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The Quaternary boundary : 1.8 or 2.6 millions years old? Contributions of early homo

Les limites du Quaternaire : 1,8 ou 2,6 millions d’années? Apports de l’étude concernant les premiers représentants du genre homo
Sandrine Prat
p. 99-107

Résumés

Le Quaternaire est souvent considéré comme une période liée à l’apparition du genre Homo et marquée par la diversité de ses représentants, ce qui lui valu parfois d’être défini comme une période « Anthropozoïque ». Mais que nous indique vraiment le registre des homininés fossiles et en particulier des premiers représentants du genre Homo? La définition du genre Homo et l’identification de ces premiers représentants sont l’objet de nombreux débats depuis 1999. Quatre hypothèses sont actuellement débattues : (1) tous les spécimens appartiennent à un seul et même taxon : Homo habilis ; (2) deux espèces peuvent être identifiées dans ce groupe : Homo habilis sensu stricto et Homo rudolfensis ; (3) ces spécimens n’appartiennent pas au genre Homo mais au genre Australopithecus ; (4) les spécimens de l’espèce rudolfensis devraient être mis dans le genre Kenyanthropus. L’émergence du genre Homo serait à 2,45 millions d’années (hypothèses 1 et 2) ; à 1,9 million d’années (hypothèse 3) ; à 1, 9 ou 2,4 millions d’années (hypothèse 4, selon l’attribution de l’espèce habilis, au genre Homo (2,4 Ma), ou au genre Australopithecus (1,9 Ma)). Nous exposons dans ce papier les problématiques actuelles et nos propres résultats (analyse cladistique) concernant l’émergence du genre Homo en Afrique, en les mettant en perspectives avec les différentes propositions concernant la limite du Quaternaire. Notre propos, basé sur les données paléoanthropologiques, est centré sur la période allant de 2,6 à 1,6 millions d’années.

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I gratefully acknowledge P.V. Tobias, L. Berger, F. Thackeray, M.G. Leakey, the Office of the President, the cultural Ministry of Tanzania, the National Museums of Tanzania, National Museums of Kenya and National Museums of Ethiopia, W. Van Neer, H. Hartman, C. Smeenk for permission to study the great apes and fossil hominins in their care. I would like also thank Jodi Blumenfeld, Kris Carlson and Anne-Marie Bacon for their helps and François Marchal and Florent Detroit for their judicious comments. This study was supported by the French Ministry of Foreign Affairs, the Singer-Polignac Foundation and the Fyssen Foundation.

1- Introduction

1The Quaternary, which is sometimes also termed the Anthropogene Period, often is linked to the beginning of humankind. The boundary of the Quaternary, thus, is connected strongly to the date of human origin. But how can you anatomically define the genus Homo? And, moreover, who are the first representatives of the genus Homo? Since 1999, debate is focused on this definition and on the date of the first occurrence of this genus.

2What is exactly the evidence from the fossil hominin record? What information is provided by the hominin fossil bones, and, in particular, specimens of early Homo? These early Homo specimens are at the centre of major debates concerning the actual number of fossils and species; and the chronological boundaries of this taxon. Three questions can be proposed, who are the first specimens of early Homo, when is the first occurrence of the genus Homo, and what are the consequences for the determination of the inferior boundary of the Quaternary? We propose in this paper to A) review and assess knowledge of the early Homo specimens; B) present our own approaches and results; C) discuss the identity of the first members of the genus Homo and the implications concerning the dating of the human origin and the Quaternary boundary (2.6 Myr or 1.8 Myr?).

2- The definition of early homo: a review

2.1 - The definition of Early homo

3Since discovery of the first specimens attributed to Homo habilis in the Olduvai Gorge in 1959, and subsequent definition of Homo habilis by Louis Leakey, John Napier and Phillip Tobias in 1964, no consensus has been achieved on allotting specimens to early Homo. In order to accommodate H. habilis into the genus Homo, Leakey and collaborators were forced to modify the definition of the genus Homo proposed originally by Le Gros Clark (1955). Specially, they reduced the lower end of the cranial capacity range from 900 cm3 to 600 cm3.

4Forty-seven dental, thirty-one cranial, twenty-one mandibular and twenty‑four postcranial fragments have been allocated to this taxon (Prat, 2000; Prat et al., 2005). These specimens have been discovered in Ethiopia (Hadar, Omo), Kenya (East Turkana, West Turkana, Chemeron), Tanzania (Olduvai), Malawi (Uraha) and the Republic of South Africa (Sterkfontein, Swartkrans, Drimolen) (fig. 1).

