Lights and shadows in the study of human evolution

Author: Carlos A. Marmelada
Published in: Aceprensa; Service 27/02
date of on-line publication : 27.II.02

Expansion of article Progress and unknowns in human evolution, published in Aceprensa; Service 27/02 of 27.II.02.

In school or educational books, and in museums or public exhibitions, diagrams of human evolution are often presented, showing the passage from one species to another. They begin with a long-armed, hairy being, walking hunched over, and the figures gradually move upwards until they take on our form. In reality, these depictions are highly hypothetical, as the certainties of science about the phylogeny of Homo sapiens (contrary to what many may think) are riddled with murkiness, and we often have to resort to more or less reasonable assumptions to fill in the many gaps in our interpretative narratives of the exciting human evolutionary history. A review of recent major advances in palaeoanthropology, as well as the unknowns that have accumulated, will provide a synthesis of the current state of our scientific knowledge on the origin and evolution of humans.

The first common ancestor of all hominids

The 21st century has begun in the most spectacular way that scientists investigating the origin of human beings could ever have imagined. Until the late 1990s, the oldest known hominid (Ardipithecus ramidus ramidus) was thought to be 4.4 million years old (ma.). This was considered an extraordinary age and, although hominids of an earlier age were not ruled out, the possibility of finding them was almost a pipe dream. That is why the world of human palaeontology was so surprised when Martin Pickford and Brigitte Senut announced in early December 2000 that they had discovered in the Tugen Hills, in the Baringo region of Kenya, the remains of a bipedal hominid six million years old!

Remains of Orrorin tugensis found by Pickford and Senut's team in autumn 2000 in the Tuguen Hills, Kenya. source: http://www.modernhumanorigins.com/lukeino.html

The specimen in question was initially known as Millenium man. However, when in February of the following year its discoverers made the official presentation of their programs of study *(1) they gave it the technical name Orrorin tugenensis. For its discoverers, Orrorin was the first common ancestor of all hominids. Through the so-called Preanthropus (a new genus of hominids composed of hitherto poorly catalogued specimens) Orrorin would have given rise to the human genus. According to Pickford and Senut, Ardipithecus ramidus, on the other hand, would be the common ancestor of Australopithecus, but not of humans.

The discoverers of Ardipithecus reject this interpretation, even questioning the hominid status of Orrorin, arguing that it could even be the last common ancestor of the genus Pan (which includes the two currently recognised chimpanzee species: Pan troglodites, also known as the "common chimpanzee"; and Pan paniscus, the misnamed "pygmy chimpanzee", which is none other than the bonobo). In addition, in July 2001, new fossils were found, assigned to Ardipithecus ramidus kadabba*(2), with an age of almost 5.8 Ma, insisting on the idea that it is the first member of the hominid family.

Far from being controversial, in the summer of 2002, the French paleoanthropologist Michel Brunet announced *(3) the finding of a skull, popularly known as "Toumaï", assigned to a new genus and a new species of hominid: Sahelanthropus tchadensis, and dated at an antiquity of almost seven million years. Spectacular indeed. For Brunet, Sahelanthropus would be the first representative of the hominid family.

Toumaï, a badly fragmented skull of Sahelanthropus tchadensis, the presumed first hominid, seven million years old. source:http://images.tvnz.co.nz/news/africa/toumai_skull.jpg.

However, some scientists have not passively accepted this proposal. Pickford, Senut and Wolpoff have denied that Sahelanthropus is a hominid *(4). Brunet has flatly rejected this*(5). For their part, the discoverers of Ardipithecus maintain that Sahelanthropus could be an archaic Ardipithecine.

As can be seen, it is still far from clear what the first hominid was. Indeed, some authors even doubt that, by definition, it will ever be discovered, since, they believe, it would be impossible to distinguish it from the last common ancestor of hominids and chimpanzees, or from the first common ancestor of all chimpanzees.

Australopithecines

Which hominid species gave rise to Australopithecus? At present we do not know. What we can say is that Australopithecus anamensis is the first specimen that we know for certain was already a hominid. Discovered by the team of Meave Leakey and Alan Walker in Kanapoi and Allia Bay, on the shores of Lake Turkana (anam means lake in language turkana), it is 4.2 million years old and was already a bipedal being *(6).

