that they still exploited some of the ecological niches in the trees, either for sleeping at night or for feeding". Cif. Kate Wong op. cit.
- Statement collected by Kate Wong; cif. op. cit.
- Lee R. Berger: Tras las huellas de Eva; Ediciones B, Madrid, 2001, pp. 192-194.
- Cif. Jonathan G. Wynn, et al.: Geological and paleontological context of Pliocene juvenile hominin at Dikika, Ethiopia; Nature, Vol. 443, 21 September 2006, pp. 332-336.
- William H. Kimbel, Meave Leakey, Donald C. Johanson, Yoel Rak, et al.: Was Australopithecus anamensis ancestral to A. afarensis? A case of anagenesis in the hominin fossil record; Journal of human evolution, Vol. 51, issue 2, August 2006, pp. 134-152.
- Lee R. Berger: Op. cit., p. 222.
- Daniel Turbón: Op. cit., p. 114.
- Ibidem; p. 122.