5Since the middle of the 80’s, phylogenetic and taxonomic studies of the genus Homo have progressed considerably in a series of steps. Some are the direct result of discovery of new specimens (OH 62, OH 65 at Olduvai Gorge (Tanzania); UR 501, Uraha (Malawi); AL 666-1, Hadar (Ethiopia)). Others result from reinterpretations of existing anatomical features and the development of new methods and analyses (cladistics, bootstraps technique, 3D coordinate data, and geometric morphometry...).

6Recent studies have been conducted, on the one hand, to re-examine the taxonomic validity and composition of the species habilis (e.g. specimen diversity representing one or two species), and on the other hand, to discuss the attribution of this species (or these species) to the genus Homo. Four hypotheses have been proposed:

7 1) all specimens belong to the same species: Homo habilis sensu lato.

8For some authors (Howell, 1978; Tobias, 1978, 1991, 2003; Blumenshine et al., 2003), all specimens belong to one species: Homo habilis. In these authors, metric and morphometric differences between the specimens can be interpreted as an intra-specific variability. They argued that Homo habilis is a polymorphic species evolutionarily between Australopithecus africanus and Homo erectus. For example, the masticatory apparatus is more reduced compared with the australopithecines, but less reduced compared to Homo erectus, and the face is less prognathic than the former and more prognathic than the latter. Homo habilis exhibits increased brain size and development of the cranial vault relative to australopithecines, but less of an increase than observed in Homo erectus.

9 2) two species could be defined: Homo habilis sensu stricto and Homo rudolfensis.

10Others researchers (Stringer, 1986; Chamberlain & Wood, 1987; Lieberman et al., 1988; Wood, 1991, 1992; Rightmire, 1993; Lieberman et al., 1996; Prat, 1997, 2000) do not support the retention of a single taxon. They argue that the degree of variation within Homo habilis sensu lato is too large relative to what can be justified for a single taxon. Different parameters have been studied: morphology, endocranial volume, sexual dimorphism, and degree and pattern of variation of the cranio-facial measurements. This point of view is the current consensus opinion, but it is important to note that the specimens included in these species differ according to the authors. A majority of the distinctive traits are located on the face. For example, Homo rudolfensis exhibits a greater absolute size of the brain case, a face that is widest in its mid-part, complex roots and large crowns, and a powerful masticatory apparatus.

11 3) the species habilis and rudolfensis do not belong to the genus Homo but to the genus Australopithecus.

12For many years, the criteria used to allocate species to Homo, and those proposed by Leakey and colleagues to define the genus Homo (Leakey et al., 1964), have been considered insufficient or inappropriate. In 1999, Bernard Wood and Mark Collard proposed alternative criteria for defining the genus Homo. Wood and Collard (1999 a, b) suggested that the definition of the genus should follow both the evolutionary systematic method of classification and the cladistic criteria.

13Genus names should correspond to both grades and clades (fig. 2). According to their criteria, a genus is a group of species that are related more closely to one another than to species assigned to another genera (cladistic interpretation) occupying an ecological situation, or adaptive zone, that is different from that occupied by the species of the genus in question (evolutionary systematic interpretation) (Wood & Collard, 1999 a).

14They tested whether or not the genus Homo (including both Homo habilis and Homo rudolfensis) is monophyletic and adaptively coherent. For them, all the species within genus Homo must share a functionally coherent adaptive strategy with H. sapiens rather than with other genera (e.g. Australopithecus). But the species habilis and rudolfensis present an australopithlike pattern rather than a modern human-like pattern for body mass (relatively low), body shape (interpreted in terms of thermoregulation as being better suited to a relatively closed environment), locomotion (combination of terrestrial bipedalism with proficient climbing), morphology of the jaws and teeth, development and brain size (Wood & Collard, 1999b). Moreover, the genus Homo is monophyletic, in the spirit of Wood and Collard, only if the species habilis and rudolfensis are excluded. Therefore, they proposed that habilis and rudolfensis should be placed in the genus Australopithecus as Australopithecus habilis and Australopithecus rudolfensis, respectively (Wood & Collard, 1999 a, b).

15 4) it would be more appropriate to put Homo rudolfensis into the genus Kenyanthropus.

