Cranial morphology of Javanese Homo erectus: New evidence for continuous evolution, specialization, and terminal extinction

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Abstract

Our current knowledge of the evolution of Homo during the early to middle Pleistocene is far from complete. This is not only because of the small number of fossil samples available, but also due to the scarcity of standardized datasets which are reliable in terms of landmark identification, interobserver error, and other distorting factors. This study aims to accurately describe the cranial morphological changes of H. erectus in Java using a standardized set of measurements taken by the authors from 18 adult crania from Sangiran, Trinil, Sambungmacan, and Ngandong. The identification of some obscure landmarks was aided by the use of micro-CT imaging. While recent studies tend to emphasize evolutionary conservatism in Javanese H. erectus, our results reinforce the theory that chronologically later groups experienced distinct morphological changes in a number of cranial traits. Some of these changes, particularly those related to brain size expansion, are similar to those observed for the genus Homo as a whole, whereas others are apparently unique specializations restricted to Javanese H. erectus. Such morphological specializations in Java include previously undescribed anteroposterior lengthening of the midcranial base and an anterior shift of the posterior temporal muscle, which might have influenced the morphology of the angular torus and supramastoid sulcus. Analyses of morphological variation indicate that the three crania from Sambungmacan variously fill the morphological gap between the chronologically earlier (Bapang-AG, Bapang Formation above the Grenzbank zone in Sangiran) and later (Ngandong) morphotypes of Java. At least one of the Bapang-AG crania, Sangiran 17, also exhibits a few characteristics which potentially indicate evolution toward the Ngandong condition. These strongly suggest the continuous, gradual morphological evolution of Javanese H. erectus from the Bapang-AG to Ngandong periods. The development of some unique features in later Javanese H. erectus supports the hypothesis that this Javanese lineage went extinct without making significant contributions to the ancestry of modern humans.

Introduction

In the past two decades, debates over the variation of Homo erectus have been centered on the question of whether or not morphological differences between African and Asian forms of this species (H. erectus sensu lato) are significant enough to recognize two distinct species; namely, H. ergaster and H. erectus (Andrews, 1984, Stringer, 1984, Wood, 1984, Wood, 1994, Groves, 1989, Rightmire, 1990, Rightmire, 1998, Bräuer and Mbua, 1992, Kramer, 1993, Bräuer, 1994, Clarke, 1994, Asfaw et al., 2002, Manzi, 2004, Villmoare, 2005, Terhune et al., 2007). The “Asian” vs. “African” morphological dichotomy has been investigated intensively in order to approach this question. In this context, Antón (2002a) correctly emphasized the need to look at regional and temporal variations within Asian H. erectus. This is reasonable because we can understand overarching themes in hominid evolution only by accumulating and analyzing valid, detailed information from each local area. In this study, we focus on H. erectus from Java and investigate chronological changes in its cranial morphology.

Fossil remains of Javanese H. erectus are sampled from several different sites with varying dates. While the Trinil and Sangiran specimens belong to the Early Pleistocene, Ngandong possibly belongs to the late Middle or Late Pleistocene. Dates of the Sambungmacan specimens are unclear, but are generally believed to be contemporaneous with, or older than, Ngandong. In spite of this wealth of well-preserved fossil materials, however, there still exist a number of controversies and unsettled questions regarding the evolutionary history of archaic Javanese hominids.

For example, many past and recent researchers infer phylogenetic continuity in Javanese H. erectus from Sangiran/Trinil to Sambungmacan to Ngandong (Weidenreich, 1943, Wolpoff, 1999, Antón, 2002a, Antón, 2003, Baba et al., 2003, Kidder and Durband, 2004). However, this scheme of linear evolution in Java is not unanimously accepted (Jacob, 1973a, Jacob, 1975, Jacob, 1976, Jacob, 1981, Santa Luca, 1980, Schwartz and Tattersall, 2000, Schwartz and Tattersall, 2005, Widianto and Zeitoun, 2003; see below for more details). The details of morphological evolution in Java are another question. In light of some morphological differences between the Sangiran/Trinil and Ngandong cranial remains, specific level distinction between H. erectus and H. soloensis was once a popular concept. Although the latter species name rarely appears in modern paleoanthropological papers, this view is still alive among some recent researchers (Zeitoun in Widianto and Zeitoun, 2003, Durband, 2004). In addition, there are old and new controversies concerning the phylogenetic relationships of Javanese H. erectus with other groups of hominids, such as aboriginal Australians and H. floresiensis.

