Australia: The Land Where Time Began
Xujiayao Hominin from the Nihewan Basin, North China: Implications for Human Evolution in the Middle Pleistocene in East Asia
The site in the Nihewan Basin, North China, is one of the most important sites dating to the Palaeolithic in East Asia. Excavations have uncovered 20 Homo fossils, which had previously been assigned to an archaic Homo sapiens group, as well as more than 30,000 lithic artefacts and about 5,000 mammalian fossil specimens. Since the Xujiayao hominin was excavated in the 1970s researchers have been dating it, but its age has remained controversial as a result of the limitations of the dating techniques that have been used on the materials that were available. In this paper Ao et al. report new ages for the Xujiayao hominin that are based on combined electron spin resonance (ESR) dating of quartz in the sediments and high-resolution magnetostratigraphy of the fluvio-lacustrine sequence. It is suggested by the magnetostratigraphy that the upper Matuyama and Brunhes polarity chrons are recorded at Xujiayao. The results of ESR dating indicated a pooled average age of 260-370 ka for the layer containing the Homo skeletal material, which is consistent with its position in the middle Brunhes normal polarity chron that was indicated by magnetostratigraphy. The Xujiayao hominin was therefore dated to the estimated age that makes it the oldest hominin from the mid-Pleistocene to have derived Neanderthal traits in East Asia. This age is consistent with the time the early Denisovans, which are a sister group of Neanderthals, first appeared and colonised eastern Eurasia. According to Ao et al. their updated age and the traits that are Neanderthal-like of the Xujiayao Homo fossils, in particular the teeth, that are Denisovan-like, make it possible that the Xujiayao hominin could possibly represent an early Denisovan.
The Nihewan Basin, North China, is one of the most important Palaeolithic areas in East Asia, containing more than 60 early Palaeolithic sites, and having more than 100,000 stone tools, and tons of mammal fossils that have been recovered. Xujiayao (also known as Houjiayao, 40o06’N, 113o59’E, 908 m above sea level) is an open air site that dates to the Palaeolithic on the northwestern margin of the Nihewan Basin, that was discovered in 1974. This site contains Oldowan-like stone tools (i.e. Mode 1 core and flake technologies) and mammal fossils as well as, more importantly; archaic fossils that have been discovered in it (Jia & Wei, 1976; Jia et al., 1979; Wu, 1980; Bae, 2010; Wu et al., 2013, 2014; Wu & Trinkaus, 20124).
Several Chinese institutions have carried out multidisciplinary studies using a variety of techniques to date the Xujiayao hominin since the 1970s. This site was suggested to date to at least more than 100 ka and possibly to the late Mid-Pleistocene (Jia et al., 1979), based on initial analyses of its faunal composition and stratigraphy. An estimated age of about 90-125 ka (Chen et al., 1983, 1984) based on uranium (U)-series dating of 6 mammal fossil teeth, which was possibly underestimated as the result of an open system of bones and teeth (Grün et al., 2014; Shen et al., 2014). Later, conventional radiocarbon dating of organic material and mammal bones indicated an age older than 40 ka (Institute of Archaeology of Chinese Academy of Social Sciences, 1991; Hayase, 2012). An age as early as 500 ka has been suggested (Løvlie et al., 2001; Wang et al., 2008), based on magnetostratigraphy and correlation between the Xujiayao environmental magnetic data and marine δ18O record. Recent 26Al/10Be burial dating of 2 quartz samples yielded a weighted mean age of 240 ± 50 ka (Tu et al., 2015). The result of the various estimates of age is intense debate about the age of the Xujiayao hominin (Tu et al., 2015 and references therein), which has complicated in-depth investigation of its evolutionary significance.
ESR dating of fluvio-lacustrine sediments has provided an opportunity to date archaeological sites from the Pleistocene, with recent improvements in ESR technology (e.g., Voinchet et al., 2010). According to Ao et al. ESR has been successfully applied in the Nihewan Basin (Liu et al., 2010a, 2013, 2014a). New age estimates are presented in this paper for the Xujiayao hominin by the use of ESR dating of sediment containing quartz and high resolution magnetostratigraphy of the fluvio-lacustrine sequence and address the paleoanthropological significance of the Xujiayao hominin in light of this dating evidence that has been updated.
