Fórmula de Bayes, Modelos de Distribución Previos y

1.5.21 In t r o d u c t io n

Modern human evolution is one of the most heavily debated issues in palaeoanthropology in recent years (see reviews in Aiello, 1993; Lahr and

Foley, 1998). The main debate stands between two polarised theories of human origins: the multiregional model; and the recent African origin model. The two theories are wholly incompatible, to the degree that if one is proven correct, the other will have to be discarded. In addition, some intermediary theories have been developed, each occupying a position on the continuum between the two poles. Two of these will be briefly discussed: the African hybridisation model; and the assimilation model. The relative position of Neanderthals and modern humans in the human evolutionary sequence, as well as the species affiliation of Neanderthals, is entirely dependent on which of the theories one chooses to accept. This section will discuss the evidence and assumptions which form the basis of these theories, and assess the relative relationships between modern humans and Neanderthals predicted by each one.

1.5.2Ü Th e r e c e n t Af r ic a n o r ig in m o d e l (RAO)

The RAO proposes the evolution of anatomically modern humans in Africa at ca. 200,000 BP, a consequent migration into other regions and a complete replacement of all more archaic populations (Cann et al, 1987; Stringer and Andrews, 1988; Stringer, 1989, 1992, 1993, 1994; Harpending et al, 1993). It does not allow for any hybridisation or genetic drift between the archaic and modern populations. This model is supported by several lines of evidence both morphological and molecular. The morphological evidence is twofold. First, as discussed in section 1.3.2, craniometric analyses of modern human populations have shown that they are very homogenous in their craniofacial morphology (Howells, 1989, 1995; Hanihara, 1996), indicating that it is highly

unlikely that they could have evolved separately from more distinct regional populations of Homo erectus (see discussion of the multiregional model below). Even given this heterogeneity, within-group variation is greatest within the African populations, relative to all other groups (Relethford and Harpending, 1994). Second, advocates of the RAO hypothesis see no evidence of regional evolution in any region other than Africa, and a marked morphological discontinuity between the archaic and modern populations in all other regions. Transitional morphologies have been proposed for fossils such as those from Border Cave (Rightmeier, 1989), and Klasies River Mouth (Deacon, 1992), in South Africa, and from Omo-Kibish in Ethiopia (Day and Stringer 1981), while those proposed in Australasia, Europe and China (see discussion of the multiregional model below), have been rejected on the basis of numerous analyses (Stringer et al, 1984; Habgood, 1989; Groves, 1989; Rightmire, 1990, 1996; Stringer, 1990; Lahr, 1994, 1996). However, a recent excavation at Abrigo do Lagar Velho in Portugal (Duarte et al., 1999), unearthed a juvenile skeleton, showing skeletal traits which have been claimed to be a mosaic of modern human and Neanderthal morphologies. The skeleton is dated to 21.500 years, which makes it possible that the individual was the result of admixture between modern humans and Neanderthals. Unfortunately the specimen was missing most of its face. Further excavations and analyses of the remains will hopefully illuminate the status of this individual in human evolution.

The most significant support for the RAO model in recent years has come from the analysis of modern and Neanderthal genetic material. In 1987, Cann and colleagues, analysed mitochondrial DNA (mtDNA), from a wide range of

human populations, and came to the conclusion that all modern mtDNA could be traced back to a single origin, in Africa, ca. 150-200 thousand years ago. This date was calibrated on the basis of the mutation rate of mtDNA, and has since been recalculated to 435-806 thousand years (Wills, 1995). Genetic diversification was the greatest within the African continent, and several of the most parsimonious trees derived from the genetic distances, could be rooted in Africa. However, as discussed in section 1.3.3, modern humans are unusually genetically homogeneous, compared to other species (Wilson et al, 1985; Maynard Smith 1990).

