One of the most important discoveries of recent times, related to our origins, is the verification of a close genetic affinity between human beings and chimpanzees. Such affinity is so significant (98.8%) that it is estimated that both phylogenetic lines that led to the two species had a common ancestor approximately 6 to 8 million years ago. Since that moment, and after the branching occurred, begins the most amazing and passionate history, that of our own family.
The “hominization process” refers to the origin and evolution of the hominid family (forms that comprise both our ancestors and ourselves). When trying to reconstruct this process, paleoanthropology acquires an especial relevance, because while giving us the historic evolutionary fundamentals of man, it shows us the same basics of anthropology as science. If the uniqueness of this is its approach to man as a bio-psycho-socio-cultural unity, that of paleoanthropology resides in its intent to answer the question of how this unity was developed.
Hominization was not only a process of biological transformation nor a simple adding of unidirectional causes and effects. On the contrary, we think that this process was a complex event, characterized by the succession of particularly significant synergic phenomena resulting from a dynamic interaction of biological, psychological, spiritual, social, and cultural variables.
Our knowledge about this process has undergone remarkable improvement during the last decades. Nevertheless, there are still some questions of complex solution. One of them derives from the fact that when we go back in time to track down our biological past in search of our own family first members, we face a great difficulty in identifying them in the fossil record. In our opinion, the main reasons for this difficulty are the following:
- The material known until now, between 4 to 8 million years ago, is barely representative due to its scarcity and to its being usually fragmented and deteriorated.
- To define which are the physical features that show hominid nature in a fossil is an act always conditioned by each historical moment ideology.
- Despite the current establishing of these features, the valuations of fossils’ anatomic traits are so varied that they do not obtain consensus among scientists.
The first reason becomes obvious if we think about the amount and quality of available material to be studied and analyzed by specialists. As for the second, we discuss below two illustrating occurrences: the famous Piltdown “discoveries” and the one performed by the so-called ramapithecines. In both cases, we will see how they were highly valued in their moment, for they clearly fulfilled the expectations of the time. Finally, we will discuss the varied interpretations supported at present concerning the finds of likely hominids with over a 4-million-year dating.
In the first decades of the 20th century, scientists thought that the most significant hominization phenomenon was the development of the brain. In this way, the existence of a decisive step or “cerebral Rubicon” was supposed, between 700 to 800 cc, the so-called step to reflection. Therefore, a skull fossil with a cranial capacity below 700 cc could not be regarded as human, while one over 800 cc could.
This explains the impact that the Piltdown “discoveries” had at their time. These “discoveries” were made in Sussex, England, including a modern-looking braincase, with cranial capacity over 800 cc, and a very primitive lower jaw. It was regarded as obviously “human” and was named Eoanthropus (“The Dawn Man”).
For 40 years, the Piltdown “discoveries” were considered legitimate. But in 1953, the British Kenneth Oakley, using the “fluorine technique” on the bones, proved that owing to their different fluorine percentages, those fragments could not belong to a single individual. In fact, it would be soon found out that those “discoveries” were fakes, and the whole affair turned out to be a big fraud. The braincase had actually belonged to an Upper Pleistocene individual, while the jaw came from a modern orangutan, deliberately altered to make it look primitive and ancient.
Some years later, “ramapithecines” would come on the scene. Ramapithecine fossil fragments were considered representative of the “first hominid” and had unanimous acceptance during the 1960s and 1970s, then they were completely abandoned. This episode’s significance in paleoanthropology history deserves more detailed discussion.
In 1932, during a Yale University expedition to Siwalik Hills (north of India), graduate student G. Edward Lewis found a maxilla with two first molars, two premolars, and canine and incisor teeth alveoli. From the alveoli shape, it could be derived that the canine barely exceeded other teeth in height and that the incisor was inserted almost vertically, reducing its prognathism.
The aforementioned features led Lewis to name this specimen Ramapithecus brevirostris (Rama, alluding to a hero in Indian mythology; pithecus, ape; and brevirostris, Latin, “short face”). When trying to place it within the family it belonged, Lewis recorded, “hominid”? This question was left without an answer because nobody cared about it in those days. It is obvious that if Australopithecus was not regarded as a hominid, much less would be the more primitive Ramapithecus. Almost 30 years were necessary for the topic to be considered again during the 1960s.
In 1961, Helson Mukuri, Louis Leakey’s first field assistant, found a fragment of a left upper maxilla that still had the canine tooth, the first premolar alveolus, the second premolar, and the two first molars. This find took place in Fort Ternan, northeast of Lake Victoria (Kenya). Leakey immediately noticed the hominid characteristics of the finding and named it Kenyapithecus wickeri, after Kenya Colony and the man who owned the site, Charles Wicker.