16More recently, in 2001, Meave Leakey and colleagues described a new species they termed Kenyanthropus platyops. For them, this new species has close affinities (especially in its facial morphology) with KNM-ER 1470, which is the holotype specimen of Homo rudolfensis. Thus, while the transfer of this species to the genus Australopithecus has been recommended by others (see preceding point), it would be more appropriate to put rudolfensis into genus Kenyanthropus (Leakey et al., 2001 p. 439). No genus attribution for habilis is mentioned by these authors.

Fig. 1: Map of distribution of early Homo specimens.

Fig. 1: Map of distribution of early Homo specimens.

Fig. 2: Definition of a genus according evolutionarysystematic (left) or cladistic (right) criteria (after Wood & Collard, 1999 b).

Fig. 2: Definition of a genus according evolutionarysystematic (left) or cladistic (right) criteria (after Wood & Collard, 1999 b).

2.2 - Geographical and chronological setting

17All habilis and rudolfensis specimens have been discovered in Ethiopia, Kenya, Tanzania, Malawi and the Republic of South Africa. Dating of these specimens, in particular those of the East Turkana region in Kenya have been at the heart of many controversies between 1970 and 1989. Indeed, these dating are linked to those of the associated tuffs. For example, the KBS tuff, which delimited the top of the upper Burgi Member, has been dated to 2.44 +/–  0.03 Myr by Fitch and Miller in 1970. But the review of the isotopic and magnetostratigraphic dating evidence demonstrates that the KBS is dated to 1.88 +/–  0.02 Myr (Feibel et al., 1989), or 1.87 +/–  0.02 Myr (McDougall & Brown, 2006). Some specimens ascribed to rudolfensis (e.g. KNM-ER 1470, ER 1801, ER 1802) and habilis (e.g. ER 3735 and the fossil from Area 123 (ER 1501, 1502, 1813)) are below the KBS tuff. This implies that during the 70’s and 80’s, these early Homo specimens were considered older than 2.5 Myr, but after the review of Feibel et al. (1989) their dates were revised to 1.8-1.9 Myr. Moreover, it must be noted that the stratigraphical position and dating of the fossils from Area 123 (in which some are allocated to habilis) are still debated (Gathogo & Brown, 2006). Indeed, for Gathogo and Brown, these fossils belong to the KBS member rather than the upper Burgi Member, as proposed by Feibel et al., 1989. These specimens are placed relative to the airfall KBS tuff and the lower Ileret Tuff, which have been dated to 1.87 +/–  0.02 Myr and 1.53 +/–  0.01 Myr respectively (McDougall & Brown, 2006). Their stratigraphic revisions result in age estimates between 1.53 and 1.75 Myr for the fossils from Area 123 whereas they were believed to be between 1.65 and 1.9 Myr in age (Gathogo & Brown, 2006, p. 478).

18In the beginning of the 90’s, additional fieldwork in Malawi, Kenya and Ethiopia, led to the discovery of new specimens of habilis and rudolfensis at 2.3-2.4 Myr. Therein, they provide new data for the chronological framework of these species. However, across Africa, few cranial specimens allocated to early Homo are known from this time period. They are represented by the temporal bone KNM-BC 1, recovered at Chemeron, Kenya, 2.4 Myr (Hill et al., 1992; Sherwood et al., 2002); the mandible UR 501 from Uraha, Malawi, ca. 2.45 Myr (2.3-2.5 Myr) ascribed to Homo rudolfensis (Schrenck et al., 1993; Bromage et al., 1995); the maxilla AL 666-1 from Hadar, Ethiopia, 2.33 +/–  0.07 Myr, allocated to Homo aff. H. habilis according to the morphology of the palate and the alveolar region (Kimbel et al., 1996); and teeth from the Omo basin, Ethiopia, 2.3-2.4 Myr (Suwa et al., 1996). More recently, in the Lokalalei complex site in West Turkana (Kenya), a first molar, dated to 2.34 +/ – 0.05 Myr also has been allocated to early Homo (Prat et al., 2005). Thus, the chronological distribution is circa 2.45-1.85 Myr for rudolfensis and 2.4-1.6 Myr for habilis.

3 - The definition of the genus homo: another perspective

19Concerning the taxonomic position of the early Homo specimens, we have proposed another point of view (Prat, 2000, 2004, 2005) based on a cladistic analysis of twenty-two original Plio-Pleistocene specimens.

