The uncertainty of human phylogenies

Author: Carlos A. Marmelada

1. Introduction

There is an amusing amecdote that illustrates very graphically the topic we want to deal with in this article. On one occasion, during a television discussion between Donald Johanson and Richard Leakey (two of the most famous specialists in human evolution), the former took out one of the several panels he had in one of those big folders used by cartoonists and said to Leakey something like: "I'd like to know what you think about this", and showed him his phylogenetic proposal for the human family tree. Richard Leakey looked at it and turned to the camera and exclaimed, "Gee... I didn't come so well prepared! I didn't come so well prepared! But my opinion is this..." He then took a thick-tipped marker pen and drew a big X on Johanson's panel crossing out its contents and then wrote a big question mark. "This is what I think!" exclaimed Leakey to the dumbfounded gaze of a bewildered Johanson.

Years later, another great scholar, Roger Lewin, warned us that: "a review of recently published physical anthropology books and articles reveals a rather diverse collection of proposed hominin phylogenies. This diversity of professional opinion is instructive, for it confirms that "the phylogeny of hominids is far from settled"". *(1). These words of Roger Lewin were written more than a decade ago, but they are still valid today.

Palaeoanthropology, or the science of human evolution, is doubly exciting. On the one hand, it arouses the passion that is inherent in any science, insofar as it allows us to expand our knowledge of reality. On the other hand, its fascination stems from the fact that its object of study is ourselves, so that the results of its research allow us to increase, and deepen, our knowledge of our biological origins and our own evolution.

At final, human palaeontology aims to unravel the mystery of our origin and development to become who we are. In other words, it aims to find out the biological pathway of our lineage, using the tools of the scientific methodology of research from disciplines as diverse as: comparative Anatomy , archaeology, palaeoclimatology, palaeobotany, geology, etc.

In order to establish the phylogenetic tree of our species, palaeoanthropologists have to make phylogenies in which the multiple specimens that appear fit together coherently. Until the mid-1980s, the phylogeny of the human lineage was very simple. Australopithecus afarensis was the oldest hominid known to date and, at three and a half million years old, was at the basis of almost all human phylogenies at the time. According to its discoverers (including the aforementioned Johanson), two branches branched off from it. One branch led to Paranthropus (australopithecines with a very robust craniofacial architecture), via Australopithecus africanus (as was thought at the time). The other led directly to the genus Homo. This genus seemed to have an evolutionary subject anagenetic evolution with a really very simple structure, in which species followed one another linearly with a clear descendant-ancestor relationship. Thus Homo habilis had originated from Australopithecus afarensis and, over time, had given rise to Homo erectus; which, in turn, had given rise to Neanderthals on the one hand and Homo sapiens (i.e. us) on the other. Admittedly, the simplicity of this phylogeny was a truly idyllic picture for a student who had to take an exam on this topic .

But this intellectually bucolic horizon has undergone a radical turnaround thanks to the new discoveries made in the last twenty years; as well as the importance of these discoveries * (2). Although it is true that: "the simple classical outline based on three genera (Australopithecus, Paranthropus and Homo) had difficulties in adequately including all known specimens assigned to Hominidae, even before the recent finding of hominid specimens and the proposal of four new genera for them (Ardipithecus, Orrorin, Kenyanthropus and Sahelanthropus)" *(3).

2. The new landscape: the difficulty of establishing phyletic relationships

The new discoveries made are, logically, very well received by the entire academic community dedicated to the study of our origins, as they have allowed us to make a great qualitative leap in the understanding of the evolutionary process of hominids in general and humans in particular, and all this in very few years. But it is also true that the new findings have not only not solved the great enigmas of human evolution, in fact not a single one of them has been solved; on the contrary, they have brought new questions, and many of them of a non-trivial significance.

To begin with, all these new discoveries have to be fitted into a phylogeny that coherently contains all the specimens of the family Hominidae. Naturally, the wish of all specialists is to be able to clearly establish the phyletic relationship between all the genera and species of our biological family. But this is by no means a simple matter. Indeed, "the most difficult problem for the palaeontologist is to establish the Degrees of evolutionary kinship" *(4).