Geological and archaeological setting
Situated about 150 km to the northwest of the Zhoukoudian Homo erectus sites, that are well-known, the Nihewan Basin is an intermontane basin of a relatively small area of about 150-200 km2 (Yuan et al., 2011). Fluvio-lacustrine sediments dating to the Mid-Pliocene to the Late Pleistocene, which are known as the Nihewan Formation (sensu lato; Barbour, 1924), were deposited widely in the Basin (Deng et al., 2008; Ao et al., 2013a). The Nihewan Formation, which is comprised of thick, stratigraphically continuous sequences that are exposed mainly along the Sanggan River, that trends southeast to northwest, and along the Huliu River. A rich source of sites from the Palaeolithic is located on the eastern margin of the Basin, with the Palaeolithic evidence that is presently the oldest known that is documented as early as about 1.7 Ma from the Majuangou and Shangshazui sites (Zhu et al., 2004; Ao et al., 2013b). There is a mean annual temperature in the Basin of 7.7oC and the precipitation is 365 mm, with more than 60% of the precipitation falling in summer.
At Xujiayao the stratified fluvio-lacustrine succession that is exposed horizontally is about 17 m thick. A well was dug down to a stratigraphic level of 24.2 m (the top of the stratigraphic level that indicates the top of the section is 0) in order to extend the magnetostratigraphic record. The sequence is dominated by silt, clay, silty clay and sand. The fluvio-lacustrine sediments about 200 m northwest of the studied section, are capped patchily by soil from the last interglacial (S1, not sampled; Wang et al., 2008). The interval, which consists of brown clay, greyish-yellow silty clay, and greyish-dark silty clay, contains the artefact layer.
Xujiayao is best known for 20 archaic Homo fossils that were excavated (Jia & Wei, 1976; Jia et al., 1979; Wu, 1980; Wu et al., 2013, 2014; Wu & Trinkaus, 2014). In 1974 the site was discovered during field surveys, which were carried out by the Institute of Vertebrate Palaeontology and Palaeoanthropology, Chinese Academy of Sciences. Between 1976 and 1979 subsequent extensive excavations yielded 2 fairly complete parietals, a number of parietal fragments, the ramus and posterior body of a lower jaw, a mandible, a juvenile maxilla, 2 upper molars, and a lower molar (Jia & Wei, 1976; Jia et al., 1979; Wu, 1980; Wu & Poirier, 1995). Also reported from the site are 30,000 pieces of stone artefacts, which included cores, flakes, burins, borers, scrapers, points, stone balls or bolas, and choppers-chopping tools (Jia & Wei, 1976; Jia et al., 1979). Also, in recent years there have been more than 10,000 stone tools that have been recovered, though they have not yet been formally published. These artefacts from Xujiayao are typical Oldowan-like technology and are primarily made of quartz (Ma, 2009). The principal stone-knapping and tool-touching technique was direct hammer percussion (Ma, 2009). Also, about 5,000 mammal fossils were recovered from the layer bearing Homo fossils. The fossils come from at least 21 taxa that have been identified, which include Struthio sp., Ochotona sp., Myospalax fontanieri, Microtus brandtioides, Canis lupus, Equus przewalskaii, Coelodonta antiquitatis, Cervus elaphus, Megaloceros ordosianus, Bos primigenius, Panthera tigris, Palaeoloxodon cf. naumanni, Equus hemionus, Cervus Nippon grayi, Spirocerus hsuchiayaocus, Spirocerus peii, Procapra picticaudata przewalskii, Gazella subgutturosa, Gazella sp., and Sus sp. (Jia & Wei, 1976; Jia et al., 1979). In the Nihewan Basin the taxonomic composition of the fossil mammals implies that the area was a cool, open savannah environment with patches of forest.