3.1 - Material and methods

20Cladistic analysis is the method of choice for reconstructing the relationships among species, when it can be assumed that the phenotype is an efficient safe proxy for genetic affinity.

21Because no consensus concerning the hypodigm of the species Homo habilis has been achieved, our analysis is based on specimens, not on species to define the Operational Taxonomic Unit (OTU). Indeed, because the aim of our analysis is to understand and define this species, the creation of an OTU based on the species would have been a circular reasoning. This approach with the specimen as OTU has been used by Capparos (1997), Zeitoun (2000), Prat (2000), Asfaw et al. (2002) and Gilbert et al. (2003). Our study, therefore, differs from prior studies concerning early Homo phylogeny, where the OTU is defined by the species (Skelton et al., 1986; Skelton & McHenry, 1992; Lieberman et al., 1996; Strait et al., 1997; Strait & Grine, 2004).

22For our analysis, the trait list comprises 122 cranial features. This list and the definition of the traits are derived both from our own observations (Prat, 2000) and a compilation list of various other studies (Chamberlain & Wood, 1987; Lieberman et al., 1996; Skelton et al., 1986; Skelton & McHenry, 1992; Strait et al., 1997; Stringer, 1987; Tobias, 1991; Zeitoun, 2000).

23We scored features on original material (see illustration in Prat, 2000, 2004, 2005). Outgroup taxa used in the present analysis are mature specimens of Pan and Gorilla. This included twenty-five males and twenty-five females of Pan (Pan paniscus, Pan troglodytes troglodytes, Pan troglodytes schweinfurthi) and fifteen males and fifteen females of Gorilla (Gorilla gorilla gorilla, Gorilla gorilla beringei, Gorilla gorilla graueri). All specimens are from the Musée Royal d’Afrique Centrale de Tervuren (Belgium) and the Powell Cotton Museum (United Kingdom).

24The ingroup includes 22 original hominin specimens in the first analysis, and 23 specimens (the same specimens plus KNM-WT 40 000, holotype of Kenyanthropus platyops) in the second one. These specimens represent the most preserved accessible Plio-Pleistocene specimens. This fossil sample is comprised of original cranial specimens discovered in Ethiopia, Kenya, Tanzania, and Republic of South Africa. For KNM-WT 40000, data were obtained from the literature. Specimens from Hadar (Ethiopia), AL 666-1 (Kimbel et al., 1996), and from the Olduvai Gorge, OH 65 (face and palate), described by Blumenshine and colleagues (Blumenshine et al., 2003), are not included in this study because their data come from the literature, and their condition is judged too fragmentary.

25All 122 traits have been used independently to maximise information. Furthermore, an effort was made to eliminate characters that redundantly describe the same underlying morphological feature. Dental data have been excluded in this analysis because many specimens are edentulous. Moreover, since cranial capacity has been estimated for only a small number of specimens in the sample, it was excluded as well.

26Character polarity has been determined by rooting the out-group. The polymorphism is coded as multiple states (0/1, 0/2, 0/1/2) with the polymorphism option of the Paup 3.1 software (Swofford, 1993). Quantitative characters were coded using the method proposed by Thiele (1993) (xs=((x-min)/(max-min))*n; n=maximum number of ordered states allowable by the algorithm used (32 for PAUP), after a logarithmic transformation of data). Data have been computed in a non-arbitrary way in order to avoid any preconceived phylogenetic hypothesis. The most parsimonious trees have been obtained using the heuristic algorithm with the options “general, starting trees, stepwise addition and branch swapping”. These trees are presented with their length, their consistency index, and their retention index. These indices measure the relative amount of homoplasy. Consistency index (CI) is calculated as the minimum possible tree-length divided by the observed tree-length (Farris, 1989). If there is no homoplasy in a tree, then its observed length equals the minimum tree-length, and the CI equals one. If homoplasy is present, then the CI is less than one. Retention index (RI) is calculated by subtracting observed tree-length from maximum possible tree-length, and then dividing this value by the difference between maximum and minimum lengths (Archie, 1989; Farris, 1989).

3.2 - Results

27- Homo / Australopithecus

28Three equally parsimonious trees have been obtained using all option analyses (« general, starting trees, stepwise addition or branch swapping ») based on 122 unordered cranial characters, which were taken on 22 mature fossil individuals. A consensus tree (fig. 3) has been constructed based on topologies of all trees. The length of this tree is 431 steps with a consistency index of 0.452 and a retention index of 0.431.