The initial claim of Cann et al. (1987), was met with great scepticism, and a large number of methodological flaws were exposed (e.g. Maddison et al, 1992; Templeton, 1993), however their initial conclusion has since been supported by many independent analyses, on various sites within the human genome, including both nuclear and mtDNA (e.g. Torroni et al, 1994; Batzer et al., 1994; Paâbo, 1995; Nei and Takezaki, 1996; Comas et al, 1996; Tishkoff et al, 1996; Stoneking et al, 1997). A recent analysis of the 565bp chromosome 21 region has also found evidence for distinctive prehistoric migrations, one to Oceania, a second to Asia and subsequently to America, and a third to Europe (Jin et al.,1999). These results are thought to support the RAO model.

In 1997, an analysis of mtDNA from the Neanderthal type specimen, revealed that the genetic code of Neanderthals was significantly different from that of modern humans (Krings et al, 1997). Whereas modern humans sequences differ by on average 8.0±3.1 substitutions, the Neanderthal and moderns differ by 27.2±2.2 substitutions. The Neanderthals were the most distant from

Australian/Oceanian and European lineages, indicating that they were no more closely related to Europeans than they were to any other modern populations (see discussion of the multiregional model below). Krings et al (1997), took these results to mean that Neanderthals had contributed little or nothing to the modern human gene pool, and advocated support of a recent African origin of modern humans.

The relatively limited degree of variation within the modern human gene pool has been interpreted by the supporters of the RAO model as the result of a severe population bottleneck in our relatively recent evolutionary past, probably reducing the ancestral population of African and non-African lineages to an effective size in the region of 10.000 individuals (Zietkiewitcz et al, 1998)

According to the advocates of the ROA model. Homo sapiens evolved as a result of a spéciation event, replacing the archaics in each region without interbreeding or genetic drift. Thus, Neanderthals should show no greater affinity with Europeans than they do with any other modern human population.

1.5.2iii Th e m u l t ir e g io n a l m o d e l (MR)

The MR model of human origins sees no single region as being the site for the evolution of anatomically modern human morphology. Instead, it advocates a gradual evolution of regional populations, derived from an initial spread of what is classically known as Homo erectus into Europe and Australasia (Wolpoff, 1985, 1989, 1992; Wolpoff et al, 1984; 1988; Frayer et al, 1993). A considerable level of gene flow between the regional groups would prevent spéciation events in the regional lineages. The effect of gene

flow on regional morphological characters is summarised in the “Centre and Edge” hypothesis (Thorne, 1981; Thorne and Wolpoff, 1981). According to this hypothesis geneflow is highest within the centre of a population and reduced at the periphery. Selection pressures would however be highest at the periphery, and thus features diagnostic of the population would first develop there, and be transmitted to the centre through genetic drift. This would be translated into greater morphological variability at the periphery of a population than at its centre.

The evidence used to support the MR model of human origins, is primarily morphological. The model makes the following predictions: fossils showing transitional morphologies should occur in regions other than Africa; certain regional morphological traits should show continuity from the earliest regional populations to the present day; and these traits should also occur exclusively, or with the highest frequency in those regions of the world where they evolved.

In keeping with the first prediction, groups of fossils from non-African regions have been singled out as showing transitional morphologies. These include the Javan fossils from Ngandong (Wolpoff, 1989), the Chinese fossils from Dali and Jinniushan (Frayer et al, 1993), and the Central European fossils from Vindija (Wolpoff et al, 1981). The evidence from Vindija is of the greatest importance to the present analysis. MR advocates see several transitional features in this sample, including a reduction in the size of the supra-orbital torus, a narrowing of the nasal cavity, a reduction in anterior tooth size, and a shorter mandibular retromolar space.

Evidence for the second and third predictions have been sought in areas such as Australasia, China, Europe and Africa, all of which are claimed to show persistence of regional features from the Lower to the Upper Palaeolithic. As an example, in Australasia (Lahr, 1994), such features include frontal flatness, posterior position of minimum frontal breadth, marked facial prognathism, presence of zygomaxillary tuberosity, deep and narrow infraglabellar notch, aversion of lower borders of the zygomatics, a large and rounded zygomatic trigone, absence of supratoral sulcus, and features of the nasal and orbital borders, while in Europe (Frayer et al, 1993), they include a horizontal-oval mandibular foramen, cranial base flattening, the retromolar space, the suprainiac fossa and the mastoid tubercle.