Estimation of age
It is indicated by this study that, as with previous magnetostratigraphic results from Xujiayao (Løvlie et al., 2001), the artefact layer is located in the Middle Brunhes normal polarity chron, which corresponds to an age in the Mid-Pleistocene. Consistent error-weighted mean ages of 374 ± 27 ka and 260 ± 30 ka for the lower (12-9.6 M) and upper (9.6-8 m) parts of the artefact layer, respectively. Rapid deposition of the lower and upper parts of the Xujiayao artefact layer is indicated by this, which is consistent with rapid burial of the Homo and mammal fossils and stone tools. The possible presence of a sedimentary hiatus between the 2 parts of the artefact layer is supported by these dates, as has been suggested (Tu et al., 2015) based on sharp lithological change. The entire artefact layer, which spans from 12 to 8 m, therefore ranges in age from about 370-260 ka. The teeth of the Xujiayao hominin support this age assignment to the Mid-Pleistocene, which are consistently larger and have morphological features (e.g., metric dimensions, trapezoidal shape of the crown outline of the M1, and shape, robusticity, and divergence of molar roots) that are more typical of Asian populations from the Early and Middle Pleistocene. This age estimate also agrees with those that were estimated for the associated mammal fossils from Xujiayao, among which are many of the same species from Zhoukoudian H. erectus sites that have been dated to the Mid-Pleistocene (Wu et al., 1985).
The age estimated in this study for the Xujiayao hominin is much younger than the about 500 ka age that was suggested by magnetocyclochronology (Løvlie et al., 2001; Wang et al., 2008) and is much older than the about 90-125 ka age that was obtained by conventional U-series dating of mammalian teeth (Chen et al., 1982, 1984). An older age from magnetocyclochronology, which assumed no stratigraphic discontinuities (Løvlie et al., 2001; Wang et al., 2008) may have resulted from the complex depositional system and the presence of erosional unconformities at Xujiayao (Jia & Wei, 1976; Jia et al., 1979; Ma, 2009; Li et al., 2014; Tu et al., 2015). It is generally considered that uranium-series dates from pure calcites are accurate and precise, but datable calcite is not present at Xujiayao. Calcite differs from bones in that bones are susceptible to post-burial U migration, therefore, the dating of bones and teeth is prone to giving ages that are substantially young ages as demonstrated by dating by U-series of intercalated calcite formations and fossil bones that are excavated in archaeological sites (Bischoff et al., 2003; Shen et al., 2004a, b, 2010: 2014; Grün et al., 2014). The initial U-series dating that were made more than 30 years ago were limited by technical issues and low resolution which therefore means that the initial U-series dating of teeth from Xujiayao may have given ages that are underestimated. Also, at about 200 m northwest of the section of the study the fluvio-lacustrine sediments are capped patchily by thick palaeosols from the last interglacial that is about 0.5 m thick of the Chinese loess sequence (Wang et al., 2008) where the age ranges from about 80-130 ka (Sun et al., 2006). The top of the Xujiayao section must therefore be older than 80 ka. Consistent with this, dating of quartz grains by optically stimulated luminescence (OSL) suggests that at 2 layers, at depths of 1 and 3 m, from the uppermost Xujiayao section are older than the conventional quartz OSL dating range of 100-150ka for fluvio-lacustrine sediments (Zander & Hilgers, 2013) and have OSL ages that are identical of about 100 ka (Li Et al., 2014).
Agreeing with this recent 26Al/10Be burial dating of the lower artefact layer at about 12 m (Tu et al., 2015), in this study the ESR dating of quartz sediments, as well as with high resolution magnetostratigraphy, suggests an age for the Xujiayao hominin of the mid-Pleistocene, which contrasts with the earlier age of the Late Pleistocene based on U-series dating of fossil remains. Dates of 267 ka at 8.7 m and 370 ka at 11 m according to ESR dating, and an age of 780 ka at 18.3 m according to magnetostratigraphy dating, Ao et al. established a relationship between stratigraphic depth and age, given an age of 100 ka at 1 m depth at the Xujiayao section, according to OSL dating (Li et al., 2014). The yellow fluvial silts in the upper section had higher rates of sedimentation than the underlying lacustrine greyish-green clays and the greyish-dark silty clays, which is consistent with lithological variations.