29The result of the analysis suggests the existence of two monophyletic groups: the Homo clade defined at node A and the Australopithecus clade defined at node B. The Homo clade is defined by (((((KNM-ER 1470((KNM-ER 1813(OH 16, OH 9))KNM-ER 730))(OH 24, OH 13)KNM-ER 3732)(KNM-ER 3733, KNM-ER 3883))OH 62) Stw 53) at node A by six unambiguous characters: asymmetrical shape of the postglenoid process, uniform petrous crest, articular eminence with two joint areas (angle superior to 90o), angle between the articular eminence and the preglenoid planum (parallel with clade C), medial position of the parietal prominence, and presence of an anterior nasal spine.

30The Australopithecus clade is defined, at node B, by ((((KNM-ER1805((KNM-ER406,OH5)(SK48,SK46))) (Stw505(Sts5,Sts71)))KNM-WT17000)AL333-45) by four unambiguous characters. The synapomorphies are anterior position of the foramen magnum relative to the bi-tympanic line, presence of temporal crests and presence of a sagittal crest (on the anterior part of the bregma-lambda arch) with a moderate expansion. The sagittal crest is not observed in Pan. This trait is only observed in some Gorilla specimens (on 35% of females, 43% of males; 70% of adult overall). The presence of a sagittal crest is influenced to a greater extent by the anatomical age rather than by the sex of the individuals (Prat & Thackeray, 2001).

31- Homo / Kenyanthropus

32The new cladistic analysis (fig. 4) includes data from the description of the new Kenyan specimen KNM-WT 40000, the holotype of the species Kenyanthropus platyops. Results indicate that the following specimens: KNM-ER 730, 1470, 1813, 3732, 3733, 3883, OH 9, 16, 13, 24, 62, Stw 53, belong to the clade Homo (defined at node A). In other words, these specimens are not linked with Kenyanthropus platyops (KNM-WT 40000).

33The consensus tree of the two most parsimonious trees has a length equal to 442 steps with a consistency index of 0.442 and a retention index of 0.431. Synapomorphies at node A are a flat glabellar region in norma lateralis, a lateral postorbital depression, absence of a sagittal crest, a weak development of the supramastoid crest, no junctions between the mastoid and supramastoid crests, an uniform petrous crest, a canine region independent of the piriform aperture, no lateral prominence of the zygomatic bone, the inferior part of the orbital region posteriorly located relative to the superior part and nasal bone eversion.

34It is worth noting in this analysis both that, KNM-WT 40000 is linked neither with Homo nor

35Australopithecus specimens, and that, with the introduction of this specimen, Australopithecus becomes paraphyletic. New analyses based on the cast or on the original specimen of KNM-WT 40000, instead of the data derived from existing studies in the literature, are required.

Fig. 3: Most parsimonious cladogram. Consensus tree, 122 traits, 22 mature specimens (L= 421 steps, IC = 0.452; IR= 0.431).

Fig. 3: Most parsimonious cladogram. Consensus tree, 122 traits, 22 mature specimens (L= 421 steps, IC = 0.452; IR= 0.431).

Fig. 4: Most parsimonious cladogram. Consensus tree, 122 traits, 23 mature specimens (L= 442 steps, IC = 0.442; IR= 0.431).

Fig. 4: Most parsimonious cladogram. Consensus tree, 122 traits, 23 mature specimens (L= 442 steps, IC = 0.442; IR= 0.431).

4 - Discussions and conclusions

4.1 - Who are the first members of the genus Homo?

36Cladistic analysis indicates that the specimens KNM-ER 1470, KNM-ER 1813, KNM-ER 3732, OH 24, OH 62 and Stw 53 belong to the genus Homo rather than the genus Australopithecus (fig. 3). Furthermore, KNM-ER 1470 is not grouped with KNM-WT 40 000 (fig. 4), holotype of Kenyanthropus platyops, as was proposed by Leakey et al., 2001.