Slightly more recent is Australopithecus afarensis, the species to which belongs the famous partial skeleton of Lucy*(7), a specimen found by Donald Johanson and Maurice Tieb's team in Hadar, Ethiopia, in 1974. Ranging in age from 2.9 to almost 4 Ma, these creatures were also quite efficient bipeds. At the Tanzanian site of Laetoli, Mary Leakey's team found 3.6 Ma old footprints in 1979. It is not known to which species the individuals that produced them belonged, and it may never be found out, but the individuals that left their footprints already had a very efficient adaptation to bipedal walking.

Another species of Australopithecus is the Africanus. It was the first known species of australopithecine and to it belongs the famous skull of the Taung child described by Raimond Dart in 1924. Recently it was announced the finding of a skeleton of africanus that would have been between 3.2 and 3.5 Ma. The age of the members of this species is approximately between 3.5 Ma. and 2.3 Ma.

In 1995, Michel Brunet discovered at Abel, a jaw remains which he attributed to a new species of australopithecine: Australopithecus bahrelghazali. This finding is very important, because "The Bahrelghazali fossils overturn a hypothesis on human evolution, defended by Yves Coppens, according to which the training of the Rift Valley divided the only ancient species, isolating the ancestors of the hominids on the east side, from the ancestors of the anthropomorphs on the west side.... But new fossils from Chad show that ancient hominids lived west of the Rift Valley" *(8). This has led Craig Feibel (partner of Meave Leakey) to ask the question core topic: "Have we so far found fossils only in the East Rift Valley because that is where man's ancestors lived or because we have not looked elsewhere? *(9).

Another species of australopithecus, also found in 1995 by Tim D. White's team in Bouri, Ethiopia, although made known a little later: Australopithecus garhi, is considered by its discoverers, to the surprise (and this is precisely what its name means) of almost the entire academic community, as the possible author of some tools found not far from its fossil remains.

With regard to Australopithecus, we do not know what the phylogenetic relationships between them were, that is to say: we do not know what the ascending-descending kinship they may be related to. As we have already said, we also do not know what species and genus of hominid they come from. Each discoverer of a supposed primitive hominid considers his discovered specimen to be the one that gives rise to Australopithecus, but, as we have already seen, the picture is far from clear on this point. The same is true of the evolutionary outcome of each of the species of australopithecine mentioned. For each of their discoverers, their species is the one that gave rise to the human genus. But the truth is that no one has been able to prove that the human genus comes from this or that species of australopithecine. Indeed, there are even those who argue that our genus does not originate from any australopithecine. So what species and genus do we humans come from?

Trying to answer this question brings us back to an extremely polemical topic , where opinions multiply and disputes become particularly heated. Traditionally, it has been claimed that the human race originated from a species of gracile australopithecine. But which one? For Meave Leakey and Alan Walker, it is Australopithecus anamensis, a species they discovered in Kanapoi (Kenya). For Donald Johanson, it is A. afarensis, which he and others discovered in Hadar (Ethiopia). Phillip Tobias, Ron Clarke and Peter Lee Berger think it is A. africanus, a species they work with in South Africa. Tim White, on the other hand, believes that it was A. garhi, a species he found in Bouri (Ethiopia), that gave rise to humankind. And Michael Brunet favours A. bahrelghazali, which he unearthed in Chad in 1995.

As can be seen, opinions multiply and only agree on two points: one, that we do not come from a robust australopithecine *(10); and the other, that for each researcher cited, its fossil is the precursor of the human genus. To make matters worse, some authors (Louis, Mary and Richard Leakey) argue that we do not come from an australopithecine but from an as yet undiscovered species of hominid. According to others (Pickford and Senut), what is happening is that we have considered australopithecines to be fossils that are not australopithecines and that they include in a new genus: Praeanthropus, which they claim derives from Orrorin and gives rise to mankind.