U-series dating of fossil bones was the main method of determining ages for hominins from the Middle and Late Pleistocene or Palaeolithic sites in China 1 or 2 decades ago, which contributed significantly to the understanding of human evolution. The considerably underestimated dates resulting from the open systems in bones and teeth (Bischoff et al., 2003; Shen et al., 2004a, b, 2010, 2014; Grün et al., 2014) has, however, complicated in-depth investigation of the Middle and Late Pleistocene hominins in East Asia and comprehensive understanding of their links to contemporaries in Africa and Europe that are well dated (Dennell & Petraglia, 2012). With improvements in technology over the past 10 years, U-series dating of intercalated calcite formations instead of bones, which were previously used, has established ages that are relatively precise for several sites from the Middle and Late Pleistocene, such as the sites containing modern Homo sapiens at Huanglong Cave (81-101 ka) in central China (Shen et al., 2013), Luna Cave (70-127 ka) Bae et al., 2014), Daoxian (80-120 ka;) (Liu et al., 2015; but see Michel et al., 2016), Liujiang (111-139 ka) (Shen et al., 2002), and Zhiren Cave (>100 ka) ( Liu et al., 2010b) in South China, as well as the sites containing archaic H. sapiens of New Cave (248-269 ka) at Zhaoukoudian (Shen et al., 2004a) and Chaoxian (310-360 ka) in eastern China (Shen et al., 2010), plus H. erectus sites at locality 1 (400-500 ka) (Shen et al., 2001) and Nanjing (580-620 ka) in eastern China (Zhao et al., 2001). Fundamental revision of understanding of evolution in China has been contributed to by these updated ages. E.g., remains of modern H. sapiens that were dated to more than 100 ka from China, though they have been debated (Dennell, 2010; Michel et al., 2016), challenge significantly the traditional view that modern humans were restricted to portions of Africa prior to 100 ka and they rapidly dispersed throughout Eurasia after 60 ka (Qiu, 2016).
A chronological framework for the many sites dating to the Pleistocene in China that is complete and updated is a long way from being established because of the poorly constrained ages of many sites that is due to the lack of volcanic products and pure calcite on which 40Ar/39Ar and U-series can be applied, respectively, to determine dates. As well as ESR technology improvements over the past decade, ESR dating of bleached quartz that is extracted from sediments has become a useful tool for dating many Pleistocene sites, which otherwise lack material which is suitable for radiometric dating. Dating of quartz sediments with ESR, combined with high resolution magnetostratigraphy, is useful for dating sites from the Pleistocene and has been successfully applied to the Dongpo site (304-333 ka) (Liu et al., 2010a) from the mid Pleistocene Palaeolithic, and to the Dongguotuo site from the Early Pleistocene, about 1.1 Ma (Liu et al., 2013), Nanshan, about 1.35 Ma, and Majuangou, 1.4-1.7 Ma, (Liu et al., 2014a), in the Nihewan Basin. Ao et al. used combined ESR and magnetostratigraphic dating of Xujiayao to represent a further step in the establishment of a complete chronological framework for sites from the Pleistocene in China, as well as making possible the study of contemporaneous Asian and non-Asian hominins.
Implications for the evolution of hominins
There are mixed characteristics of the Xujiayao Homo associated with European Neanderthals, Asian H. erectus and modern H. sapiens (Jia & Wei, 1976; Jia et al., 1979; Wu, 1980; Bae, 2010; Wu et al., 2013, 2014; Wu & Trinkaus, 2014Xing et al., 2015), which makes affiliation of Xujiayao hominins to “classic” H. erectus, modern humans or Neanderthals difficult. The Xujiayao hominins were therefore assigned to the archaic H. sapiens (Jia & Wei, 1976; Jia et al., 1979; Wu, 1980; Wu & Poirier, 1995), though in China this unique term is controversial as noted by later studies (e.g. Rightmire, 1998; Dennell & Petraglia, 2012). The updated chronology of Ao et al. makes Xujiayao Homo fossils among the earliest archaic H. sapiens remains in eastern China from Chaoxian, 310-360 ka (Shen et al., 2010). When the remains of archaic H. sapiens from New Cave (Shen et al., 2014a) at Zhoukoudian, Dali, about 270 ka (Xiao et al., 2002) and Jinniushan, about 260 ka (Rosenberg et al., 2006), it appears that the mid-Pleistocene archaic H. sapiens occupied a vast area across China. At Xujiayao archaic H. sapiens and other East Asian sites, e.g. Jinjiushan and New Cave, differed from some African Homo individuals from the mid-Pleistocene in that they were associated with an Oldowan-like technology that was relatively simple (Bae, 2010), whereas the Africans were associated with Acheulian stone tools (Rightmire, 2008). The Xujiayao hominins were capable of successfully obtaining regular sources of animal fat and protein that probably helped their survival in the harsh mid-latitude northeast Asian winters. Surface modifications that are observed on the midshafts of long bones indicate that the Xujiayao hominins were skilled at hunting large animals such as horses and had access to high-utility (meat-bearing, marrow-rich) long bones (Norton & Gao, 2008), which was important for overwintering in the temperate zone at more than 40oN.