Recently, several researchers performed multivariate analyses on the cranial measurements of Asian H. erectus (Antón, 2002a, Antón et al., 2002, Baba et al., 2003, Kidder and Durband, 2004, Durband et al., 2005, Liu et al., 2005). However, the number of measurement variables included in these studies was limited to five to eight due to the nature of multivariate analysis, which basically requires a complete dataset from all the specimens examined. Consequently, these studies failed to document some of the potentially more dynamic aspects of temporal change in Javanese hominids, which have been in part suggested through other morphological observations and univariate studies (Weidenreich, 1951, Jacob, 1975, Jacob, 1976, Santa Luca, 1980; see also Delson et al., 2001, and Baba et al., 2003). Instead, Antón, Kidder, and Durband stressed the morphological consistency of the entire Javanese assemblage in comparison to the northern Chinese assemblage. In the most recent comprehensive review of H. erectus, Antón (2003) mentioned a limited number of characteristics that might show temporal changes in Java, and discussed them only in the context of chronological brain size increases.

Another restriction in these recent metric studies is the use of published measurement sets from different researchers. None of these works sufficiently examine the possible influence of interobserver error, but this factor should not be neglected. In addition, measurements are often influenced by distortions of the fossil specimen, unclear landmarks, and sometimes by indeterminate or confusing measurement definitions. Therefore, the accuracy of measurements should be examined even when the researcher's own measurements are being used.

Subgrouping of the fossil sample is another issue to be considered. The H. erectus cranial collection from Indonesia encompasses remains from the early to middle/late Pleistocene, but the exact and relative dates of each specimen are often controversial or confusing. There is a general consensus that all the Ngandong hominid remains collected in the 1930s are largely contemporaneous and derive from a single group, if not a single population, because they are from the same stratum of the spatially restricted single paleontological site and their morphology is relatively homogeneous (Weidenreich, 1951, Santa Luca, 1980, Antón, 2003). However, researchers disagree on how to treat the Trinil, Sangiran, and Sambungmacan remains, and the basis for each subgrouping is often not clearly explained.

Santa Luca (1980) compared the crania of Trinil 2/Sangiran 2, Sangiran 4, and Sangiran 12/Sangiran 17 separately to the Ngandong crania. Antón (2002a) partially adopted Rightmire's dichotomy of ‘small-brained’ and ‘large-brained’ crania (Rightmire, 1990), which was designed by the latter author primarily in order to compare African and Asian archaic Homo and recognized small-brained (Trinil 2, Sangiran 2, 3, 4, 10) and large-brained (Sangiran 12, 17) groups in the early fossil record of Indonesian H. erectus. In Antón's extensive review of H. erectus, she modified this scheme and allocated all the Indonesian fossils to ‘earliest’ (e.g., Sangiran 4, 27), ‘early’ (e.g., Trinil 2, Sangiran 2, 10, 12, 17, Skull IX), or ‘later’ groups (Antón, 2003). As for the three crania from Sambungmacan (Sambungmacan 1, 3, 4), Antón regarded all of these as largely contemporaneous with the Ngandong specimens (<100 ka), and allocated them to her later Indonesian group. However recent morphological and chronological evidence offers a slightly different subgrouping for the Trinil, Sangiran, and Sambungmacan remains (see below).

In summary, a balanced selection of accurate measurements taken from appropriate fossil samples is needed to further understand morphological variation in Javanese H. erectus. With such a goal in mind, this study examines the temporal variation of cranial morphology in Javanese H. erectus based on conventional, and some new, two dimensional (2D) measurements (chord, arc, and angle).

Measurement variables are selected and devised to provide a framework within which the “total morphological patterns” of this cranial series can be examined. That is to say, instead of scoring each morphological character without attempting to understand intercharacter correlation, we first investigate measurable characters (overall cranial size and shape, basic dimensions of each cranial bone, and other surface structures) to grasp the basic cranial architecture; this then serves as a foundation on which the background of other detailed character variation can be understood. Of course, our measurements do not cover every detail and the available sample is still insufficient to accomplish such an ambitious task, but the attempt was made possible thanks to the relative wealth of the fossil materials from this region.

Recent advances in 3D measurement and analytical techniques are remarkable. Still, conventional 2D measurement remain as the important, fundamental technique in the field of paleoanthropology because of its relative easiness and straightforwardness, and the accessibility it provides to a larger set of comparative data. If the above-mentioned difficulties in data collection can be overcome, 2D measurement analysis would become a truly powerful and effective way to document the fossil morphology.