In his first scientific communication, Leakey called attention to the resemblance between Kenyapithecus and Ramapithecus, pointing out that both had features that put them closer to hominids than to pongids. Yet he gave greater significance to Kenyapithecus.
Meanwhile, Elwyn Simons, of Yale University, would revitalize the Ramapithecus significance as probable first hominid, due to its small canines, its reduced diastema, its thick dental enamel, and its nonprotruding short face (unlike apes). Later, in 1963, David Pilbeam joined the team. Together, they would carry out a remarkable synthesis task by observing that not only were Ramapithecus brevirostris and Kenyapithecus wickeri alike, despite the very long geographic distance between them, so were fossils found in other locations (Greece, Turkey, Hungary, to name a few). By this reason, they proposed to include all of them into the generic name Ramapithecus (which had priority because of its being coined first).
Furthermore, Simons and Pilbeam endorsed the idea, speculative but in agreement with the thoughts of the time, that the canine reduction in Ramapithecus was not an isolated trait. It was, as Darwin had expressed it, a result of a greater use of the hands owing to the manufacturing of tools and weapons. This would suggest bipedalism.
By then, the “granivorous hypothesis” was known, proposed by Clifford Jolly of the University of New York, who, in the 1970s, provided valuable data gathered from his studies on the gelada baboon (Theropithecus gelada) of Ethiopia. These data reasserted the significance of the canine reduction (thus consolidating Ramapithecus as likely first hominid). However, they also showed that this reduction was not a result of a greater use of the hands (as the Simons and Pilbeam hypothesis propounded), but of ingesting tough foods, such as grains and seeds. To eat this kind of food requires premolar and molar movement from side to side, for grinding it, an impossible procedure with very large canines.
In the meantime, during the 1960s, there were interesting innovations in the biochemistry, molecular biology, and genetics fields. These innovations would have a strong influence regarding Ramapithecus within the primate phylogeny. In 1962, Emile Zuckerkandl and Linus Pauling proposed to apply molecular biology to the study of human evolution. They thought the molecular study could provide equal or greater information than fossils in matters involving the construction of genealogical trees. Thus, “molecular anthropology” was born. By then, biologist Morris Goodman, of Wayne State University (Detroit), had published some data about the immunological properties of albumin. These properties showed that chimpanzees, gorillas, and human beings were closely related, while gibbon and orangutan were more distant relatives of that group.
Some time later, in 1967, American biochemists Vincent Sarich and Allan Wilson, of the University of California (Berkeley), studied the chemical structure of protein molecules in gorillas, chimpanzees, and human beings. As a result of this work, they concluded that the perceived differences among them could give some accuracy about the time their evolutionary lines drifted apart. In this way, the “molecular clock” was created, based on the principle that when these two groups began to split, each of them started to accumulate mutations that would create differences between them. Therefore, the greater the elapsed time, the more accumulated differences between the two groups would exist.
By applying the “molecular clock,” estimates were made about the branching of the lines that led to present chimpanzees and hominids. This branching would have occurred approximately 5 million years ago. Also, taking this split into account, for molecular biologists, it was clear that to search for a hominid in the fossil record before this branching was useless, despite appearances that might suggest the contrary. This obviously made Ramapithecus, at 14 million years ago, lose every chance for being regarded as a hominid, although most paleoanthropologists still believed that at the time.
In fact, paleoanthropologists regarded Ramapithecus as a hominid for almost 20 years. But in the early 1980s, some revealing finds would be made, particularly face fragments of two fossils named Sivapithecus. One of these was found in Turkey, and Pilbeam and his team found the other one in Pakistan. At once, the resemblance between Sivapithecus and Ramapithecus was recognized. Furthermore, attention was paid to the new fossil resemblance to the present orangutan. Then, it became obvious that Ramapithecus was more related to the phylogenetic line that led to present orangutans than to hominids.
This corroboration, along with the available biochemical, molecular, and genetic evidences, convinced David Pilbeam and most paleoanthropologists in the following years that Ramapithecus could no longer be regarded as a hominid. Moreover, nowadays its name has been completely lost, being absorbed by Sivapithecus.
Today, there is ample consensus in the international scientific community that bipedalism alone is a clear identifying trait for the hominid nature of a being. It is an anatomic change of major importance not only due to what it means in itself but because it became a “trigger” of significant later changes. Yet even agreeing with this point, some authors consider the possibility that early hominids did not develop bipedalism, although they had dental modifications (for example, the canine reduction) that could be regarded as an indicator of the beginning of the evolutionary process that led to modern humans.