i) East Asia is suggested by fossils from Yuanmou Basin, about 1.7 Ma, (Zhu et al., 2008), Nanjing, 580-620 ka, (Zhao et al., 2001), Hexian, 400-420 ka (Grün et al., 1998), and Yunxian, 0.936 Ma (Dennell, 2015), in South China and Gongwangling, 1.62-1.63 Ma (Zhu et al., 2015), Chenjiawo, 0.65 Ma (An & Ho, 1989), and Zhoukoudian, 0.4-0.77 Ma (Shen et al., 2001, 2009) in North China, that occupation by Homo erectus began 1.7-1.6 Ma, then persisted to about 400 ka.
ii) Africa, During the terminal Early Pleistocene to the earliest Middle Pleistocene, about 600-800 ka (Rightmire, 1998, 2008, 2009, 2013), H. erectus was giving way to H. heidelbergensis,
Homo fossils that had unambiguous affinities to H. heidelbergensis have not been reported from East Asia (Bae, 2010), though some divergences of the Yunxian crania from the standard H. erectus pattern imply links to H. heidelbergensis (Rightmire, 1998; Stringer, 2002). Whether there was a dispersal of H. heidelbergensis to East Asia has remained enigmatic. In order to assess the history of H. heidelbergensis in Asia further in-depth study of Homo fossils and more material are needed. H. erectus persisted in East Asia until at least 400 ka, the time when H. heidelbergensis was giving way to H. neanderthalensis in Europe (Rightmire, 1998). However, according to Ao et al. this does not support the replacement of H. erectus by H. heidelbergensis in East Asia (Groves & Lahr, 1994; Etler, 2004). It is possible that H. heidelbergensis coexisted with H. erectus if the presence of early mid-Pleistocene H. heidelbergensis in East Asia is documented. Based on recent ages that are more precise for various fossils of Homo that have been established in recent years, and with disregarding previous ages that are not precise, that were underestimated by the use of U-series dating on bones, Ao et al. suggest it is possible that archaic H. sapiens (370-250 ka) may not have interacted with the older H. erectus (1.7 ma – 400 ka) or younger modern H. sapiens (<150 ka) in East Asia, as is indicated by the updated Chinese Homo chronostratigraphy by this study.
It is indicated by recent studies of the Xujiayao Homo fossils (Wu et al., 2012, 2014; Wu & Trinkaus, 2014; Xing et al., 2014) that they show more similarity to Neanderthals than to H. heidelbergensis or H. erectus. The bi-level nasal floor in the maxilla of the Xujiayao individual I (PA1480), e.g., and the presence of a median pterygoid tubercle, that is well developed, a retromolar space, and a mandibular notch that is asymmetrical, in the Xujiayao 14 mandibular ramus are characteristics typically found in Neanderthals (Wu et al., 2012; Wu & Trinkaus, 2014). In Xujiayao 15, the morphology of the temporal labyrinth and the relative proportions of their anterior, posterior, and lateral canals, are consistent with traits found in Neanderthals (Wu et al., 2014). The teeth of Xujiayao, likewise, have some Neanderthal features (such as high degrees of shovel shape and labial convexity of the anterior dentition and continuous distal trigonid crests in the molar teeth (Xing et al., 2015). Studies were prevented from affiliating the Xujiayao hominin to the classic European Neanderthals, however, by several other features that are common to H. erectus and anatomically modern H. sapiens (Jia & Wei, 1976; Jia et al., 1979; Wu, 1980; Bae, 2010; Wu et al., 2012, 2013, 2014; Wu & Trinkaus, 2014; Xing et al., 2015).