This study overlaps extensively with the work of Santa Luca (1980) in terms of characters measured and samples used but involves various improvements that reflect practical and theoretical advances made during the last quarter century. We attempt to collect accurate measurements based on observation of the original fossil specimens, partly with the help of high resolution micro-CT images. Reference to the previous reports and examinations on interobserver differences of measurements enabled us to refine our data in a way that a single researcher or research team can never achieve if working in isolation. The measured specimens include the newly discovered Sambungmacan 4 (Baba et al., 2003) and the Sangiran 38 and the Bukuran skulls from Sangiran (Indriati, 2004), whose basic measurements are reported here for the first time. This study presents the first systematic metric comparisons of the cranial base and some other aspects of Javanese H. erectus.

Section snippets

Materials and their chronology

In this study, we compare the adult crania from Trinil, Sangiran, Sambungmacan, and Ngandong (Table 1). The prefixes of “T”, “S”, “Sm”, and “Ng” are used here to refer to individual specimens from these regions, except for the three Sangiran crania with no formal specimen numbers: Skull IX, Bukuran, and Grogol Wetan. We recognize the Bapang-AG of Sangiran, Sambungmacan, and Ngandong groups in the present study. The contents of each group and the rationale for these divisions are as follows.

As

Measurement

Those measurements considered useful in evaluating basic cranial architecture (overall cranial size and shape, basic dimensions of each cranial bone, and other surface structures) are selected and devised. Most of them are conventional items defined by Martin (Bräuer, 1988, Baba, 1991: Japanese translation of Bräuer, 1988 with expanded notes and illustrations), Howells (1973), and Wood (1991), but we have established some additional items to complement them (Table 2). When the positions of the

Overall size and shape

The results of two PCAs (PCA1 and PCA2) are presented in Table 6 and Fig. 8. PCA1 is based on maximum cranial length, maximum biparietal breadth, and porion-bregma height, and the size parameter used to standardize each variable is the cubic root of the product for these three measurements (SIZE1). PCA2 includes four additional breadths taken at the anterior, basal, and posterior portions of the cranium (SOT breadth, postorbital breadth, maximum bimastoid breadth, biasterionic breadth), and

S 3, Ng 5, and Ng 9

The above comparisons are based on the unquestionable adult subsamples, which do not include S 3, Ng 5, and Ng 9. However, the inclusion of these specimens does not affect the morphological contrasts listed in Table 7 and Fig. 10 in any significant ways.

The inclusion of these older adolescent/young mature adult specimens has the effects of lowering the breadth values in both the Bapang-AG and Ngandong samples only slightly. The only change of the statistical results in Table 5 accompanied by

T 2, S 2, and S 4

Among the 25 characters in Table 7 which were metrically examined by this study, 19 were found to differ between Bapang-AG and Ngandong. Conditions of S 4, T 2, and S 2 can be examined for a part of them, and they are summarized in Table 8. Because the characters 8, 9, and 16 in Table 7 (frontal breadth and length, parietal length) largely reflect the variation in overall cranial size (see 23/1, 25/1 in Table 5), the formers are integrated to the character 1 (cranial size) in Table 8.

The

Sambungmacan specimens

Table 8 also lists the character states for the three Sambungmacan crania. Besides those characters that show differences between Bapang-AG and Ngandong, the characters 2 and 17 in Table 7 are included because some of the Sambungmacan specimens exhibit unique conditions in these traits.

In overall cranial vault size and shape, Sm 1 conforms to the variation of Ngandong (Fig. 8), except that its upper face may be relatively narrow. The specimen also shows similarities with Ngandong in the total

Discussion

As documented above, differences in landmark identification and method occasionally produce a considerable degree of interobserver measurement error (Table 3). We have attempted to minimize the chance of landmark misidentification by repeated observation of the original specimens, comparisons with published measurements, and the use of micro-CT images where available. Measurement definitions also occasionally vary among workers. The definitions chosen by us may not necessarily be superior to

Conclusions

The measurement data collected in this study are useful in documenting cranial morphology and variation in Javanese H. erectus. The following main conclusions are drawn from our analyses of these data.

  • (1)

    Javanese H. erectus maintains its basic cranial architecture from the Bapang-AG through Ngandong periods, but at the same time shows distinct evolutionary changes in various regions of the cranium.

  • (2)

    Individual specimens from Sambungmacan show affinities with the Ngandong specimens, but at the same

Acknowledgements

We would like to thank John de Vos, Friedmann Schrenk, Ottmar Kullmer, Christine Hertler, and Denise Donlon for access to the specimens in their care. We are grateful to three anonymous reviewers and Susan Antón, the editor, for their invaluable suggestions and comments. We also thank Yuji Mizoguchi, Shuji Matsu'ura, and Megumi Kondo for comments and information, Gen Suwa, Reiko T. Kono, Daisuke Kubo, and Hitoshi Fukase for CT-scanning, and Koeshardjono for assistance in the laboratory.

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