In the opinion of Juan Luís Arsuaga, establishing the Degrees of evolutionary kinship is, precisely, the maximum to which we can aspire according to the cladistic school; and, according to the opinion of the members of this school: "it is impossible to be sure that a fossil species is the ancestor of another, fossil or living, since no one is truly capable of travelling back in time to follow the course of evolution. The only thing that can be scientifically established is the Degree of kinship between species"*(5). The point is, as the co-director of the research work at Atapuerca points out, that "the veracity of an evolutionary scenario as such cannot be proven" *(6).

At this point, a necessary distinction must be recalled. Indeed, scientific hypotheses are one thing and "evolutionary scenarios" quite another. Scientific hypotheses are based on the objective description (as objective as possible) of fossils, trying to reduce interpretations to a minimum. Evolutionary scenarios, on the other hand, are highly speculative hypothetical constructs, which attempt to bring together into a coherent whole the various isolated evolutionary hypotheses that we derive from the study of fossils. Evolutionary scenarios are interpretations and therefore easily susceptible to error. In evolutionary scenarios we have to replace the gaps in our knowledge with imaginative proposals based on reasonable but unprovable assumptions. Thus, for example, as we see that chimpanzees today use small twigs impregnated with saliva to "fish" for termites, we project this same behaviour onto australopithecines three million years ago*(7 ) , assuming that at this point they had a behaviour similar to that of the panidae. Of course, we will never be able to prove that australopithecines did anything similar to what we have just described.

This way of proceeding, or method, is called actualism. Although actualism is very suggestive, it has its limitations. Actualism has to be applied with caution, since nowadays: "doubts have grown about the Degree confidence that any 'account' of human evolution can inspire. How accurate and reliable can such reconstructions be? [Because] when one tries to approach [the study of] hominids from the perspective of the past from today's perspective, irresolvable problems arise" *(8).

In fact "some palaeontologists have mistakenly acted as if actualism consisted in looking for a living species, the most similar to the fossil species we are investigating, and attributing all aspects of the biology of the living species to the extinct one (...) The proper way to apply actualism consists, instead, in discovering in the present living world the laws that govern it, and using them to interpret the fossils" *(9).

It is very important to always bear in mind that when carrying out programs of study on human evolution one should not confuse the objective data obtained from the study of fossils and the context in which they have been found with the hypotheses that are elaborated from them and, even less so, with the evolutionary scenarios configured from the attempt to harmonise a set of hypotheses with each other. In other words, in palaeoanthropology, when drawing conclusions, one must be very careful to avoid, as far as possible, confusing description with interpretation, and both with supposition. This is why Jean Chalin says: "discoveries are one thing and their interpretations and explanations are another" *(10). That is to say: it is one thing to describe, another to interpret, and quite another to suppose or speculate. All three are necessary to try to obtain a coherent vision of human evolution, but we must not forget that they imply different levels of certainty Degrees .

However, when all this is forgotten, controversy seems inevitable, which is why Tim D. White, co-discoverer of several hominin genera, species and subspecies, reminds us that: "The controversy [over phylogenies] revolves around very different interpretations of the fossil record" *(11).

3. Biological species, palaeospecies and time-travelling machines

Why can we not be sure of the veracity of an evolutionary scenario in which we are told that these fossils belong to this species and those to that species? The reason is very simple and stems from the very concept of species. Indeed, since Ernst Mayr proposed it, a biological species is defined as a set of individuals that are capable of reproducing among themselves and having fertile offspring. In other words, two individuals belong to the same species if they are able to interbreed genetically and have grandchildren. We leave aside the question of hermaphroditism and asexual reproduction.

This criterion cannot, of course, be applied to extinct species. But then: how can we be sure that, for example, a male Homo rudolfensis could not mate with a female Homo habilis and have offspring capable of producing offspring? At present, there is no way to prove that this was possible or impossible. This is why human evolutionary scholars do not use the biological species concept as we have defined it here. Instead, they resort to the concept of "palaeo-species" or "chrono-species"; so they categorise individuals into different genera and species according to the greater or lesser morphological difference. And, for the moment, this is what we have to make do with, since: "until we have a machine that allows us to go back in time, no hypothesis concerning the relationships of subject ancestor-descendant can be confirmed" *(12).