Ao et al. say they were prompted to re-evaluate the Xujiayao hominins from a global perspective by the recent discovery in southern Siberia of the Denisovan hominins (Reich et al., 2010; Meyer et al., 2012; Sawyer et al., 2015) and the Sima de los Huesos hominins in Spain (Arsuaga et al., 2014; Meyer et al., 2016). It remains enigmatic what the Denisovans looked like because of a lack of crania, though it is indicated by DNA studies of their teeth and bones that they were a sister group of Neanderthals and they were distributed broadly across Siberia (Reich et al., 2010, Meyer et al., 2012; Sawyer et al., 2015). It is indicated by cranial and DNA evidence that the Sima de los Huesos hominins, about 430 ka, were early Neanderthals (Arsuaga et al., 2014; Meyer et al., 2016). Following divergence at about 430 ka from a common ancestral population it is possible Neanderthals colonised Europe and Denisovans possibly colonised eastern Eurasia (Arsuaga et al., 2014; Sawyer et al., 2015, Meyer et al., 2016). The Nihewan Basin is just over 2,000 km from southern Siberia; therefore it is reasonable that early Denisovans could have expanded from high-latitude Siberia southeastwards to the mid-latitude Nihewan Basin that had ecological and climatic conditions that were better than southern Siberia. Ao et al. suggest that the hominins at Xujiayao were possibly early Denisovans; just as the Sima de los Huesos hominins were ancestral to Neanderthals in Europe, based on the Neanderthal traits present in the Xujiayao Homo fossils and the presence of a sister group of Neanderthals (Denisovans) northeastern Eurasia. Also, the molar teeth of Xujiayao hominins (Xing et al., 2015) have roots that flare massively, and complex crowns that are relatively large, which are reminiscent of Denisovans (Sawyer et al., 2015). Also, the updated age from this study of 370-260 ka for the Xujiayao hominins is consistent with the time of the appearance of the oldest known early Denisovans, about 430 ka, who then colonised eastern Eurasia (Arsuaga et al., 2014; Sawyer et al., 2015, Meyer et al., 2016). After the divergence of the Neanderthals and Denisovans that occurred at about 430 ka (Meyer et al., 2016), Denisovans in eastern Eurasia had Neanderthal traces as well as developmentally derived features. This could be the explanation of why Xujiayao hominins had mixed Neanderthal and characteristics that were modern human derived. Understanding potential relationships between Xujiayao and other archaic Asian hominins is a new topic that is important and contentious (Smith et al., 2017), which must be explored intensively in order to develop an in-depth understanding of human evolution in eastern Eurasia in the Middle Pleistocene. In order to test whether Denisovans were distributed broadly across China in the Middle Pleistocene, DNA study of Chinese mid-Pleistocene Homo fossils is crucial, particularly the Neanderthal-like Xujiayao hominin, and comparison with Denisovan fossils. In establishing the taxonomic affinity of Chinese Homo fossils with characteristics that are transitional and for a comprehensive understanding of hominin evolution in Asia and globally, such studies will be significant. In-depth assessment of their global evolutionary position is made difficult by this and is likely to produce confusion as the same term is sometimes used to describe the earliest examples of H. sapiens in Africa (Dennell & Petraglia, 2012).
It is indicated by magnetostratigraphic results that the Xujiayao records the upper Matuyama and Brunhes chrons. The Xujiayao hominin has a pooled average quartz ESR age of 260-370 ka, which is in agreement with its position in the middle Brunhes normal polarity chron, as indicated by magnetostratigraphy. Motivated by significant recent progress in Neanderthal lineage studies in Europe and northeastern Asia, the results of this study shed new light on the Xujiayao hominin from a global perspective. Combined with traits that are Neanderthal-like for the Xujiayao Homo fossils, the updated age of this study, in particular the molar teeth that are Denisovan-like, imply there is a possible affinity of the Xujiayao hominin to early Denisovans, a sister group of European Neanderthals, that diverged from Neanderthals. It appears that this study supports the idea that Denisovans may have become widely distributed across eastern Eurasia during the Middle Pleistocene, though there is clearly a need for more Denisovan fossils and detailed DNA studies.
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