Australia: The Land Where Time Began

A biography of the Australian continent 

Human Occupation of Wallacea – Isotopic Evidence for Initial Coastal Colonisation and Subsequent Diversification

Wallacea is comprised of a group of islands that are poor in resources that has been considered to be a major adaptive obstacle to hominins that expanded into Australasia. It has been hinted at by archaeological evidence that coastal adaptations in Homo sapiens enabled rapid island dispersal and settlement; however there has been no means to test this proposition directly. In this study Roberts et al. applied stable carbon and oxygen isotope analysis to the tooth enamel of humans and fauna from 6 archaeological sites that dated to the Late Pleistocene across Wallacea. The earliest human forager that has been found in this region was demonstrated by the results to date from ~42 ka made significant use of coastal resources earlier than the subsequent niche diversification that has been shown by later individuals. It is argued by Roberts et al. that their data provides clear insights into the huge adaptive flexibility of humans, which includes our ability to specialise in the use of varied environments, particularly when compared with other species of hominin from island Southeast Asia.

It is being demonstrated by recent, high profile studies of symbolic material culture (e.g., Aubert et al., 2019), technological complexity (e.g., Akhilesh et al., 2018), fossil morphology and chronology (e.g., Rizal et al., 2020), and genetics (Reich et al., 2011) an increasingly complex and dynamic picture of the capacities and interactions of different hominin populations in the Late Pleistocene (125-12 ka), in particular in Asia. If the uniqueness of Homo sapiens is to be determined, the last extant hominin on the planet, it is becoming apparent that the way in which its ecological adaptations differed from those of the genus Homo needs to be examined (Rabett, 2012; Roberts & Stewart, 2018). It has been suggested that populations of Homo sapiens that were expanding around the world in the Late Pleistocene were able to not only exploit flexibly varied, and often extreme, environments – which included deserts, tropical rainforests, high-altitude settings, and deep sea maritime habitats – while also specialising in the occupation of them, and so enable our species as a whole to proliferate even while local communities sometimes failed (Roberts & Stewart, 2018). Earlier and contemporaneous species of Homo that were expanding into Eurasia in the Early and Middle Pleistocene (2.6 Ma – 126 ka) made genera use of forest and grassland mosaics (Zhu et al., 2008; Gamble, 1993), which made them potentially vulnerable to more extreme environmental changes that occurred in the Late Pleistocene (e.g., Rizal et al., 2020) and incapable of surviving on islands that were depauperate in large terrestrial fauna (Gamble, 1993).

Given recent finds that other species of hominin may have ventured into challenging adaptive settings (Reich et al., 2011; Chen et al., 2019), testing this hypothesis is particularly timely. An ideal ‘island laboratory’ setting is provided by Wallacea in which to do so in the increasingly paleoanthropologically significant Southeast Asian region. Wallacea is an isolated series of islands that have never been connected to neighbouring Pleistocene landmasses of Sunda or Sahul, which necessitates a waster crossing to reach (O’Connor et al., 2017; Bird, Taylor & Hunt, 2005; Bird et al., 2019; Norman et al., 2017).  It has been hypothesised that these islands host depauperate island forest environments, that lack reliable terrestrial protein and carbohydrate resources (O’Connor, Ono & Clarkson, 2011; O’Connell & Allen, 2012; Samper Carro et al., 2016). It is significant that while these islands are home to some of the earliest evidence of earliest H. sapiens east of Africa and the Middle East ~45 ka (O’Connor, Ono & Clarkson, 2011; Hawkins et al., 2017; Shipton et al., 2019), and it is also suggested by finds of artefacts that earlier members of the genus Homo were present on the island of Flores from ~1 Ma (Morwood et al., 2005; Van den Bergh et al., 2016), Luzon from 0.7 Ma (Ingicco et al., 2018), and Sulawesi from ~0.2 Ma (Van den Bergh et al., 2016). More direct assessments of overall hominin resource reliance and palaeoenvironmental changes in the region have been lacking, though some insights into the ecological niches of different populations in Wallacea have been provided by zooarchaeological records.

In this paper Roberts et al. examined the adaptations of the earliest known fossil members of Homo sapiens that have been found in Wallacia by the use of isotopic analysis of archaeological human tooth enamel from 2 islands, Timor and Alor. The earliest known dated material culture and fossil evidence for H. sapiens in Wallacia was found at the sites of Asitau Kuru (formerly Jerimalai) and Laili on the island of Timor (Hawkins et al., 2017; Shipton et al., 2019). It is suggested by faunal remains and cultural artefacts at Asitau Kuru that during the Late Pleistocene humans relied on marine shellfish and fish, which were partly obtained by fishing (O’Connor, Ono & Clarkson, 2011). Evidence of early reliance on marine resources is also provided at Laili (Hawkins et al., 2017). This contrasts with the generalised adaptations to mixed grasslands and woodlands that were associated with other hominins in the region (O’Connor et al., 2017; Roberts & Amano, 2019; Brumm et al., 2019). Human reliance on fishing at Asitau Kuru has, however, been questioned (Louys et al., 2018). There remains, moreover, the possibility that giant rat taxa, which has been proposed to have had a preference for closed forest environments and to have had a body weight of adults of up to 6 kg, which represented significant food resources; and they have been identified in early coastal and inland archaeological contexts in Alor and Timor (e.g., Louys et al., 2018).