Indeed, it seems that only by travelling back in time to check in situ whether, for example, an afarensis male could or could not mate with an anamensis female, having (or not) offspring and these offspring, will we be able to say, without any doubt, whether or not they belonged to different species. Obviously this is not possible; so the palaeoanthropologist, as we have already pointed out, will have to make use of other resources when establishing evolutionary relationships, i.e. phylogenetic trees.

But it is not surprising that this is so, since: "if there are problems of classification with living species, let the reader judge what problems the palaeontologist encounters when working with fossils" *(13).

4. Another source of problems: the precariousness of the fossil record

Certainly not financial aid much of the fact that the hominid fossil record is sometimes, in fact too often, very thin indeed. And there have been many, many discoveries in the last two decades. But, without a doubt, there is still a lack of material.

This state of affairs means that human palaeontology sometimes becomes: "a discipline in [which] sometimes an entire species is named and classified on the basis of a single tooth" *(14). There are few occasions when we have a good collection of remains belonging to the same individual. In fact, when studying hominids older than two million years: "most of the fossil specimens discovered are small fragments: a piece of skull, a cheek bone, a portion of the arm bone and many teeth. Species identification from this subject of such scanty evidence is no easy task and sometimes impossible" *(15). And status is not much better for most species less than two million years old.

It is curious, but it seems as if this status is very specific to palaeoanthropology, since: "in any other discipline it would be adventurous to use such a small sample to try to determine a universe" *(16). This status is very complex and delicate and, in fact, makes: "the task of constructing a phylogenetic tree showing how each group descends from the previous ones is difficult and arduous.... The first difficulty is the very partial and fragmentary character of the fossils..... It is practically impossible to state with certainty, in any case, that they are the true ancestors or descendants in the tree sought. The palaeoanthropologist is confronted with a jungle of data which is very difficult to relate to each other... Each supposed evolutionary step makes it necessary to resort to unknown intermediate forms.... Hypotheses are necessary for each of the hypotheses that lead from one being to another in the first primates, and the same is true for the origin of the simiformes, the catarrhines, the hominoids and the hominids" *(17).

No wonder, then, that there are specialists who warn us that: "phylogenetic reconstruction of long extinct organisms from the incomplete fossil record is a dangerous exercise" *(18). This is why it is worth remembering that: "the task of establishing an evolutionary link on the basis of extremely fragmentary evidence is more difficult than many people realise, and there are many traps for the unwary" *(19) . *(19). 

Given this situation: "placing fossil finds on this framework and establishing the relationships between the fossils themselves are subjects of ongoing and usually heated debate, with little consensus" *(20).

On the other hand, and for the last fifteen years or so, almost every time a team from research makes an important finding they shake up a good portion of the phylogenetic tree of hominids and make a new proposal . For this very reason, one ends up with the feeling that: "somewhere, somehow, another fossil will appear that will force a revision of the prevailing theories" *(21). In other words, there are no certainties and all our knowledge about human evolution seems to be purely provisional. This is why some people draw pessimistic conclusions from these views and argue that: "the uncertainties of the hominid fossil record easily discourage us and make us doubt that we know certain facts about our ancestors" *(22).

The precariousness of the fossil record leads to an uncomfortable and unscientific status , according to which: "in human palaeontology there are often more opinions and interpretations than fossils" *(23). This state of affairs means that, as we said before, interpretations are given excessive value, so that evolutionary scenarios can end up being defended tooth and nail; sometimes even with a truly dogmatic attitude, far removed from the weight and equanimity that is supposed to be part of the critical (and self-critical) attitude that a scientist is supposed to have. And the fact is that: "palaeontology, being such an interpretative science, leads to personal rivalries" *(24). There are occasions when this status is so strong that: "the interpretation of a single muscle could lead to fierce public confrontations between scientists" *(25); so that: "at conferences, some were excluded from their group for the simple fact of addressing a member of the opposing side" *(26).

At the other extreme, there are those who imply that we already know almost everything about human evolution and that only a few specific details remain to be known. However, the truth is that "there are still many important questions about human [evolutionary] history for which there is no answer final, for example: what is the exact shape of the human family tree? *(27).