37The Homo genus is monophyletic even if the specimens of the species habilis and rudolfensis are included (Prat, 2000, 2004, 2005). These results have been confirmed by other analyses (e.g. Strait & Grine, 2004), but contrast with additional studies (e.g. Wood & Collard, 1999 a, b) that use the same methodological approach (cladistic analysis). For the latter authors, the earliest species of Homo is ergaster. They also argued (Wood & Collard, 1999 a, b; Wolpoff, 1999) that an important change in terms of body proportions, brain size and facial morphology occurs between habilis/ rudolfensis and ergaster (referred as early Homo sapiens by Wolpoff (1999)). However, one specimen from Dmanisi (D2700) exhibits facial morphology and brain size approximating those associated with Homo habilis (Vekua et al., 2002). Thus, any discussion about substantial morphological change between habilis and ergaster requires further examination of the Dmanisi specimen.

38Moreover, it is important to note the existence of a methodological problem in comparing different cladistic results. Differences between tree topology could be due to species attribution of specimens or to the traits included in the analyses. For example, allocation, for example, of the Kenyan specimen KNM-ER 1805 to the species Homo habilis has some important consequences for the character state assignment of this species. Indeed, “presence of a sagittal crest and presence of a temporo-nuchal crest” are coded “yes” for habilis (Skelton & McHenry, 1992, p. 321; Strait et al., 1997, p. 26), simply because of the inclusion of KNM-ER 1805 in this species (for temporo-nuchal crest « the H. habilis state assignment is based on KNM-ER 1805 », Strait et al. 1997, p. 69). However, if we do not consider KNM-ER 1805 to be the average male Homo habilis (Prat, 2002), then these traits are coded « no » and all topology of the consensus tree can be modified.

39Concerning significance of the morphological traits used in the comparative and cladistic studies, it is essential to test whether the traits are homologous (Lieberman, 1999; Lockwood & Fleagle, 1999) or independent [i.e. not functionally- or structurally-related (Skelton & McHenry, 1998; Strait & Grine, 1998;

40Strait, 2001)]. It is also important to exclude non-informative diagnostic traits (Capparos, 1997). Moreover, it is important to test whether the traits are related to developmental age or to sex-specific characteristics of the specimen (Prat, 2000; Prat & Thackeray, 2001). Indeed, numerous traits, that distinguish the two groups in this study, much less others (Lieberman et al., 1996; Strait et al., 1997), could be related to the sex-specific characters or to the age of the specimen. Re-examination of the significance of traits is necessary for all comparative and phylogenetic studies.

Fig. 4: Most parsimonious cladogram. Consensus tree, 122 traits, 23 mature specimens (L= 442 steps, IC = 0.442; IR= 0.431).

Fig. 4: Most parsimonious cladogram. Consensus tree, 122 traits, 23 mature specimens (L= 442 steps, IC = 0.442; IR= 0.431).

4.2 - Implications for dating the first occurrence of the genus Homo

41The four main hypotheses concerning taxonomic allocation of early Homo (cf. supra paragraph 2.1. and 2.2.) have some important implications for dating the earliest occurrence of the genus Homo. The appearance of genus Homo is at:

  • 2.45 Myr for hypothesis 1 (all the specimens belong to the same species Homo habilis sensu lato (fig. 5a)) and hypothesis 2 (two taxa can be distinguished in genus Homo: Homo habilis sensu stricto and Homo rudolfensis (fig. 5b)). Our results are in accordance with the latter hypothesis;

  • 1.9 Myr for hypothesis 3 (the two species habilis and rudolfensis do not belong to genus Homo, but belong instead to genus Australopithecus (fig. 5c)). Homo ergaster and the Dmanisi specimens (Gabunia & Vekua, 1995; Gabunia et al., 2000; Vekua et al., 2002; Lordkipanidze et al., 2005) are the first members of the genus Homo, at 1.9 Myr. Dates of Homo ergaster in Africa are between 1.45 to 1.9 Myr, with majority of specimens dated between 1.45 and 1.65 Myr, if we refer to the new age estimations of the specimens from Koobi Fora (Gathogo & Brown, 2006; Brown et al., 2006).

  • 1.9 Myr or 2.4 Myr for hypothesis 4 (it would be more appropriate to put rudolfensis into the genus Kenyanthropus (fig. 5d)). According to the genus attribution of the habilis specimens, the first appearance of genus Homo is at 2.4 Myr if habilis is allocated to Homo genus or at 1.9 Myr if habilis is allotted to Australopithecus genus.

Fig.5 : Hypotheses concerning the first appearance of genus Homo.