Fortunately, the last 12 years have been especially plentiful for findings of probable hominids 4 to 7 million years old. Here, we will proceed from the more recent to the older ones, starting with the only one that is indisputably hominid. We speak about the one found in 1995 by Meave Leakey, of the Kenya National Museum, and her team, southeast of Lake Turkana in Kenya, named Australopithecus anamensis ( anam means “lake” in the Turkana language). The found materials were a jaw, some face and forearm fragments, and a tibia, dated between 4.1 and 3.9 million years ago. It is the oldest of all australopithecines known until now.
Unlike chimpanzees, which have a deep oval depression in the lower part of the humerus against which the ulna is flexed to improve the elbow articulation strength and make possible the typical displacement on the knuckles, A. anamensis lacks this feature. Moreover, the anterior ridge of its tibia is a clear indicator of bipedalism.
Also in the 1990s, more precisely between 1992 and 1993, Tim White, of the University of California (Berkeley), directed a research project in Middle Awash, Ethiopia, where his team discovered hominid fossils in the Aramis Basin. Afterward, these fossils were named Ardipithecus ramidus (Ardipithecus means “ground primate,” and ramidus means “roots” in the Afar language). It was dated 4.4 million years ago, and from the kind of related fauna, its habitat was a densely wooded zone.
At first, the occipital hole location led the finders to think that Ardipithecus was biped, which would have made it the oldest hominid known up to that moment, but new studies do not seem to corroborate this idea. However, the hominid nature of Ardipithecus could reside in its canine reduction.
Between 2001 and 2004, Tim White, together with Gen Suwa, of the University of Tokyo, and Ethiopian paleontologist Yohannes Haile Selassie, of the Natural History Museum of Cleveland (Ohio), found teeth and several bone fragments. At first, they were attributed to an older and more primitive variety of Ardipithecus ramidus. Afterward, these fossils were confirmed to belong to a different species, Ardipithecus kadabba, with an established antiquity of 5.2 to 5.8 million years. In the opinion of these authors, A. kadabba could have been the last common ancestor of chimpanzees and hominids.
In October 2000, a French-Kenyan team directed by Martin Pickford, of the Community Museums of Kenya, and Brigitte Senut, of the National Museum of Natural History (Paris), found some fossils within the Lukeino Formation at Tugen Hills (Kenya). These fossils comprised small mandibular fragments, isolated teeth, manual phalanxes, and upper and lower limbs bones of several individuals. They were named Orrorin tugenensis (Orrorin means “original man” in the Tugen language) and dated about 6 million years ago.
While its canines are long and the arms bones and the fingers show climbing adaptations, Orrorin tugenensis was probably biped. Its lower limbs were strong, and it had a long femoral neck, with a groove in its back, a possible consequence of the pressure exerted by the obturator externus muscle during bipedal locomotion.
In July 2002, a finding of the prior year was published. Michel Brunet, of the University of Poitiers (France), and its French-Chadian team made this finding in the Toros-Menalla site, Djurab Desert, northern Chad (Central Africa). A well-preserved skull, two maxilla fragments, and three teeth were named Toumai (“hope of life” in the Goran language) and scientifically classified as Sahelanthropus tchadendis. Potassium-argon or paleomagnetism could not been applied, but the surrounding fauna were compared to that of Lukeino, at Tugen Hills (6 million years), and that of Nawata, at Lothagam (5.3-7.4 million years). As a result of this, Toumai antiquity was estimated between 6 and 7 million years ago (closer to the last figure).
The fossil shows a real “mosaic of characters,” a mix of both primitive and advanced features. In the opinion of Bernard Wood, of George Washington University, the front of the Toumai skull is that of a true australopithecine, while the back is that of a chimpanzee. For Michel Brunet, the traits that suggest its hominid condition are the face structure, the kind of superciliary arches, and the small canines. Since there are not postcranial bones, nothing can be said about its locomotion mode, save the fact that by its occipital hole location, Brunet maintains that Toumai probably had been biped.
However, Milford Wolpoff, of the University of Michigan, John Hawks, of the University of Wisconsin, and the aforementioned Martin Pickford and Brigitte Senut disagree with Brunet; they claim that the indicated traits do not ensure a hominid diagnostic in any way. Thus, the canine smallness, for example, would not be such because its width is similar to the mean of chimpanzee males or females and of gorilla females. On the other hand, while these authors set aside the continuous supraorbital torus as a result of strong sexual selection, as Brunet states, they think that the size and form of the supraorbital structures are a mechanical response to strain from anterior teeth.