5. (Eternally) missing links

Perhaps the notion of the "missing link" is the most popular scientific concept in human palaeontology; the one that has achieved the greatest diffusion and popularity among the general public. The reality is that in the phylogenetic tree of humans, and in that of hominids in general, there are a good issue of missing links; and not just one as, very simplistically, it is said. So: "rather than speaking of the "missing link", we must speak of many "known small pieces of possible links"". *(28). Therefore: "it would be wise to take good note grade of an observation by Gould, which is certainly serious, since it refers to concrete facts from his specialization program and concerns the basic evidence for evolutionism: "the family trees of the lines of evolution that adorn our manuals contain data only at the extremities and at the nodes of their branches; the rest are deductions, certainly plausible, but not confirmed by any fossil". It should therefore be clearly pointed out that the lines and arrows linking these extremes are hypothetical, and not presented as certainties or as the only possible explanation" *(29).

work The fact that there are still so many missing links to be discovered leads Juan Luís Arsuaga to state that "there is still a lot of work to be done in the field of palaeontology to have a clearer idea of how we got here" *(30). Our lack of knowledge about these missing links means that there are still "several questions about the possible "process of hominisation", which are very difficult to answer with the current scientific data , which are clearly insufficient to do so" *(31).

It is true that it was the publication in 1859 of Darwin's On the Origin of Species that gave a definitive boost to the theory of evolution. However, despite such an ambitious degree scroll , the reality is that to this day there are still serious discrepancies as to which is the most accurate explanatory model for this fact. Moreover, there are authors of undoubted prestige who even state that: "we still don't know exactly how evolution works" *(32). What is certain is that: "although the degree scroll of Darwin's work is The Origin of Species, it is a topic that continues to present enormous difficulties: more or less plausible suppositions are made, but, of course, no one has ever seen the transformation of one species into another" *(33).

The method of scientific research requires the ability to test, by experiment, the explanatory hypotheses of experience, but: "since evolution is a process on a scale far beyond the human scale, no one can actually experiment with it and claim to have observed how it occurs" *(34).

When trying to specify the phylogenetic tree of our lineage, we are faced with a plurality of gaps and, therefore, uncertainties involving both the bifurcation points and some of the linear series of these phylogenies. The unknowns appear at the very base of the tree, since: "the phylogenesis and, consequently, the taxonomy of hominoids is one of the most controversial questions in primate palaeontology. How to classify humans, their ancestors and their closest relatives is still, a century and a half after Darwin pointed out the close relationship between the African great apes and our species, a controversial question" *(35). Roger Lewin is of a similar opinion and considers that: "given the fossil gap that precedes the fossil hominoid record and the even greater hiatus that precedes the modern African great apes, any conjecture about the identity of the ancestor of modern hominoids can only be a conjecture" *(36).

Things do not improve when we try to find out which hominoid primate gave rise to the hominid family, in which our genus is encompassed, since: "establishing connections between hominids and fossil primates is a difficult task; in fact, between both phases there is a real palaeontological gap where fossils only allow us to establish a hypothetical and uncertain continuity" *(37). This status derives from the fact that: "the fossil record provides very little information about the evolution of the human lineage during the late Miocene, between 10 and 5 million years ago (...) Unfortunately, the late Miocene fossil record tells us little about the creature that connects the great apes of the jungle with modern humans" *(38). Moreover, when discussing the hominoids of the second half of the Late Miocene it is always worth remembering that as far as topic is concerned: "the divergence between gorillids, panids and hominids (...) we do not have fossils of the first beings of these three families. In fact, there are authors such as Greenfield and Ciochon who, with Darwin, maintain that we would never find them, or rather, that if we had them in front of us, we would not be able to distinguish them as such. Their features would not have differed sufficiently to enable us to identify them as panidae, gorillids or hominids" *(39).