Insights into the environments that were present, as well as human reliance on different ecosystems is difficult to resolve by the use of traditional zooarchaeological methods alone due to the preservation biases and the role of nonhuman predators in site taphonomy (e.g., Samper Carro et al., 2017).

In this study Roberta et al. applied stable (δ¹³C) and oxygen (δ¹⁸O) isotope analyses to the enamel of human and faunal teeth from 6 archaeological sequences dating to the Late Pleistocene/Holocene on Timor and Alor in order to determine reliance of early human colonisers of Wallacea on tropical forest and terrestrial versus marine resources. Stable isotope analysis of faunal (including hominin) tooth enamel in tropical regions has been used in the assessment of proportion of C₃ dominated woodland/forest and C₄ grassland biomass in diets (Lee-Thorp, Sealy & van der Merwe, 1989; Levin et al., 2008; Roberts et al., 2017). In regions such as Wallacea in the Pleistocene, where it has been suggested by some researchers that tropical forests dominated terrestrial environments (Monk, de Fretes & Reksodiharjo-Lilley, 1997) with grasslands considered to be absent the canopy effect will be the most significant driver of stable isotope variation, whereby low light and respired CO₂ cause plant biomass that dwell in forest and its consumers to have more negative ¹³C values than their counterparts in more open habitats (Levin et al., 2008; Roberts et al., 2017). Meanwhile, marine producer biomass has higher ¹³C than all C₃ plants (Smith & Epstein, 1971; Fry, 2006), which allows marine consumers to be distinguished from terrestrial C₃ consumers (Kusaka et al., 2015). Roberts et al. expect, based on research carried out in East Africa (Levin et al., 2008), Sri Lanka (Roberts et al., 2017), and Japan (Kusaka et al., 2015), including extensive modern studies (Levin et al., 2008), preindustrial humans that relied completely on tropical forest, open C₃ resources, and marine resources to have tooth enamel δ¹³C values of about -14‰, about -11‰, and about -4‰, respectively.

Additional palaeoecological information about water and food is provided by stable isotope (¹⁸O) measurements from the tooth enamel of animals, and has also been argued to distinguish terrestrial from marine consumers (Clementz & Koch,  2001). The deposits at Asitau Kuru, Matja Kuru 1 and 2, Lene Hara, Makpan, and Tron Bon Lei provide a unique suite of human and associated faunal samples that span the earliest fossil appearance of H. sapiens in Wallacea through the Last Glacial Maximum (LGM) and across the terminal transition from the Pleistocene to the Holocene (Hawkins et al., 2017; Shipton et al., 2019), based on existing, published and available chronological information. Coastal and hinterland habitats are both covered by them. The building of the first detailed palaeontological and palaeoenvironmental records for Wallacia in the Pleistocene is allowed by ample marine and terrestrial animal remains as well as test assumptions in relation to:

1)   Pure C₃ terrestrial environments on Timor and Alor in the past;

2)   The ¹³C distinction between available marine and terrestrial resources; and

3)   Environmental shifts across the Pleistocene-Holocene boundary that has been proposed elsewhere in Southeast Asia (e.g., Rabett, 2012; Cannon, Morley & Bush, 2009).

Fourier transform infrared spectroscopy (FTIR) as per Roberts et al. (Roberts et al., 2017; Roberts et al., 2018), was also used to check the preservation of a subsection of the analysed tooth enamel sample.

It is demonstrated by the extensive faunal baseline of Roberts et al. that terrestrial and marine environments can be clearly distinguished isotopically in Wallacea on the basis of stable isotope analysis of fossil tooth enamel. It is shown by a tooth of the H. sapiens that was the earliest known that has been preserved that was found from the region of ~42-39 ka, demonstrates that this individual made significant use of coastal resources. Human populations show an increasing reliance on interior, terrestrial environments on the islands of Timor and Alor at a time when there was increasing forest expansion in Island Southeast Asia more generally, though there were some individuals that continued to use marine resources intensively. It is argued by Roberts et al. that their data demonstrates further the huge adaptive flexibility of our species, which is acutely visible as it rapidly and persistently colonised Wallacean environments. The ability of H. sapiens to the use of more extreme environments seems to contrast with that of other hominin species that are known from Southeast Asia based on current evidence.