But Brunet disregards that Sahelanthropus had a powerful masticatory complex. Based on experimental investigations that seem to demonstrate that the strains exerted by mastication on primate eyebrow arches are always very small, too small as for generating bone growth responses, he firmly claims that the supraorbital size is due to a strong sexual selection. For Brunet and his colleagues, Toumai is indisputably the oldest hominid ancestor known up to date. If it were so, it would show that first hominids had a much greater geographic dispersal than what was thought until now.
Undoubtedly, it is very difficult for authors to establish a distinction among the members of the oldest ancestors of present chimpanzees, the members of the oldest hominids, and the ways these two lines drifted apart. The whole is a real “Gordian knot” if we consider its remarkable complexity. The distinction between these two groups always depends on the evaluations of different physical characters authors could make. When something is significant for some researchers, sometimes it may not be so for others. Owing to this, the chances of conciliation in these matters are rather complex because, in common practice, authors tend to overvalue the significance of their own findings.
Thus, Martin Pickford and Brigitte Senut think that Orrorin tugenensis would be the common ancestor of all hominids, while Ardipithecus would be the common ascendant of australopithecines, not of Homo. On his part, Tim White rejects this interpretation, questioning the Orrorin hominid status and regarding it as a likely ancestor of present chimpanzees. On the other hand, for Michel Brunet, Sahelanthropus is the oldest hominid ancestor known until now, while White regards it as a kind of archaic ardipithecine, and Wolpoff, Hawks, Pickford, and Senut disregard it as a hominid candidate, assuming it is an ape, probably a gorilla female ascendant.
These varied interpretations of the same facts show the high degree of subjectivity existing in the studies of our origins. Certainly, in paleoanthropology there is not an objectivity concept equal to the Newtonian physics-mathematics one. Is all this a display of paleoanthropology vulnerability as a scientific discipline? We do not think so. On the contrary, we are sure that the variety of opinions and discrepancy among authors is proper to science and, without doubt, enriches the discussion. Advancements of knowledge of our past have always happened, and they will keep happening, as a consequence of hypotheses modifications and their adjustment to new data or new available information. Only in this way will paleoanthropology keep achieving increasing acknowledgment at the scientific level.
In time, the best, most-convincing explanations are the ones that last, while the others are put aside. But frequently, an explanation that had been put aside some time before is revitalized afterward with more available information, or a good explanation is replaced by a better one. If this is absolutely natural in science, we must understand that the same will happen in such a young discipline as paleoanthropology.
The available fossil documentation related to first-hominid origin shows great complexity. No morphological discontinuity is observed, and, as we know, in continuity it is very hard to establish clear differences. Besides, if their evolution was not linear but treelike, as most authors propound nowadays, we must think of a significant diversification of forms, in such a way that some hominid-like anatomic traits could have existed in several groups at the same time. If it were so, it would be extremely hard to identify which of them first became Homo.
Surely, the discussion about the “first hominid” will not end. Nevertheless, we think it is important to point out that we run the risk of “not seeing the woods for the tress,” as the saying goes. Hence, our opinion is that beyond any singularity of character and relevant significance of the “first hominid” determination (if this were possible), we must not lose sight of an even more significant fact: the process as a whole. The significant thing is the process, and this is indisputable. Very slight differences at the beginning would later increase progressively in changing the phylogenetic line, which led to something absolutely original. It is the process as a whole, and not just a moment of it, that explains our present nature. We are what we are because we have a long and complex evolutionary history behind us.
References:
- Brunet, M., et al. (2002). A new hominid from the Upper Miocene of Chad, Central Africa. Nature, 418, 145-151.
- Haile Selassie, Y. (2001). Late Miocene hominids from the Middle Awash, Ethiopia. Nature, 412, 178-181.
- Leakey, M. G., et al. (1995). New four-million-year-old hominid species from Kanapoi and Allia Bay, Kenya. Nature, 376, 565-571.
- Senut, B., Pickford, M., et al. (2001). First hominid from the Miocene (Lukeino Formation, Kenya). Académie des Sciences de Paris, Sciences de la Terre et des planetes, 332, 137-144.
- Wolpoff, M., Senut, B., Pickford, M. Y, & Hawks, J. (2002). Sahelanthropus or Sahelpithecus? Nature, 419, 581-582 (includes Brunet et al. reply).