The same is true when we want to determine which species of pre-human hominid gave rise to our genus. It is usually believed that it was some species of australopithecine that gave rise to Homo; but: "only when we have more fossils will we be in a position to affirm that the genus Australopithecus was the direct ancestor of man; for now the fossils allow us to give the same Degree of veracity to the different phylogenies" *(40)

This state of affairs sometimes translates into a structural scepticism that leads some palaeoanthropologists to take the view that: "it is perhaps shocking to say, but there is a possibility that none of the fossils so far discovered in eastern, southern or north-west Africa are the direct ancestors of living humans" *(41). Similarly, Roger Lewin is of the opinion that: "establishing phylogenetic relationships vertically or within any particular time period therefore becomes extremely risky, if not downright impossible, with today's patchy fossil record" *(42). And speaking: in absolute terms the fossil collections are quite sparse" *(43). This may be a very extreme sceptical stance, but the fact is that: "it is not easy to know how things happened in the remote past (...) and it is not even easy to reconstruct the tree of human evolution, the genealogy of the species" *(44). For this very reason: "until we have a clearer understanding of the taxonomy, phylogeny and chronology of hominids, we cannot seriously answer questions such as the coexistence of the ancestor and descendant species, which is why, for the time being, they remain problems to be solved in the future" *(45).

Thus, "in the evolutionary paths leading ultimately to our own species, we occasionally find branches that multiply the traces, but we also find gaps in the fossil record that hide them, making it very difficult to reconstruct the phylogenesis of a given period" *(46).

The topic of the multiple missing links is not a trivial issue. In fact: "the gaps in the palaeontological record are too large to draw definitive conclusions" *(47).

Of course, this status could be overcome: "if we could have one fossil specimen for every six months or one year of development of all hominid species, we would be able to solve the complicated puzzle of the phylogenetic relationships of all these species" *(48). In other words: "only when we have different infant and juvenile forms of all hominin species can we have a more accurate picture of the phylogenetic relationships between these species and the processes that have determined the changes in shape, size and morphology throughout our evolutionary history" *(49).

Obviously, this is an idyllic but unrealistic status , since it will never happen; so we have to get used to the idea that the fossil record will always have many irremediably structural gaps, and this by virtue of the very internal dynamics of evolution, whether gradual or punctuated.

In any case, it would be wrong to conclude that the picture is extremely dark and that at subject of human evolution we know very little about which we have any certainty. What is certain is that we know many things, and very important things; and, fortunately, in the last fifteen years we have made a great leap forward, both quantitatively and qualitatively, as far as the knowledge of our evolutionary history is concerned.

Nor do we believe that it is correct to state, without further ado, that each finding sinks us deeper into ignorance: "if we conclude that as the number of available specimens increases, so does the perplexity, we would be doing an injustice. result In reality, the problems caused by new discoveries are, in many cases, the result of having previously held hypotheses that were excessively speculative and lacking in basis (...) When risky assumptions are made, it is not difficult for the appearance of new data to make them doubtful, sometimes giving the impression that whatever is said about the fossils will end up being wrong. The truth is that this is not the case (...) within the multitude of dubious hypotheses, there are more than a few well-established principles about how the process of differentiation between the ancestors of human beings and those of our closest living relatives went" *(50).

What is really happening is that as more and more discoveries are made, we realise more and more that the twists and turns of our evolutionary history were much more complex than we thought until a few years ago, so that the finding of new fossils always contributes to a better understanding of our evolution, at least grosso modo; But at the same time, new questions arise that are precisely the result of the increase of our knowledge, and this is due to the simple fact that we are entering areas of which we previously had no idea or, at best, only a cursory glimpse reference letter. However, the main issue is different. The important thing is that, for the moment, we still do not know the answer to the fundamental questions of human evolution, despite the great discoveries that are being made. In this sense, it can be said that we are not much further from where we were a century and a half ago.

6. Intellectual a priori in evolutionary theories

Incredible as it may seem to the uninitiated subject, there are times when difficulties in establishing phylogenies arise not only from gaps in the fossil record but also from personal prejudices, i.e., from the preconceived ideas of researchers. Some say that, for example, "anthropologists disagree agreement on how to classify Middle Pleistocene hominins. This disagreement over classification stems from differing ideas about how humans evolved" *(51). A famous historical example of the influence of ideological bias is the controversy over the "Taung Boy".

These intellectual a priori that come into play in the elaboration of our interpretative hypotheses of human evolution and our evolutionary scenarios are even reflected in the names given to genera and species. This is why: "discussions among specialists about the scientific names of fossils may, for the rest of us, be mere academic discussions. But in reality they are of great ideological importance" *(52).

The thinking in vogue at the time can strongly influence the formulation of evolutionary hypotheses *(53); to the extent that, in the opinion of some authors, "you find what you look for" *(54). The influence of intellectual and cultural a priori, memes, is a fact, so that "in establishing a phylogeny there is always the danger of being influenced by our expectations and taking an invalid characteristic for granted or leaving out a valid characteristic. As scientists we are obliged to be objective, although this is not always easy, because the morphology of the fossils we deal with is often very poor" *(55).

It seems inevitable that the interference of subjective ideological factors in the elaboration of evolutionary scenarios (and even in the formulation of evolutionary hypotheses) cannot disappear completely, because although "science set out, starting with the so-called scientific revolution of the Baroque (in the 17th century), to eliminate all emotion and all ideology (religious or political) from its work, with the aim of reaching knowledge goal . Despite this good purpose, scientists are human beings and are conditioned by our environment and our Education. We do our best not to be influenced by what surrounds us, but we must recognise that it is easier to do objective science by studying the atom, butterflies or volcanoes, than by tackling the thorny question of the human condition" *(56). In this sense, Jordi Agustí reminds us that "a palaeontologist does not work with fossils, but... with ideas about fossils" *(57). And the fact is that "the finding" is not epistemologically neutral, but its incidence depends largely on the ideological environment in which it is framed" *(58). The truth is that, a priori, it seems simple to draw up a cladogram, but the reality is quite different and when the moment of truth and: "Despite claims of objectivity, the question of which cladogram is the "best" depends very much on the choices made at each step" *(59).

7. Conclusion

We have already said above that, in our opinion, the conclusion to be drawn is not one of pessimism about the possibilities of our knowledge goal on our own biological evolution. However, it is true that: "a review of recently published physical anthropology books and articles reveals a rather diverse collection of proposed hominin phylogenies. This diversity of professional opinion is instructive, for it confirms that "hominid phylogenesis is far from settled"." *(60)

In this sense, the words of Juan Luís Arsuaga are of particular interest when he reminds us that: "the phylogenies that we palaeontologists draw up may look very impressive from the outside, but those of us who construct them know how much of a house of cards they are, and how much of pure speculation" *(61). For this reason, there are authors who believe that "with the discoveries of recent years, one can be less certain than ever about the shape of the tree of human evolution" *(62).

Notes

1. Roger Lewin: Evolución humana; Ed. Salvat, Barcelona, 1994, p. 240.

2. Cf. Carlos A. Marmelada: La evolución humana. Los descubrimientos más recientes; http://www.educarm.es department de Evolución Humana; and Universidad de Navarra: http://www.unav.edu/web/ciencia-razon-y-fe/evolucion-humana-los-descubrimientos-mas-recientes.

3. Francisco Ayala and Camilo José de Cela Conde: Los géneros del linaje humano. The Spanish version is published in F. Ayala: La evolución de un evolucionista; Universitat de València, Valencia, 2006, p.340. The English version is published in Procedings of the National Academy of Science, PNAS, vol. 100, no. 13, 24 June 2003, pp. 7684-7689.

4. Juan Luis Arsuaga and Ignacio Martínez: La especie elegida; Ediciones Temas de Hoy, Madrid, 1998, p. 144.

5. Juan Luis Arsuaga: El collar del Neandertal; Ediciones Temas de Hoy, Madrid, 1999, p. 37.

6. Ibid.

7. Juan Luis Arsuaga: The Aborigines. La alimentación en la evoluicón humana; RBA Libros, Barcelona, 2002, p. 21.

8. David Pilbeam: Origin of hominoids and hominids; research and Science, no. 92, May 1984, p. 49.

9. J. L. Arsuaga: The Aborigines; op. cit., pp. 73-74.

10. Jean Chalin: A million generations. Towards the origins of humanity; Ediciones Península, Barcelona, 2002, p.221.

11. Tim D. White: Australopithecines; Scientific World, January 1983, no. 21, p. 27.

12. Juan Luis Arsuaga and Ignacio Martínez: La especie elegida; op. cit., p. 143.

13. Ibidem; p. 41.

14. Robert Boyd and Joan B. Silk: How Humans Evolved; Ariel, Barcelona, 2001, p. 290.

15. Richard Leakey: The Origin of Humanity; Ed. discussion, Madrid, 2000, p. 44.

16. Lee R. Berger and Bertt Hilton-Barber: In the Footsteps of Eve. El misterio de los orígenes de la humanidad; Ediciones B, Barcelona, 2001, p. 43.

17. Mariano Artigas: Las fronteras del evolucionismo; Ediciones Palabra, Madrid, 1992, pp. 59-62.

18. Robert Boyd and Joan B. Silk: op. cit., p. 310.

19. Richard Leakey: op. cit., p. 27.

20. Lee R. Berger: op. cit. p. 42.

21. Ibidem; p. 64.

22. Robert Boyd and Joan B. Silk: op. cit; p. 328.

23. Leslie C. Aiello: The African Cradle of Man, Conocer, No. 175, August 1997.

24. Lee R. Berger: op. cit.; p.67.

25. Ibidem; 194.

26. Ibid.

27. Richard Leakey: op. cit.; pp. 17-18.

28. Mariano Artigas: op. cit.; p. 63.

29.  Ibidem; pp. 95-96.

30. Juan Luis Arsuaga: El enigma de la esfinge; place & Janés, Barcelona, 2001, p. 351.

31. M. Artigas: op. cit.; p. 166.

32. Yves Coppens: Lucy's knee. Los primeros pasos hacia la humanidad; Ed. Tusquets, Barcelona, 2005, p. 47.

33. Ibidem; p. 107.

34. Juan Luis Arsuaga: The Enigma of the Sphinx; op. cit., p. 93.

35. Francisco Ayala and Camilo José de Cela Conde: Senderos de la evolución humana; Alianza publishing house, Madrid, 2001, p. 86.

36.  Roger Lewin: Evolution; op. cit., p. 167.

37. José Alcázar Godoy: El origen del hombre; Ed. Palabra, Madrid, 1986, p. 43.

38. Robert Boyd and Joan B. Silk: op. cit., p. 290.

39. Francisco Ayala and Camilo José de Cela Conde: Paths of human evolution; op. cit., pp. 144-145.

40.  José Alcázar Godoy: op. cit., p. 71.

41. Lee R. Berger: op. cit., p. 344.

42. Roger Lewin: op. cit., p. 167.

43. Ibidem, p. 231.

44. J. L. Arsuaga: The Enigma of the Sphinx; op. cit., p. 9.

45. Ibidem, p. 316.

46.  Francisco Ayala and Camilo José de Cela Conde: Paths of Human Evolution; op. cit., p. 98.

47. Jean Chaline: A million generations; op. cit., p. 281.

48. José María Bermúdez de Castro: El chico de la Gran Dolina. En los orígenes de lo humano; Ed. Crítica, Madrid, 2002, p. 14.

49. Ibidem, p. 267.

50. Francisco Ayala and Camilo José de Cela Conde: Paths of Human Evolution; op. cit., p. 86.

51. Robert Boyd and Joan B. Silk: op. cit, p. 377.

52. Juan Luis Arsuaga: op. cit., p. 176.

53. F. Ayala and C.J. Cela Conde: Paths of human evolution; op. cit., p. 173.

54. Ibidem, p. 148.

55. Meike Köhler, in VV. AA.: Antes de Lucy. El agujero negro de la evolución humana; Tusquets Editores, Barcelona, 2000, p. 274.

56. J.L. Arsuaga: The Aborigines; op. cit., pp. 129-130.

57. Jordi Agustí: El secret de Darwin; Ed. Rubes, Barcelona, 2002, p. 44.

58. Ibid.

59. Francisco Ayala and Camilo José de Cela Conde: Los géneros del linaje humano; op. cit., p. 341.

60. Roger Lewin: op. cit., p. 240.

61. Juan Luis Arsuaga: The enigma of the sphinx; op. cit., p. 338.

62. Antonio Rosas, in Mónica Salomé: La cuna africana del hombre; Conocer, nº 175, August 1997, p. 37.