Discussion

Direct assessment of the ecological reliance on different categories of resources that accompanied the arrival of H. sapiens and subsequent settling of Wallacea was allowed by the stable isotope data of Roberts et al. When the earliest humans arrived in this part of the world they seemed to be specialised in the use of coastal resources. Though it is currently not possible to distinguish between pelagic and the use of other offshore resources (O’Connor, Ono & Clarkson, 2011), Roberts et al. have confidence that the δ¹³C value for this individual indicates that he relied on marine resources. A clear diversification of human use of resources across Wallacea emerged after 20,000 cal. BP. Most individuals demonstrated broader use of interior environments, which included closed tropical forest habitats, though it is indicated that 1 individual at Matja Kuru 2, and possibly also Makpan, showed some coastal reliance. Roberts et al. suggest that this may be considered surprising, particularly given the ongoing presence of fish and shellfish (O’Connor et al., 2017), the presence of fish hooks in a symbolic burial of an individual at Tron Bon Lei (Supplementary note 1), as well as archaeological evidence for transfer of material culture between islands from time to time (Reepmeyer et al., 2019; Shipton et al., 2019). It is reflected, however, δ¹³C from tooth enamel that the entire diet of an individual, and the data of Roberts et al., highlights the necessity of paying more attention to the contribution of plant (and terrestrial animal) to the diets of humans on tropical islands, especially as those populations become more established – something that has recently been urged for the study of the later Lapita expansion in the Pacific (Maxwell et al., 2016; Tromp et al., 2020).

The first detailed palaeoenvironmental information for Wallacea in the Late Pleistocene-Holocene is presented in this study, which is associated directly with records of human behaviour. It indicates by the data of Roberts et al. that the tropical forest environments on Timor remained prevalent throughout the past 45,000 years, and decreased only in the Late Holocene with the arrival of deforestation during the Iron Age (Louys et al., 2018; O’Connor & Aplin, 2007). There is no evidence of C₄ grassland environments being present in the vicinity of any of the sites that were studied. On Alor, by contrast, C₄ resources may have been available to some small mammals, and presumably to humans as well, during the earliest period of occupation (40,000 to 21,000 years ago). These resources disappear as tropical forests expanded across the Terminal Late Pleistocene-Holocene boundary, while it remains possible that elevated ¹³C for some fauna in the early phase represents consumption of marine resources, this pattern of increasing tropical forests during the Terminal Pleistocene has also been documented in other places in Island as well as Mainland Southeast Asia (Rabett, 2012). The expansion of tropical forest during the Terminal Pleistocene and Early Holocene has also been associated with humans that were increasingly associated with specialised hunting of arboreal and semi arboreal mammals and the use of plants from the tropical forests (Rabett, 2012),

Human colonisation models of Wallacea and Australia suggesting rapid, initial coastal colonisation that was followed by later inland settlement (Bird et al., 2019; Kealy, Louys & O’Connor, 2017), at least with the sites that were studied here, and is supported by the isotopic evidence of Roberts et al. This mode of colonisation is distinct from the isotopic and material evidence from the rainforests of the Wet Zone of Sri Lanka in the Late Pleistocene (Roberts et al., 2017; Wedage et al., 2019) and archaeological evidence that was recovered from Niah Cavs in Borneo (Barker & Farr, 2016) which indicate dedicated, specialised foraging in tropical forests by early human populations in these regions that began about 45,000 years ago. This highlights further the potential role of sophisticated seafaring in the colonisation of eastern Wallacea and Australia by early humans (Bird et al., 2019; O’Connor & 2011; Balme, 2013). During the Terminal Pleistocene and Early Holocene a later, increased focus on terrestrial resources or nearshore coastal resources has also been argued based on zooarchaeological evidence from Timor, as well as elsewhere in Island Southeast Asia (Hawkins et al., 2017; Boulanger et al., 2019). Across Wallacea from the Terminal Pleistocene this subsistence pattern occurs alongside an increase in occupation intensity, as well as an increase in long distance trading networks that are formalised, and is likely to be representative of exchange between settled groups in the region (Reepmeyer et al., 2019; Shipton et al., 2019). The adaptive flexibility displayed by humans in the colonisation of nearly all the continents of the Earth in the Late Pleistocene stands in stark contrast to the other species of hominin (Rizal et al., 2020; Roberts & Stewart, 2018; O’Conner et al., 2017; Roberts & Amano, 2019).  Where present, existing zooarchaeological/palaeontological and palaeoenvironmental evidence suggests a general, albeit diverse, focus on mixed grassland and woodland environments, with dispersals and contractions often relying of environmental change that was climatically driven (Roberts & Amano, 2019; Brumm et al., 2016), though there is some evidence that other hominins made water crossings (Morwood et al., 2005; Van den Bergh; 2016; Ignicco et al., 2018; Van den Bergh et al., 2016) or ventured into high altitude environments (Chen et al., 2019). In order to test this distinction, in Wallacea and beyond, requires that isotopic analysis and more detailed zooarchaeological work in the future be carried out. There is clear evidence, however, that different populations of H. sapiens were able to specialise in a variety of extreme environments even as humans as a whole generalised in the use of multiple settings (Roberts & Stewart, 2018). This flexibility that was possibly supported by unique capacities of innovation and social communication (Boyd, Richardson & Henrich, 2011), enabled adaptation to a variety of conditions, through time as well as space, that would eventually leave humans as the last hominins standing.

Roberts, P., et al. (2020). "Isotopic evidence for initial coastal colonization and subsequent diversification in the human occupation of Wallacea." Nature Communications 11: 2068.