The human race

As can be seen, we do not know where the human race came from. But the matter is much more complicated, because we are not even sure which were the first humans. The most orthodox academic tradition attributes this primacy to Homo habilis (2.4-1.8 Ma.). But other authors believe that the earliest human species is H. rudolfensis (2.5-1.8 Ma.). Some claim that habilis and rudolfensis actually belong to the same species, only that there was a strong sexual dimorphism, so that the most robust fossils belong to rudolfensis and the most gracile to habilis.

However, all this is called into question by other specialists. Thus, Bernard Wood argues that H. habilis is not only not the first human species, but does not even belong to the genus Homo; according to the British scientist, it would be more appropriate to place it among the australopithecines. Meave Leakey and Allan Walker, discoverers of the genus Kenyanthropus*(11)H. rudolfensis is the one that is not human, since, according to them, it would be one of the species belonging to the genus Kenyanthropus, to which the species K. platyops (literally: "flat-faced Kenyan man") would also belong.

The first species whose Anatomy is accepted worldwide as human is H. erectus. But here, too, we run into serious questions: Do all the fossils from between 1.2 and 1.8 Ma, found in Asia and Africa, belong to the species erectus? According to some authors, this would be the case if we accept a marked sexual dimorphism. For others, on the other hand, they are two different species. The youngest and most gracile fossils would correspond to erectus, and the oldest and most robust to H. ergaster.

There is also no unanimity as to who the first toolmakers were. Louis and Mary Leakey, discoverers of H. habilis, named it as such because they considered it to be the first to carry out this activity. But voices are already beginning to be heard claiming that some australopithecines, Garhi in particular, could have developed lithic industries. The controversy is on; although, for the moment, the evidence in favour of garhi is not very solid.

The colonisation of the world from Africa

Humans had already left Africa 1.8 Ma ago, as evidenced by remains found at the Dmanisi site in the Georgian Caucasus. These humans, originally attributed to H. ergaster but now assigned to a new human species, Homo georgicus*(12), are the oldest known humans to have approached Europe. But we do not know when man first penetrated the European continent. The most remote human remains found on European soil and whose dating is uncontroversial are those belonging to the TD 6 level at the Gran Dolina site in Atapuerca (Burgos), which belong to a new species: H. antecessor. They are almost 800,000 years old, one million years younger than those from Dmanisi.

The next remains found in Europe are those from Ceprano (Italy), dated to around 600,000 years ago. Although they were initially of doubtful specific assignment, they are now attributed to Homo antecessor. They are followed in time by the Boxgrove fossils, 500,000 years old, which belong to H. heidelbergensis. Did antecessor give rise to heidelbergensis? According to the current directors of the Atapuerca excavations, yes; but in the opinion of other authors, it would not be necessary to assume this phylogeny, since heidelbergensis could have descended directly from the African ergaster.

As far as H. sapiens is concerned, it seems to have left Africa much later. According to programs of study of Genetics molecular, our ancestors belonged to a population somewhere in Africa that lived around 150,000 to 200,000 years ago, perhaps more *(13). However, modern human behaviour, which includes elements such as abstract thought and symbolism, appears to be at least 70,000 years old, as indicated by the remains found at the Blombos Cave site in South Africa, which appear to be the oldest evidence of symbolic behaviour.

What is the origin of Neanderthals and Sapiens?

At the moment we are unable to answer that question. For some, the species from which we emerged must have been H. rodhesiensis, which is disputed as to whether it is a "good species" or whether it is really just the heidelbergensis-African. What we do know is that heidelbergensis most probably gave rise to neanderthalensis; at least this is indicated by the anatomical similarity between the two, although the Neanderthals were less corpulent. But the big question is whether Neanderthals were a different species from us (H. neanderthalensis), as most researchers believe; or whether, on the contrary, they and we formed two subspecies of the same species: H. sapiens neanderthalensis and H. sapiens sapiens. programs of study recent genetics support the thesis that they belong to different species.

In any case, 28,000 years ago, and after having coexisted in Europe for just over 10,000 years with our ancestors, Neanderthals completely disappeared from the fossil record.

