Climate change and human origins in southern Arabia

Proceedings Of thE SEMinar fOr arabian StUdiES VOLUME 38 2008 Proceedings of the seminar for arabian studies Volume 38 2008 Papers from the forty-first meeting of the Seminar for Arabian Studies held in London, 19-21 July 2007 seminar for arabian studies archaeoPress oxford Orders for copies of this volume of the Proceedings and of all back numbers should be sent to Archaeopress, Gordon House, 276 Banbury Road, Oxford OX2 7ED, UK. Tel/Fax +44-(0)1865-311914. e-mail bar@archaeopress.com http://www.archaeopress.com For the availability of back issues see the Seminar’s web site: www.arabianseminar.org.uk Steering Committee of the Seminar and Editorial Committee of the Proceedings Dr. R. Carter (Chairman) Prof. A. Avanzini Dr. M. Beech Dr. N. Durrani Dr. R. Eichmann Prof. C. Holes Dr. R.G. Hoyland Dr. D. Kennet Mr. M.C.A. Macdonald Dr. A. MacMahon (Secretary) Prof. K. Al-Muaikel Dr. V. Porter Prof. D. Potts Prof. C. Robin Dr. St.J. Simpson (Joint Editor-in-Chief) Mr. A. Thompson (Treasurer) Prof. J. Watson Dr. L. Weeks (Joint Editor-in-Chief) Seminar for Arabian Studies c/o the Department of the Middle East, The British Museum London, WC1B 3DG, United Kingdom e-mail seminar.arab@durham.ac.uk Opinions expressed in papers published in the Proceedings are those of the authors and are not necessarily shared by the Editorial Committee. Typesetting, Layout and Production: David Milson The Proceedings is produced in the Times Semitic font, which was designed by Paul Bibire for the Seminar for Arabian Studies. © 2008 Archaeopress, Oxford, UK. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior permission of the publisher. ISSN 0308-8421 ISBN 0-9539923-4-9 Proceedings of the Seminar for Arabian Studies 38 (2008): 25–42 Climate change and human origins in southern Arabia a.g. Parker &. J.i. rose Summary Over the past few years, prehistorians have begun to consider South Arabia with increasingly greater interest. As the corpus of genetic data grows, scholars now realize the prominent role the “Arabian Corridor” must have played in modern human origins. Unfortunately, Palaeolithic investigations throughout the peninsula have lagged sadly behind; at the time of writing there are only three dated, stratified Palaeolithic sites that fall within the Upper Pleistocene time period (Shi’bat Dihya, al-Hatab, and Jebel Faya 1). While there are meagre data to discuss the human footprint upon the landscape, we possess abundant information to describe the land itself. This paper is intended to synthesize and present the palaeoenvironmental record throughout the late Quaternary in South Arabia, thereby presenting the landscape across which the earliest humans traversed during the initial expansion from their ancestral homeland. We present the HOPE ENV database, which is a composite sum probability curve that incorporates several hundred proxy signals used to discern ancient climatic conditions. This paper considers shifts in the terrestrial landscape morphology, as well as reconfiguration of the shorelines due to eustatic and isostatic sea levels change. We discuss how this record of environmental change might have affected human emergence, from the first appearance of anatomically modern Homo sapiens to the development of complex civilization in the middle Holocene. Keywords: human origins, climate change, Arabian Peninsula, palaeoenvironment, prehistoric archaeology Introduction The pendulum of environmental change in Arabia has oscillated between climatic extremes throughout the Quaternary period. The landscape is riddled with evidence for ancient pluvials, apparent in the lacustrine sediments, alluvial fans and gravels, palaeosols, and speleothems (e.g. McClure 1976; Schultz & Whitney 1986; Parker et al. 2006; Lézine et al. 2007; Fleitmann et al. 2007). Conversely, there are numerous signals that Arabia was subjected to extremes in aridity, most obviously manifested in the expansive sand seas comprising the Nafud, Rub’ al-Khali, and Wahiba deserts, as well as hyperalkaline springs (Clark & Fontes 1990) and petrogypsic soil horizons (Rose 2006). The earliest western explorers to penetrate the Rub’ al-Khali often described a series of small buttes standing out in stark white or grey against the seemingly endless wasteland of monotonous rust-coloured sand. During his pioneering journey across the desert in 1932, St John Philby recognized these features as small eroded lake basins comprised of marl terraces and hardened evaporitic crusts, noting associated freshwater shells and lithic implements scattered around the edges (Philby 1933). The occurrence of ancient stone tools near relict lake beds is ubiquitous throughout South Arabia and hints at a rich prehistoric past, one of which archaeologists have only yet encountered the tip of the iceberg. The aim of this paper is to present the backdrop of South Arabian prehistory by providing an overview of the mosaic of shifting landscapes during the Quaternary. These data provide a useful framework for understanding the role of the climate in patterning the ebb and flow of hominin occupation across the Arabian corridor — a critical geographic zone that has recently been established as a conduit bridging early human populations in Europe, Africa, and Asia. Geography, geology, and climate The Arabian Peninsula is bounded on the west by the Gulf of Aqaba and the Red Sea, on the south by the Gulf of Aden and the Arabian Sea, and on the east by the Gulf of Oman and the Arabian Gulf. The subcontinent measures 26 A.G. Parker &. J.I. Rose linked to the same tectonic movement as well. As the Arabian plate travels toward Asia, the Indian plate is sliding beneath it and forcing the ancient bed of the Tethys Sea to thrust upward. Consequently, the Hajar Mountains form the spine of south-eastern Arabia, reaching 3000 m above sea level in north-western Oman. This relatively long chain of mountains stretches from Ras al-Hadd in eastern Oman to the tip of the Musandam Peninsula at the Strait of Hormuz, a distance of over 600 km. Large, low angle alluvial fans coalesce at the mountain front, from which an extensive network of widis flow inland from the Hajar Mountains into the Rub’ al-Khali, the Umm asSamim, and the Haushi-Huqf basins. Perhaps the most profound outcome of the AfricanArabian tectonic rifting that has affected the course of human history is compression of the Arabian plate as it pushes against Eurasia. This process has led to geological subsidence throughout the eastern portion of Arabia, most notably a shallow depression that comprises the AraboPersian Gulf basin. In addition, buckling sedimentary strata created a series of north–south folds, thereby creating the world’s largest oil reservoirs beneath the Arabian Shelf. Under the present arid climatic regime, the low-lying basins of eastern Arabia are dominated by aeolian deposition, blanketed by massive sand seas. The Wahiba Sands are located in eastern Oman, north-east of the Haushi-Huqf Depression. This desert is comprised of linear dunes oriented on a north–south axis that run parallel to one another for several hundred kilometres. The dunes reach up to 100 m in elevation and are separated by swales 1–3 km wide (Glennie & Singhvi 2002; Preusser, Radies & Matter 2002). The most recent aeolian deposits in Wahiba formed during the last glacial maximum, at which time the emerged continental shelf provided abundant unconsolidated carbonates available for aeolian transport (Glennie 1988; Preusser, Radies & Matter 2002). Encompassing nearly 600,000 km2, most of the interior of southern Arabia is blanketed by the Rub’ al-Khali sand sea. This massive basin slopes from an elevation of approximately 1200 m above sea level in the west to nearly sea level in the east. The dunes of the Rub’ al-Khali include a variety of types, resulting from the alternating wind patterns and diverse sources of sand. The dunes are tallest in the south-western portion of the basin, as much as 200 m in height. Like the Wahiba, the extant features of the Rub’ al-Khali dunes formed during the late Pleistocene, comprised primarily of reworked Pleistocene sediments above a bed of Pliocene alluvial gravels (McClure 1978). 2100 km from north to south along the Red Sea coast, and nearly 2000 km across at its maximum width from the westernmost region of Yemen to the easternmost point in Oman. The littoral is characterized by tropical and subtropical ecosystems, while the basin-shaped interior is dominated by alternating steppe and desert landscapes. Three major sand seas are found in Arabia: the Rub’ alKhali (600,000 km2), Nafud (72,000 km2), and Wahiba Sands (12,500 km2) (Goudie 2003). Arabia is skirted by mountainous terrain along the western, southern, and eastern edges of the peninsula. The ‘Asir Highlands run along the western flank of the Kingdom of Saudi Arabia, called the Yemen Highlands where they extend into the Republic of Yemen. This mountain chain reaches nearly 4000 m above sea level in the south — the highest point on the entire peninsula; as a result, it receives up to 1000 mm of rainfall per annum. The coastal plain of southern Arabia is bounded by the ДaΡramawt (in Yemen) and Nejd (in Oman) plateaus. Extending north from the Dhofar escarpment, sedimentary beds rise sharply to an elevation of 1000 m above sea level, gradually levelling off northwards onto the Nejd. The entire region is comprised of uplifted Tertiary limestone that gradually slopes into the Rub al‘Khali basin. The ridge of the Dhofar escarpment marks the watershed divide; southwards-flowing drainages are seasonally active under present conditions, incising the limestone cliffs at a steep grade and creating springs and lagoons as they pool onto the coastal plain. Presently, the northwards-flowing drainages receive almost no storm flow but, during pluvial cycles, the magnitude of the monsoon was sufficient enough to produce high-energy fluvial systems. The tectonic plate that constitutes the Arabian Peninsula is derived from Africa. For most of geological history, both landmasses formed part of a pan-AfroArabian continent. Then, around 30 million years ago the Arabian plate broke off from the African Shield and began to slide to the north-east, rotating in a counter-clockwise direction. This event triggered a chain reaction of seismic transformations that have had an indelible effect on the course of palaeoenvironmental and palaeoanthropological history. One significant modification was the formation of the Red Sea trough. This narrow, elongate depression is more than 2000 km long and varies in width from 180 to 300 km along the main channel. The rifting of these two plates also triggered volcanic activity along the western edge of Arabia, producing the jagged basalt peaks of the ‘Asir-Yemen Highlands. The genesis of Arabia’s eastern mountain range is Climate change and human origins in southern Arabia The peninsula is subject to two different weather regimes (Barth & Steinkohl 2004). From the north come Atlantic late-winter north-westerlies, which move eastwards over the Mediterranean Sea, down the Arabian Gulf, and eventually dissipate over the Rub’ al-Khali desert and Musandam Peninsula, bringing cool gentle winds and light precipitation (Parker et al. 2004). The second weather regime consists of summer storms brought by the south-west Indian Ocean monsoon system. From June to September, the highlands of Yemen and Oman receive relatively heavy rainfall as the mountainous terrain of southern Arabia traps moisture from the monsoon (Lézine et al. 1998; Glennie & Singhvi 2002). Consequently, the ‘Asir and Dhofar Mountains receive between 200–1000 mm annually; while areas closer to sea level seldom collect more than 100–200 mm per year (Schyfsma 1978). 27 Flora and fauna The environmental gradients across Arabia, the floral varieties in adjacent territories, and the legacy of Quaternary climate change have all had a significant effect upon the distribution of plant types throughout the subcontinent (Parker et al. 2004). In southern Arabia, the sparse vegetation cover is grouped within the following biotopes: coastal habitat, interior basin, ‘Asir-Yemen Highlands, Dhofar Mountains, and Hajar Mountains. Flora such as Cressa cretica (cressa), Nitraria retusa (salt tree) and Juncus maritimus (sea rush) are found growing on the marine shores and salt flats. Wild rue, mangrove, indigo, date palms, henna, tamarinds, mistletoe, and ilb prosper around coastal wadi banks, particularly those along the Tihama (Red Sea) and al-Batinah (Gulf of Oman) coasts (Miller & Thomas 1996). In the interior, plants such as tamarisks, poplars, acacias, and several other species of reeds, grasses, and small shrubs are found scattered near depressions and seasonal drainage systems that receive a limited degree of moisture (Hugh & Mason 1946). Due to heavy precipitation deposited by the summer monsoon, there is a wide variety of flora in the ‘Asir-Yemen Highlands. Wild figs, leguminous trees, tamarisks, date palms, indigo, qat, myrrh, and a variety of flowering bushes and herbs are found along the wadi banks, while forests of juniper cover the mountain slopes between 2500 and 3000 m elevations (1946). Pollen samples taken from the wooded Yemeni highlands show a predominance of acacia, Zygophyllum (Syrian bean caper), and several species from the family Chenopodiaceae (goosefoot) (Lézine et al. 1998). A similar distribution of flora has been identified in the Dhofar Mountains and described as rolling grasslands and dense and verdant copses and woodlands, reminiscent rather of upland regions in the African savannah. Indeed, the close resemblance of South Arabian floral varieties to that of Africa is due to the fact that many of these taxa spread eastwards from Africa. These woodland and grassland ecosystems belong to the Saharo-Sindian and Saharo-Arabian phytogeographic zones (Mandaville 1985; Ghazanfar & Fisher 1998; Ghazanfar 1999). The composition of plant communities and the morphology of endemic species suggest a close botanical relationship between Africa and Arabia throughout the Quaternary. Examples of East African-derived plant types include Acacia sp. (Acacia), Ziziphus ziziphus (Jujube), and Apocynoideae rhazya (dogbane). Plant types in south-eastern Arabian (the modern territories of northern Oman and the UAE) show strong ties to the flora of Iran and south-western Pakistan (Baluchistan), comprising the Omano-Makranian subzone of the Nubo-Sindian centre of endemism (Mandaville 1985; Ghazanfar & Fisher 1998; Ghazanfar 1999). Floral elements that are linked to Asian taxa include Euphorbia larica (succulent spurge), Prunus amygdalus (almonds), Ficus carica (figs), Lawsonia inermis (henna), and Indigofera tinctoria (true indigo). These pronounced affinities in plant distribution across the OmanoMakranian sub-zone are attributed to the fact that during much of the Quaternary the basin of the Arabian Gulf was exposed, forming one continuous territory from Arabia into South Asia (Williams & Walkden 2002). Of the large mammals, animals belonging to the family Bovidae are by far the most prominent on the Peninsula. These include one species of oryx, three species of Gazella, two species of Capra, one belonging to the genus Hemitragus (wild goat), and one of the genus Ovis (wild sheep). These animals typically occupy areas that receive moderate to high amounts of rainfall such as the Yemeni highlands, Dhofar, and the Hajar Mountains. Gazelle have been noted from more arid settings such as the high plateaus, while the desert-adapted oryx were once ubiquitous throughout the interior, even within the Rub’ al-Khali (Harrison 1980). Small mammals include various species from the family Soricidae (shrews), order Rodentia (rodents), and order Chiroptera (bats). There are carnivores such as mongooses, genets, dogs, wolves, and foxes (Harrison 1980). A variety of felines are present, including Felis silvestris (wild cat), Felis margarita (sand cat), Caracal 28 A.G. Parker &. J.I. Rose figure 1. Laminated lacustrine strata at Wahalah Lake, Ras al-Khaimah, UAE. caracal (Caracal lynx), Panthera pardus (leopard), and Acinonyx jubatus (cheetah) (Harrison & Bates 1991). Like the Bovidae family, these animals are typically found in and around the montane zones. There are some examples of Lagomorpha (rabbits, hares, and pikas) reported from the interior — on the desert plateaus and along the margins of the Rub’ al-Khali (1991). Much like the pattern of floral distributions, there are strong inter-regional links between Arabian fauna and adjacent areas. Terrestrial snails of northern Oman are primarily Palaearctic taxa, while snails found west of Dhofar have East African affinities (Mordan 1980). Fernandes et al. (2006) report mtDNA evidence for a recent genetic divergence between African and Arabian genets. They note several other small and medium-sized carnivores (e.g. mongooses, desert foxes, honey badger, caracal, jungle cat, golden jackal) that occur on both sides of the Red Sea that may also be genetically linked. Harrison (1980) corroborates these African and Arabian faunal connections, observing that Crocidura somalica (shrew) and Genetta granti (genet) are closely related to East African varieties. Another indicator of faunal connections across the Red Sea is the presence of the primate Papio hamadryas — Sacred Baboon — in Yemen. Presently, these primates are indigenous to the rocky hill country of Somalia, Ethiopia, and Yemen. Papio hamadryas are arid-adapted creatures that forage protein-rich insects, hares, and other small mammals; they obtain water from shallow pools and by digging small wells in desert regions with a high water table (Nowak 1991). Analyses of Papio hamadryas mtDNA lineages on both sides of the Red Sea suggest that they originated in East Africa sometime between 150,000 and 50,000 years ago, and subsequently migrated into Climate change and human origins in southern Arabia 29 figure 2. Relict fluvial terrace in Wadi Arah, southern Oman. Arabia (Wildman 2000; Wildman et al. 2004; Fernandes, Rohling & Siddell 2006). That the East African baboon lineage is older than the Arabian implies that there must have been a demographic bottleneck release sometime during the Upper Pleistocene. This spike is attributed to the disappearance of snow and ice cover over central Asia, Tibet, and the Himalayas, suggesting that one of the primary mechanisms driving monsoon fluctuations are climatic conditions at glacialinterglacial boundaries. Biogeochemical and lithogenic data from Arabian Sea cores spanning the last 350,000 years also support the notion that monsoon winds were sensitive to changing glacial climates. The retreat of ice sheets, the rise in continental albedo (solar radiation reflected off the earth’s surface), and the increase of water surface temperatures in the western Indian Ocean triggered spikes in magnitude (Clemens et al. 1991). Computer simulations have been used to estimate the average wind speed of the south-west monsoon during such phases of intensification. Speeds currently average around 10 m/sec, while increased periods of activity the saw wind speeds reaching 15 m/sec. Precipitation would have been 50 % greater than its present value, growing from 5 mm/day to 7.5 mm/day. Northwards-shifting insulation patterns drove the monsoons further into the Arabian Peninsula, with evidence for seasonal storms reaching as far north as Bubiyan Island in the Arabian Gulf (Sarnthein 1972; Kutzbach 1981). Researchers have attempted to model the rate of change during shifts in monsoon magnitude. Analysis of 15 N isotopes was conducted on an interval of Arabian Sea Pleistocene and Early Holocene climate change in Arabia Most of the precipitation that falls over Arabia is brought by the afore-mentioned south-west Indian Ocean monsoon system, considerably more so than from “north-westerly” winter storms. Consequently, the environmental fate of the region, amelioration or desiccation, rests upon the intensity of the monsoon, which has been in flux for at least the last quarter of a million years (Clemens et al. 1991; Muzuka 2000; Fleitmann et al. 2004). Indian Ocean monsoon cycles: life and death of the South Arabian landscape Marine cores from the Indian Ocean, Gulf of Oman, and the Arabian Sea provide a detailed history of the south-west Indian Ocean monsoon system throughout the Quaternary. Analysis of dinoflagellate cyst content from Arabian Sea deep sea cores during the last glaciation reveals an abrupt fluctuation at 12,500 years ago (Zonneveld et al. 1997). 30 A.G. Parker &. J.I. Rose figure 3. Late Pleistocene dune profile at ash-Shwaib, UAE. core spanning the bracket of time between 43,000 and 42,000 years ago — a well-established climatic boundary. Significant changes in mean strength occurred within a span of 200 years, which is relatively instantaneous on a Pleistocene time scale (Higginson 2004). While the onset of the intensified monsoon was rapid, evidence has been presented suggesting that the shift back towards aridification was a more gradual process occurring on a millennial scale, at least during the most recent wet/dry shift (Lückge et al. 2001). The stable oxygen isotope record of various planktonic foraminiferal species (i.e. Globigerinoides ruber, Globigerina bulloides, and Neogloboquadrina dutertrei) attests to temporal variations in marine palaeoproductivity. Analysis of species frequency distribution over the last glacial cycle shows a direct correlation between palaeoproductivity in the Arabian Sea, the strength of the monsoon, and the global oxygen isotope curve. Scholars note the onset of intensified monsoon episodes can lag up to 1000 years after shifts in glacial conditions, possibly due to the threshold necessary for sufficient amounts of snow and ice to melt and affect Indian Ocean insulation patterns (Reichart, Lourens & Zachariasse 1997; PetitMaire et al. 1999; Ivanova et al. 2003). Fluctuations in monsoon intensity are evidenced by a variety of signals upon and within the landscape. The most complete records come from a series of dated speleothems in the Hajar and Dhofar mountains. Pronounced pluvial conditions are also signalled by remnants of ancient lake deposits (Fig. 1), travertines, fluvial terraces (Fig. 2), and alluvial fans spreading along the piedmont regions. The expansive sand seas found throughout Arabia’s desert are a testament to the hyperarid phases that have occasionally swept across the peninsula (Fig. 3). We have compiled all of these palaeoenvironmental signals together to build a comprehensive database of Climate change and human origins in southern Arabia 31 figure 4. HOPE ENV sum probability curve depicting wet/dry signals throughout Arabia during the Upper Pleistocene. climate change in Arabia, referred to as HOPE ENV. These data are derived from published sources as well as new evidence collected by the authors in the field. Based on a total of 396 absolute dates, we present a composite sum probability curve (Fig. 4) to illustrate climatic oscillations over the past 175,000 years, from MIS 6 to present. Peaks in sum probability represent periods of increased wetness, while troughs highlight drier phases. All dates are reported in calendar years BP. Climatic conditions during the late Middle and Upper Pleistocene are of particular interest to our study, given that this span of time formed the backdrop of early human emergence from Africa. Was the climate arid enough during the last few glacial maxima to preclude hominin habitation and, conversely, were conditions humid enough to facilitate human occupation during the Last Interglacial? