New data, new questions:The growing complexity of human evolution

Author: Carlos A. Marmelada. carlosalbertomarmelada@yahoo.es
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Human evolution is very complex

"I am convinced that human evolution between three and two million years ago is much more complex than we suspected" * (1). These words were written by the South African-based American palaeoanthropologist Lee R. Berger at the end of the last century, and he is certainly right.

Indeed, it is absolutely certain that human evolution is proving in recent years to be a much more complicated event than we had assumed until a few years ago. This is why the above-mentioned scientist, who works at the University of the Witvatersrand (Johannesburg, South Africa) sample , was very wise to observe that: "the more fossils that come to light, the less our family tree resembles a tall, magnificent sequoia with well-defined branches rising to the pinnacle of human achievement. On the contrary, it is more like a jagged hawthorn tree whose intertwining and lacerating branches would be dangerous to tangle. Many a scientific reputation has been torn by the spikes of this bush of human genealogy" * (2).

But it is not only Berger who sees things this way. In fact, there are already many great specialists in palaeoanthropology who are beginning to publicly recognise that: "human evolution is much more complex than was first thought" * (3). Thus Robert Boyd and Joan B. Silk consider that: "hominid evolution is more complex than we had imagined (...) Evolutionary theory gives us no reason to believe that phylogenies must be simple, nor that they are likely to be simple. They probably have many branches and look more like a bush than a tree" * (4). Roger Lewin is of the same opinion when he argues that: "the true phylogenetic tree of the hominid family, the only one that has actually occurred in evolutionary history, is almost certainly more branched than the one usually drawn by anthropologists" * (5).

According to Juan Luis Arsuaga, award Prince of Asturias and one of the three co-directors of the exceptional sites of the Sierra de Atapuerca: "the reason why we are still debating evolutionary theory is the enormous complexity of the problem" * (6). It follows that the fact that "the dynamics and rules of evolution are not completely known to us should not surprise anyone given the complexity of the problem" * (7).

The increasing complexity we are beginning to discover in human evolution means that now "less than ever we can be sure about the shape of the tree of human evolution (...) It is no longer so clear which are direct ancestors of Homo and which are lateral lines" * (8). This is why: "many anthropologists are beginning to accept that the hominid phylogeny is certainly more complex than has usually been described" * (9).

New discoveries new questions

There is one reality that seems to be a constant in the programs of study of human evolution. Every time a major finding is made in palaeoanthropology, it certainly provides invaluable information for a better understanding of our evolutionary past. But, automatically, two other things happen concomitantly. On the one hand, we discover that our ideas related to this fact were simpler than we initially supposed; and, on the other hand, many more questions appear than answers to the existing ones. José María Bermúdez de Castro is of the same opinion when he states that: "each finding comes to answer one or more questions, but always raises new questions" * (10).

The fact is that this is not an isolated opinion. On the contrary, it is gaining more and more adherents among specialists. In fact, it is not only the aforementioned co-director of the exceptional sites in the Sierra de Atapuerca who sees things this way. Other prestigious researchers, such as Francisco Ayala and Camilo José Cela Conde, also see things this way. These two scientists state that: "Each finding of a previously ignored fossil form usually resolves some of the previous doubts, but at the price of raising new ones that often produce the sensation that the panorama of our origins is something very confusing about which there is a lack of reliable knowledge. The authors sometimes question whether the traditionally accepted suggestion that finding of new fossils can resolve controversies about the interpretation of the evolutionary process is in fact reasonable. In the face of an already impressive and rapidly growing record, many classically trained palaeontologists are more confused than ever" * (11).

A concrete example of what we are talking about is what happened in the summer of 2007. Indeed, the publication, from mid-August to mid-September 2007, of four scientific articles* (12) has forced specialists to rethink whether, indeed, the understanding we have had so far of some of the stages of human evolution is really the correct view.

In principle, recently published works (dealing with discoveries of various human fossils made in recent years) should facilitate our understanding of the real evolutionary history of human beings, but this is not the case. On the contrary, the finding of new remains of Homo erectus and Homo habilis in Kenya, the study of the postcranial skeleton (i.e. from the neck down) of four humans from Dmanisi that were 1.77 million years old (from now on Ma.), the study of the shoulder and wrist of Homo floresiensis and the finding of some human fossils in Atapuerca (Spain) that could be up to one and a half million years old, are perplexing specialists in human evolution. This is to the extent that they are beginning to question whether a whole series of convictions strongly rooted in our traditional view of human evolution can continue to be upheld.

The new discoveries force us to reconsider whether Homo erectus is descended from Homo habilis or whether the origin of Homo erectus really lies in Africa rather than in Asia. Moreover, what is the role of Homo ergaster, is it still correct to consider it a good human species * (13), or is it nothing more than the African hypodigm of the erectus clade? * (14).

It doesn't end here. Indeed, the new discoveries made in the Sima del Elefante during the campaign force us to ask ourselves about the relationship between the humans who lived in Atapuerca 1.2 million years ago and those who were in Dmanisi. But, in addition, two new works published in the period mentioned above suggest that Homo floresiensis is really a human species different from ours, instead of microcephalic Homo sapiens with other diseases, but these same works raise new questions about their phylogeny. From which human species do the Homo floresiensis come from? Until now it was thought that Homo erectus came to the island of Flores more than 800,000 years ago, but new research is beginning to seriously question this evolutionary scenario.

The bewilderment caused at academic community by the finding of humans who are contemporary to us but so different morphologically reminds us of the great truth contained in the words of Juan Luis Arsuaga when he reminds us that: "the better we know about human evolution, the more we realise how extraordinarily complex it was, in issue of branches" * (15). It is for this very reason that: "today, rather than the "missing link", we must speak of many "small known pieces of possible links"" * (16). * (16).

Thus, as Antonio Rosas rightly says, what is happening is that "we now have more fossils, and that means we know more about some things, but there are also more unknowns" * (17). In any case, it is quite natural for this to happen because, as Jaume Bertranpetit and Cristina Junyent rightly point out: "the more we know, the more detailed we want to analyse the processes of change and, therefore, new gaps appear in the knowledge" * (18). In other words, it seems as if we are inevitably immersed in a dynamic that inevitably involves a whole new set of questions each time we ask a new important question finding . This leads us to the conviction that: "the hominin phylogenies that will emerge will begin to be more ramified, probably as a prelude to the recognition of greater complexity" * (19). Or as Lee R. Berger puts it: "Inevitably each new finding raises as many questions about the nature of early hominins and their relationships as it provides answers. It is possible that, while providing more complete data about early mankind, future findings will leave gaps in the fossil knowledge whose existence we do not now suspect" * (20).

Although this is true, the reality is that the publication of a small group of four papers has been enough to force specialists to rethink key aspects of the vision we have had until now of the history of our evolutionary past. Faced with this subject of situations, Lee R. Berger says that there is a feeling as if: "somewhere, somehow, another fossil will appear that will force a revision of the prevailing theories" * (21).

Thus, the reality of the facts is that: 'although some reports present human evolution as a well-known issue, the judgements of specialists are very different and much more cautious (...) The impression that everything is clear in this field is false, however often it is asserted' * (22). In fact: "the impression one gets when attending a scientific discussion like this is that there are many unanswered questions, and this is normal, because the more we know, the more questions we ask ourselves. I would divide them into two subgroups: a few recurrent questions that remain unanswered, and a multitude of new questions that only a few years ago we did not even ask ourselves, the result of new findings and new interpretations" * (23).

A common ground

The new works to which we have alluded all agree on the same point: they challenge the classical paradigm's view of the first and last stages of human evolution. What does this paradigm say?

Until now, it has been thought that our genus originated in central-eastern Africa between 3 and 2.5 Ma. ago, although the genus and species of pre-human hominid from which the first representatives of our lineage would have arisen is unknown. In any case, for the last twenty years or so, it has been thought that the first humans are members of the species Homo rudolfensis and Homo habilis. However, there are those who believe that neither of them are, strictly speaking, the first humans, since rudolfensis should be included in the genus Kenyanthropus (announced in 2002) and the latter in the genus Australopithecus. In any case, it is assumed that Homo habilis would have given rise in East Central Africa (possibly in the Lake Turkana region of Kenya) to Homo ergaster, who, on leaving Africa 1.8 Ma ago, would have transformed into Homo erectus during their journey to Indonesia.

This classical paradigm, which had been prevailing since the mid-1980s, began to be slightly challenged by the programs of study that appeared at the beginning of this century and which dealt with the humans who inhabited Dmanisi. Indeed, the disconcertingly small size of their skulls, as well as other archaic characteristics, related them more to Homo habilis than to Homo erectus; although, strictly speaking, they were not fully identified with either, so their discoverers decided to assign them to a new human species: Homo georgicus.

As far as the last stages of human evolution were concerned, it was believed that with the extinction of the Neanderthals 28,000 years ago, we had been left alone on the planet.

However, status suffered an unexpected twist when in October 2004 it was announced the finding of a new human species found on the island of Flores (Indonesia) and which had as main characteristics a very short stature leave (adults measured around one metre) and a very small brain (the skull recovered in the Liang Bua cave, belonging to an adult female that lived 18,000 years ago, had an endocranial volume of 417 cc). Their tiny brain is smaller even than that of the Dmanisi humans and the first members of our genus, the aforementioned Homo habilis. In fact, the brain of the floresiensis is more similar in size to that of the australopithecines of three million years ago.

New discoveries

The new work to which we are referring challenges this classical paradigm.

Indeed, on 9 August, the journal Nature published a article signed by Fred Spoor, Meave Leakey and others, in which they reported finding, in the year 2000, a Homo erectus calotte and a fragment of the right maxilla of Homo habilis, both found in the Kenyan locality of Ileret, on the eastern shore of Lake Turkana.

At first glance, these findings did not seem to be exceptional. After all, many other fossils of both species had already been found in the area. But the truth is that each of them had a peculiarity that made them unique in the entire world fossil record.

Indeed, the Homo erectus calotte, technically known as KNM-ER 42,700 (or, in other words, fossil no. 42,700 from the National Museum of Kenya found on the eastern shore of ancient Lake Rudolph), was the smallest of all those discovered so far and attributed to that species. Its endocranial volume is 691 cc and its age is 1.55 Ma, which raises a double question.

On the one hand, the brain volume is so small that one cannot avoid the comparison and (possibly) the relationship with the Dmanisi humans, who also had brains of a similar size (the smaller 600 cc. and the larger 780 cc.) Does this mean that the Turkana Homo erectus had something to do with the Dmanisi humans? Is it possible that, instead of being two different human species, they are two different populations of the same species?

The second issue relates to the Homo habilis mandible. Known technically as KNM-ER 42.703, it is 1.44 Ma* (24). This means that Homo habilis and Homo erectus coexisted in the same space for 200,000 years longer than previously assumed. Spoor, Leakey and collaborators have interpreted this fact as a test in favour that Homo erectus cannot descend from Homo habilis, as proposed in the classical paradigm, but that both species should come from a common ancestor and, from there, undergo a divergence that would lead them to follow parallel evolutionary histories, sharing the same space but without competing for the same resources, which would have made this coexistence possible for so many hundreds of thousands of years.

All this leads us to agree with Meave G. Leakey and Allan Walker when they state that: "a major question in palaeoanthropology today is how the anatomical mosaic of early hominids evolved" * (25), and the same applies to the study of the evolution of the anatomical mosaic of early humans. This statement is valid for the early stages of the evolution of the human race, as well as for its later stages. This is why Ian Tattersall is right when he argues that: "the emergence of Homo sapiens was not a linear transformation of one species into another, but the product of a very complex and branching evolution" * (26).

We have wondered before about the relationship between Dmanisi and Turkana humans. Well, on 20 September, the journal Nature published a work * (27) in which David Lordkipanidze and collaborators set out the conclusions of their study of 32 postcranial remains of three adults and an adolescent who had existed 1.77 Ma. The conclusion they reached was that they were about 1.5 metres tall and weighed between 40 and 50 kg, They had a combination of archaic characteristics (a small brain, leave stature and walking with the palms of their hands facing forward instead of towards the thigh of the leg), together with other modern characteristics that made them similar to us (such as the proportions between the upper and lower limbs or the structure of their feet, which allowed them to walk long distances).

In the light of the Dmanisi and Kenyan discoveries, Daniel E. Lieberman (paleoanthropologist at Harvard University in Massachusetts, USA) argues that those prehistoric humans from Georgia and the first Homo erectus from Africa belong to distinct populations (or hypodigms) of the same species that would have a high morphological variability* (28).

Ann Gibbons, in a article published in Science, gathers together the opinions of various specialists who disagree with the new proposals being made regarding the possible first steps of our genus. In response to the suggestion that archaic Homo erectus originated in the Caucasus or elsewhere in the Near East and that from there it migrated to Africa to give rise to theerectus, which then migrated to the Far East and Southeast Asia, paleoanthropologist Allan Walker (of Pennsylvania State University and co-discoverer of the Nariokotome Child, a fairly complete skeleton of Homo erectus from 1.5 Ma. ago) thinks that this is an unlikely scenario. In his view, the most plausible scenario is that H. erectus originated in Africa and migrated from there to other places (Dmanisi, Java, China, etc.) where local populations evolved separately, giving rise to distinct groups, but all of them from the same parent species.

As if all this topic were not tangled enough, there are specialists who question whether the Dmanisi hominids are really human or not. Anthropologist Bernard Wood (George Washington University, Washington D.C., USA) believes that the Georgia specimens are in fact intermediate forms between pre-human hominids and early Homo * (29).

The conclusion to be drawn from all this is that as we learn more about the early stages of human evolution, it is proving to be much more complex than we had previously thought. In this respect, Ian Tattersall's words are very apt: "In recent years it has become clear that the later stages of human evolution were much more eventful than has long been accepted. But the same is true of the early stages" * (30).

Degree All this gives the feeling that: "Doubts have grown about the confidence that any "account" of human evolution can inspire. How accurate and reliable can such reconstructions be? When one tries to approach the hominids of the past from today's point of view, irresolvable problems arise" * (31).

Indeed, in Washburn's opinion: "Most of the problems concerning human evolution remain unresolved.... As has happened in the past, researchers are likely to be wrong in those areas where they are most certain to be right. It seems advisable, therefore, to accept more than one hypothesis, and, rather than blindly corroborating their conclusions, simply to bet on them (...) In presenting my views in this way, I have tried to show that our ideas about human evolution are based on data sometimes very unreliable" * (32).

At final and... "as always, new evidence from the more remote human past has served mainly to underline the complexity of events in our evolution" * (33).

Notes

1. Lee R. Berger & Bertt Hilton-Barber: In the Footsteps of Eve. El misterio de los orígenes de la humanidad; Ediciones B, Barcelona, 2001, p. 345.
2. Lee R. Berger & Bertt Hilton-Barber: Op. cit.; p. 30.
3. Lee R. Berger & Bertt Hilton-Barber; op. cit., p. 62.
4. Robert Boyd and Joan B. Silk: How Humans Evolved; Ariel, Barcelona, 2001, p. 328.
5. Roger Lewin: Evolución humana; Ed. Salvat, Barcelona, 1994, pp. 238-239.
6. Juan Luis Arsuaga: The enigma of the sphinx. Las causas, el curso y el purpose de la evolución; place & Janés Editores, Barcelona, 2001, p. 26.
7.  Juan Luis Arsuaga: Op. cit; p. 11.
8. Antonio Rosas, in Mónica Salomé: La cuna africana del hombre; Conocer, nº 175, pp. 37 and 39.
9. Roger Lewin: Human Evolution; Op. cit., p. 239.
10. José María Bermúdez de Castro: El chico de la Gran Dolina; Ed. Crítica, Madrid, 2002, p. 27.
11. Francisco Ayala and Camilo José de Cela Conde: Senderos de la evolución humana; Alianza publishing house, Madrid, 2002, p. 86.
12. The papers in question are: Susan G. Larson et al.: Homo floresiensis and the evolution of the hominin shoulder; Journal of Human Evolution (2007). M.W. Tocheri et al.: The primitive wrist of Homo floresiensis and its implications for hominin evolution; Science, Vol. 317, 21 September 2007, pp. 1743-1745. Fred Spoor, Meave G. Leakey, Ian McDougall, et al.: Implications of new early Homo fossils from Ileret, east of Lake Turkana, Kenya; Nature, Vol 448, 9 August 2007, pp. 688-691. David Lordkipanidze et al.: Postcranial evidence from early Homo from Dmanisi, Georgia; Nature, Vol 449, 20 September 2007, pp. 305-309.
13. That is to say: different from the others.
14.  That is: the African population of group of the erectus.
15. Juan Luis Arsuaga: El collar del Neandertal; Ediciones Temas de Hoy, Madrid, 1999, p. 83.
16. Artigas, Mariano: Las fronteras del evolucionismo; Ed. Palabra, Madrid, 1992, p. 63.
17. Antonio Rosas, in Mónica Salomé: La cuna africana del hombre; Conocer, nº 175, August 1997, p. 36.
18. Jaume Bertranpetit & Cristina Junyent: Journey to the origins. A biological history of the human species; Ed. Península, Barcelona, 2000, p. 109.
19. Roger Lewin: Op. cit., p. 245.
20. Lee R. Berger & Bertt Hilton-Barber: Op. cit.; p. 42.
21. Lee R. Berger & Bertt Hilton-Barber: Op. cit.; p. 64.
22. Mariano Artigas: Las fronteras del evolucionismo; Ed. Palabra, Madrid, 1992, pp. 57-63.
23. Jaume Bertranpetit; in VV. AA.: Antes de Lucy. El agujero negro de la evolución humana; Tusquets Editores, Barcelona, 2000, p. 286.
24.  F. Spoor, M.G. Leakey, I. McDougall, et al.: Op. cit. This very recent antiquity makes it the fossil of this species that is closest in time to us.
25. Meave G. Laekey and Allan Walker: Ancient fossil hominids in Africa; research and Science, August 1997, p. 73.
26.  Ian Tattersall: Contemporary hominids; research and Science, no. 282, March 2000, p. 18.
27. D. Lordkipanidze et al.: Op. cit.
28. D. E. Lieberman: Homing in on early Homo; Nature, Vol. 449, 20 September 2007, p. 290.
29. A. Gibbons: A new body of evidence fleshes out Homo erectus; Science, vol. 317, 21 September 2007.
30. Ian Tattersall: From Africa... again and again?; research and Science, June 1997, p. 20.
31. David Pilbeam: Origin of hominoids and hominids; research and Science, no. 92, May 1984, p. 49.
32. Sherwood L. Washburn: The Evolution of the Human Species; Ed. Labor, Barcelona, 1979, p. 137.
33. Ian Tattersall: Out of Africa ... and back again; Op. cit.