A recent reanalysis of the regional traits proposed for the East Asian and Australasian sample (Lahr, 1994), found that features proposed as characteristic of those populations were not exclusive to those regions and some occurred more frequently in other populations. The features which did show regional continuity were found to be plesiomorphic and interdependent and thus of little use in determining evolutionary relationships.

Advocates of the MR hypothesis interpret the molecular evidence quite differently from those supporting the RAO model (Prayer et al, 1993; Wolpoff and Caspari, 1997). They point out the flaws with the initial mtDNA analysis, most notably with the tree structure and molecular clock calibrations. Since not all the most parsimonious trees in Cann et al’s (1987) original analysis had roots in Africa (Templeton, 1993), or indeed any other region, they reject the idea of a single area of origin for modern humans. In addition they point to flaws in the molecular clock used to calculate the time of the regional split

(Templeton, 1993). A recalibration of this clock using the ‘MR’ standards, puts this time back to 1.3 million years BP (Frayer et al, 1993), which is consistent with the migration of Homo erectus out of Africa. They also attribute the amount of genetic variation within the African continent to larger population sizes in that area (Templeton, 1993; Wolpoff and Caspari, 1997). This theory has been supported by work by Relethford (1997; 1998), who also sees the amount of excess genetic diversity to be partially a function of mutation rates. Thus, those genetic traits with high mutation rates show high levels of African heterozygosity, while those with low mutation rates show lower levels. As for the evidence from Neanderthal mtDNA (Kring et al, 1997), Wolpoff (1998), has pointed out that although the Neanderthal is separated from the modern human mean by ca. 3 times more substitutions than separate the modern groups from one another, the most distant modern groups were separated by more substitutions (24) than separated the Neanderthal and the closest modern human population (22). He sees this fact as contradictory to the claimed species difference between the two groups based on this data.

The MR hypothesis sees no spéciation event to have taken place since the original dispersal 1 million years BP. Thus what is commonly classified as

Homo erectus and all subsequent hominid morphologies (including

Neanderthals), are included in Homo sapiens (Wolpoff, 1992). Under this model Neanderthals would have been direct ancestors to modern Europeans, and show greater affinity with them than with any other modern human group.

1,5.2iv Th e Af r ic a n h y b r id is a t io n a n d r e p l a c e m e n t m o d e l (AHR)

Although this model predates the RAO hypothesis, it has regained importance in recent years as an intermediary between RAO and MD. It is similar to RAO in that it proposes a recent evolution of modern humans, starting in Africa at ca, 70.000 BP (Brauer, 1989), as a result of accumulated regional genetic changes. Unlike the RAO it emphasises a certain degree of hybridisation between the modern and archaic populations through complex interactions of natural selection, gene flow, migration and drift (Brauer 1984a, 1984b, 1989, 1992). Treisman (1995), has suggested that complete replacement of archaic mtDNA can occur in a population produced by a recent admixture of archaic and modern humans. This would explain the lack of evidence for archaic genetic origins in the molecular data. AHR recognises certain specimens in the Far East and Central Europe as showing transitional morphology between archaic and modern forms. Under this hypothesis Neanderthals would have contributed some of their genetic profile to Europeans, and so should show closer affinity with them than with any other modern group.

1 .5 .2 V Th e As s im il a t io n MODEL (A M )

AM is another attempt at combining elements of both the MR and RAO. It proposes an initial emergence of anatomically modern humans in one region, which then spread throughout the rest of the world (Smith et al, 1989; Smith, 1992). However, the anatomically modern humans did not completely replace the archaic forms in the area to which they migrated, but rather the two were assimilated through geneflow. This model is different from that proposed by Brauer, in that it emphasises the overall continuity rather than the overall

replacement of the regional archaic populations. Smith et al (1989), proposed the region of origin to be Africa, and saw transitional specimens in Africa, Eastern Asia and in particular in Central Europe (Smith, 1992). This model sees no spéciation event as having taken place at the archaic/modern transition, and thus Neanderthals are classed as a subspecies of Homo

sapiens, i.e. Homo sapiens neanderthalensis. Since Neanderthals would

have contributed considerable elements of their genetic makeup to the European genepool, it would be expected that they showed a greater affinity with modern humans in that region than in any other.

1 .5 .3 Ne a n d e r t h a l f a c ia l g r o w t h a n d d e v e l o p m e n t

1.5.3i Introduction

Although the Neanderthal fossil record contains the largest number of immature remains of any fossil human species, there are great limitations to the developmental inferences which can be drawn from this material. Of the 70 or so known sub-adult individuals, only 8 have largely complete craniofacial skeletons, and between them they represent 5 distinct geographical areas (Tillier, 1996). In addition, many of the remains have been badly reconstructed, making them difficult to interpret, although new computer-assisted reconstruction techniques are starting to overcome that problem, as well as maximising the information gleaned from each individual specimen (Ponce de Leon and Zollikofer, 1998,1999). Given these limitations a considerable number of studies have been carried out with the aim of

gaining insight into the Neanderthal growth process, the results of which will be summarised in this section.

1 .5.3Ü Ne a n d e r t h a l p r e-n a t a l d e v e l o p m e n t

The modern human infant goes through a period of rapid brain growth, at foetal rates, for a number of months after birth. This is due to the constrained size of the human pelvis limiting the size of the infant’s head, so that it will have to complete its foetal growth stage outside the womb. Prior to the discovery of the complete Neanderthal pelvis at Kebara in the early 1980s (Bar-Yosef et al, 1986), Trinkaus (1976, 1983), had conducted a study of partial Neanderthal pelves from Shanidar, La Ferrassie, Krapina and Tabun. He found that the shape of the Neanderthal pelvis was distinct from that of modern humans, in that the pubic ramus was longer and narrower, indicating an unusually wide pelvic inlet to the birth canal. The size of the canal seemed to suggest that the Neanderthals had not yet evolved the mechanism of giving birth to infants prior to the cessation of the foetal growth rate. The Neanderthal brain would have finished its rapid growth in the womb, making the infant less vulnerable in the initial post-natal period, and as a consequence the gestation period would have been longer, and put greater strain on the mother. Dean et al (1986), on the other hand, argued that the larger pelvic inlet was a function of an increased pre-natal growth rate of the Neanderthal infant, which made the brain larger at birth, without an increase in gestation length.

The Kebara 2 skeleton has a complete pelvis, which indicates that the entire Neanderthal pelvic morphology is considerably different from that of modern

humans. Despite it being a male skeleton, it is clear that the shape of the Neanderthal birth canal would have been quite different from that predicted on the basis of pelvic fragments (Rak and Arensburg, 1987). The narrow and elongated ramus was associated, not with a larger pelvic inlet, but with a differently oriented sacrum. Thus the size of the Neanderthal birth canal was no larger than that of modern humans, indicating that Neanderthal pre-natal growth rate was not unusually precocious, nor did they have a longer gestation period.

1.5.3iii Po s t n a t a lg r o w t h o f t h e Ne a n d e r t h a l f a c ia l s k e l e t o n

Traditional studies of Neanderthal growth and development have focused on two main issues: the rate of growth and the morphological pattern of growth. The following sections give an overview of the present views on both these topics, concluding with a discussion on the possible factors controlling and influencing rate and pattern of Neanderthal facial growth.

1 .5 .3 iiia Ne a n d e r t h a l Gr o w t h r a t e s

The present study estimates ages of modern and Neanderthal individuals on the basis of dental calcification and eruption. This method is adequate if one is interested in comparing craniofacial form at equivalent stages of dental development, but does not necessarily bear a simple relation to absolute age, and does not allow comparisons of developmental rates. Early studies by Legoux (1970), Wolpoff (1979), and Heim (1982), indicate that dental development in Neanderthals might not be directly comparable to that of modern humans. In particular Wolpoff (1979), found that the formation of the

M3 was accelerated in Neanderthals as compared to modern humans, so that the root of the third molar was almost complete at the time of eruption. However, neither differences or similarities in tooth formation times, in themselves indicate a difference or similarity in growth rate (Van Gelder, 1978). Ageing of individuals through perikymata counts (see section