Fig.5 : Hypotheses concerning the first appearance of genus Homo.

a) 2.45 Myr, one species; b) 2.45 Myr, two species, hypothesis selected in this article; c) 1.9 Myr; d) 1.9 or 2.4 Myr.

4.3 - Implication for the definition of the Quaternary

42Our prior results based on morphological and metric comparisons (Prat, 1997, 2000, 2004), as well as this current cladistic analysis (cf. supra), emphasize, on the one hand, that two species could be distinguished and defined habilis and rudolfensis, and on the other hand, that they belong to genus Homo. This implies an earliest appearance for the genus Homo at or before 2.45 Myr.

43This date corresponds to a major climatic event (at 2.6 Myr) as well as the earliest known stone tools. Indeed, the earliest known occurrence of stone tools is at 2.3-2.6 Myr with the stone tools assemblages from Gona, Ethiopia (Roche & Tiercelin, 1977; Semaw et al., 1997), and with the first knapping site (Lokalalei 2C; Roche et al., 1999). Early Homo, present in this time period, could be a good candidate from knapping this lithic assemblage. But the presence of other species such as P. aethiopicus and A. garhi ought not to be ignored (Prat et al., 2005). Concerning climatic changes, the marine sediment sequences demonstrate that climate variability and aridity increase near 2.8 Myr (deMenocal, 2004). Faunal assemblages portray more varied and open habitats at 2.9-2.3 Myr. The 2.9-2.5 Myr interval corresponds to an increase in the proportion of arid-adapted fauna and a change to open grassland as the dominant type of environment (Reed, 1997; Bobe et al., 2002; Alemseged, 2003). Soil carbonate stable isotopic analysis (Cerling et al., 1994; Wynn, 2004) and marine paleoclimatic records (deMenocal, 1995) indicate another intensification of aridity that is associated particularly with more open habitats near 1.8-1.6 Myr.

44If we consider that the beginning of humankind is one point that defines the Quaternary, then the boundary of the Quaternary must be extended back to at least 2.45 Myr. It is in accordance with a boundary of the Quaternary at 2.6 Myr, which has been proposed previously by Pillans & Naish (2004), corresponding to major sedimentological and climate changes, with a strong influence by bi-polar glaciation.

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Table des illustrations

Titre Fig. 1: Map of distribution of early Homo specimens.
URL http://journals.openedition.org/quaternaire/docannexe/image/1313/img-1.jpg
Fichier image/jpeg, 47k
Titre Fig. 2: Definition of a genus according evolutionarysystematic (left) or cladistic (right) criteria (after Wood & Collard, 1999 b).
URL http://journals.openedition.org/quaternaire/docannexe/image/1313/img-2.jpg
Fichier image/jpeg, 28k
Titre Fig. 3: Most parsimonious cladogram. Consensus tree, 122 traits, 22 mature specimens (L= 421 steps, IC = 0.452; IR= 0.431).
URL http://journals.openedition.org/quaternaire/docannexe/image/1313/img-3.jpg
Fichier image/jpeg, 45k
Titre Fig. 4: Most parsimonious cladogram. Consensus tree, 122 traits, 23 mature specimens (L= 442 steps, IC = 0.442; IR= 0.431).
URL http://journals.openedition.org/quaternaire/docannexe/image/1313/img-4.jpg
Fichier image/jpeg, 42k
Titre Fig.5 : Hypotheses concerning the first appearance of genus Homo.
Légende a) 2.45 Myr, one species; b) 2.45 Myr, two species, hypothesis selected in this article; c) 1.9 Myr; d) 1.9 or 2.4 Myr.
URL http://journals.openedition.org/quaternaire/docannexe/image/1313/img-5.jpg
Fichier image/jpeg, 36k
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Sandrine Prat, « The Quaternary boundary : 1.8 or 2.6 millions years old? Contributions of early homo »Quaternaire, vol. 18/1 | 2007, 99-107.

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Sandrine Prat, « The Quaternary boundary : 1.8 or 2.6 millions years old? Contributions of early homo »Quaternaire [En ligne], vol. 18/1 | 2007, mis en ligne le 01 mars 2007, consulté le 16 juillet 2024. URL : http://journals.openedition.org/quaternaire/1313 ; DOI : https://doi.org/10.4000/quaternaire.1313

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Sandrine Prat

UPR 2147 du C.N.R.S., 44 rue de l’Amiral Mouchez, 75014 Paris. E-mail: sandrineprat@ivry.cnrs.fr

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