Why did the Neanderthals become extinct? This remains one of the many unresolved questions in human palaeontology. Some have argued that we wiped them out by force, but there is no conclusive evidence to support this. Others believe that we gave them diseases that decimated their population. This is a reasonable hypothesis, but we have no evidence for it either. It is impossible to suppose that the cause was maladaptation to a worsening climate, as some have proposed; we must not forget that the Neanderthals had more than 200,000 years of existence behind them in a Europe that had gone through several episodes of cold climates, even glaciers; in any case, we were the ones who had it worse, since it is assumed that the first H. sapiens who populated Europe came from temperate regions of Africa.

Some authors suggest that Neanderthals did not actually become extinct, but that they interbred with the anatomically modern humans that arrived in Europe, and the two species mixed their gene pools to give rise to today's Europeans. This is a romantically appealing hypothesis, but the genetic programs of study conducted to date not only does not support it, but contradicts it.

However, Erik Trinkaus argues that a 24,000-year-old skeleton, called the Lapedo do Lagar Velho, found near the Portuguese city of Leiria, is a hybrid of Neanderthalensis and Sapiens, since, along with anatomically modern characters, there are hints of Neanderthal morphology. But many scientists do not accept that the Lapedo menino is truly a hybrid. They claim that it is the fossil of a child who died at the age of four, so that its young age prevents a clear appreciation of the morphological features that really specify an adult individual.

Evolutionism does not contradict creation

Sometimes the scientific data provided by the sciences in charge of studying human evolution are used for ideological purposes to disqualify the religious and metaphysical speech . However, many scientists believe that there is no incompatibility between the scientific view of human evolution, religion and philosophical anthropology.

Thus, Giovanni Carrada maintains that: "Biblical revelation clarifies for us the reason for the appearance of man and, in passing, places us before our dignity as God's creatures. The two explanations (scientific and religious) are compatible; what is more, neither of the two separately is sufficient to satisfy man's desire to know his origins and the meaning of his life" *(14). Along the same lines, Antonio Fernández-Rañada asks: "Why can evolution not be the means chosen by God to create the world in a continuous process? *(15). He goes on to add: "Christian doctrine does not imply the separate creation of species, but its central idea, the truly important one, is that everything owes its existence to a God transcendent to the natural order, and this is not affected by Darwin's theory. After all, why can evolution not be God's chosen way of creating the world?" *(16).

Even a physicist like James S. Trefil argues that: "The fact that human beings evolved from lower forms of life does not harm the fundamental tenets of anyone's religious beliefs. If we take Christianity as an example, evolution is simply irrelevant to the doctrine of salvation through faith or to any other important teaching " *(17).

Annexes

New hominin genus discovered: Kenyanthropus Platyops

In recent years, several new hominin species have been discovered, turning what was once an austere winter trunk into a flourishing bush with more branches being added every day. One of the most striking episodes came on 22 March 2001 when Meave Leakey presented to the media at speech : Kenyanthropus platyops ("Flat-faced Kenyan Human") *(18). The name is somewhat misleading because it is not a human species. Moreover, Fred Spoor (co-author of research) has stated that it is not yet known whether or not K. platyops is included in the evolutionary line leading directly to our species, although - according to the research team - it is most likely not.

Its discoverers propose the constitution of a new genus of hominids, the Kenyanthropus, which would be added to the other four so far proposed: Ardipithecus, Australopithecus, Paranthropus and Homo. And if it is confirmed that Orrorin tugenensis (the controversial and famous "Millenium Man") is also a hominid, as proposed by its discoverers (Martin Pickford and Brigitte Senut) yet another genus and species should be added to the list, which should be increased if the hominid status of Sahelanthropus tchadensis is confirmed.

Between 1998 and 1999 Meave Leakey's team found numerous fossils in the Lake Turkana area. Of particular note was a skull (known as KNM-WT 4000) found by Justus Erus near the Lomerkwi River. The skull had very striking features. Indeed, the ear Anatomy , for example, is archaic and reminiscent of the ear of the oldest Australopithecus and even chimpanzees. On the other hand, the high cheekbones and flat cheeks are surprisingly modern features. It is dated to be between 3.2 and 3.5 million years old, so it is a species coeval with the famous Lucy. According to Meave Leakey, the finding of platyops confirms that the hominin family tree is highly diversified and that several different hominin species coexisted over time and even shared spaces and habitats*(19).

But that's not all, the platyops discoverers propose that Homo rudolfensis, so far considered by some to be the first human being, should leave the genus Homo and be included in the genus Kenyanthropus. proposal which will undoubtedly be the talk of the town.

proposal a new human species

The exceptional Georgian site of Dmanisi (85 km southwest of Tbilisi, the capital of the Republic of Georgia) in the Caucasus continues to surprise academic community. In the last decade, a number of human remains have been found that until now have been considered to belong to the species Homo ergaster. However, programs of study recently found remains indicate that they may belong to a new human species: Homo georgicus.

In 1991, when archaeological excavations were being carried out in the medieval city of Dmanisi, a very well preserved human jawbone was discovered in situ (and not in the drawers of a museum, as has sometimes been written). From the outset, the fossil aroused the interest of academic community as it was a human remains that could provide clues about the settlement of Europe. At that time, the current paradigm postulated that this occupation had taken place before half a million years ago. The fossil discovered in Dmanisi was, according to its discoverers, about 1.8 million years old! (c. 1.8 Ma.). But it was precisely this fact that provoked the most misgivings, so that, as happens in these cases, the finding was received by the academic community with scepticism and mistrust. Even so, it was estimated to be no less than 1.2 million years old, 700,000 years older than the oldest European fossil known at that time: the Mauer jawbone, belonging to a Homo heidelbergensis and dated at 500 k. (500,000 years old).

In May and July 1999, two human skulls were discovered at the same site*(20). By then, the disbelief of academic community about the age of the Dmanisi human fossils was giving way to perplexity, as all the programs of study data from various independent teams confirmed the figure of almost 1.8 Ma. The age of the remains and their structure meant that they were initially assigned to Homo ergaster, but the volume of the skulls*(21) puzzled researchers. David Lordkipanidze, one of their discoverers, already stated at the time that this fact suggested that the skulls might belong to a new human species.

Two years later, a new skull (in many respects even smaller than the previous ones) was found associated with a mandible. Although scientists initially classified it as Homo erectus/ergaster, they warned that it might belong to a Homo habilis *(22). However, at the beginning of October this year, eminent scientists such as Marie-Antoinette de Lumley (from the high school of Human Palaeontology in Paris), David Lordkipanidze (from the National Museum of Georgia) and Abesalom Vekua (from the Georgian Academy of Sciences), published in Les Comptes Rendus de l'Académie des Sciences a article in which they affirm that all the remains found in the Georgian site of Dmanisi must be assigned to a new human species: Homo georgicus.

In principle, they were the first humans to leave Africa, 300,000 years earlier than previously thought. The small size of their brain volume is surprising. Furthermore, the technology found so far at the site is very simple (Technical Mode I), which contradicts the classical hypothesis that the first humans to leave Africa must have had a high level of encephalisation and relatively developed technology *(23).

Initially it could be assumed that Homo georgicus could be a descendant of Homo rudolfensis/habilis and an ancestor of the Asian Homo erectus. Now its relationship to Homo ergaster, the species to which the remains of Homo georgicus have hitherto been attributed, is more difficult to establish. Nor is its relationship with the oldest human fossils found in the interior of the European continent clear, which are those of Homo antecessor, found in the Sierra de Atapuerca in Burgos. Their anatomies are different, so we will have to wait for new human fossils to be discovered in Europe to have more information and to know whether or not Homo georgicus spread throughout western Europe.

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Notes

1. Brigitte Senutt, Martin Pickford et al.: First hominid from the Miocene (Lukeino Formation, Kenya); C. R. Acad. Sci. Paris 332, pp. 137-144, 2001.

2. Y. Haile-Selassié: Late Miocene hominids from the Middle Awash, Ethiopia; Nature, Vol. 412, pp. 178-181, 2001.

3. Cf. Michel Brunet, David Pilbeam, Yves Coppens et al.: A new hominid from the Upper Miocene of Chad, Central Africa; Nature, 418, pp. 145-151, 11-VII-2002. Cf. also Patrick Vignaud et alt.: Geology and palaeontology of the Upper Miocene Toros-Menalla hominid locality, Chad; Nature, 418, pp. 152-155, 11-VII-2002.

4. Milford H. Wolpoff, Brigitte Senut, Martin Pickford, John Hawks: Paleoanthropology (communication arising): Sahelanthropus or 'Sahelpithecus'?; Nature, 419, pp. 581-582, 10.

5. M. Brunet: Paleoanthropology (communication arising): Sahelanthropus or 'Sahelpithecus'?; Nature, 419, p. 582, 10.X.02.

6. Bipedalism has not yet been conclusively demonstrated in Ardipithecus, is questioned in Orrorin, and is presumed in Sahelanthropus, although, strictly speaking, it has not yet been irrefutably demonstrated in any of these three specimens.

7. For a detailed study of their skeleton, see Tim White: The Australopithecines. Scientific World; no. 21; pp. 18-31; 1983.

8. Cf. Meave Leakey and Alan Walker: "Ancient hominid fossils in Africa"; research and Science, August 1997; p. 75.

9. Conocer magazine; nº 175; August 1997; p. 56.

10. Paranthropus, literally: apart from man, are sometimes considered as an independent genus of hominids, while at other times they are considered simply as Australopithecus of subject robust, as opposed to those of subject gracile, which would be the species cited so far. The difference between gracile and robust would not make reference letter to the postcranial skeleton, but only to the cranial anatomical differences derived from the feeding subject , which in the case of the robust, being of an abrasive nature, would require a much more powerful and "robust" chewing apparatus. The Paranthropus species identified to date are: P. Aethiopicus (between 2.6 and 2 ma.), P. Boisei (between 2.6 and 1 ma.) and P. Robustus (between 2 and 1.2 ma.). Dates are always approximate.

11. We have not commented on this genus of hominids in the text, but the reader interested in more information about it can consult the appendix corresponding to Kenyanthropus platyops, at the end of this article.

12. Cf. the annex: proposal a new human species.

13. Cf. Aceprensa, service 177/01.

14. Giovanni Carrada: La evolución del ser humano, Editex, Madrid, 1999, p. 8.

15. Antonio Fernández-Rañada: Los científicos y Dios, Ediciones Nobel, Oviedo, 1994, p. 131; see Aceprensa, service 86/01.

16. I bidem, p. 127.

17. James S. Trefil: At the moment of creation. Del Big Bang hasta el universo actual; Salvat, Barcelona, 1986, p. 274. Cf. Aceprensa, service S6/87.

18. Cf. Meave Leakey & alter: "New hominin genus from eastern Africa shows diverse middle Pliocene lineages".Nature; 410; 433-440; 22.03.2001. Cf. also Daniel E. Lieberman: "Another face in our family tree". Ibidem, pp. 419-420. Cf. also Lisa Krause: "New face added to humankind's family tree". National Geographic; 21.03.01.

19. As time goes by, this thesis is gaining a larger issue of followers. Cf. Ian Tattersall: Contemporary Hominids. research and Science; no. 282; pp. 14-20; March 2000. According to this author Paranthropus Boisei, Homo Rudolfensis, Homo Habilis and Homo Ergaster coincided around Lake Turkana (northwestern Kenya) 1.8 Ma ago.

20. Cf. Leo Gabunia, Abesalom Vekua, David Lordkipanidze, Carl C. Swisher, Marie- Antoinette de Lumley, et alt.:Earliest Pleistocene Hominid Cranial Remains from Dmanisi, Republic of Georgia: Taxonomy, Geological Setting, and Age. Science 2000 May 12; 288: 1019-1025.

21. The smallest is 600 cc, less than half the current average of our species, while the largest is about 760 cc, a volume slightly less than that of Homo ergaster and markedly less than that of Homo erectus.

22. Cf. Abesalom Vekua, David Lordkipanidze, Jordi Agustí, Marcia Ponce de Leon, Christoph Zollikofer, et alt.: A new skull of early Homo from Dmanisi, Georgia. Science 2002 July 5; 297: 85-89.

23. Cf. Valéry Zeitoun: The first men who left Africa; Scientific World, no. 218, December 2000, pp. 37-41.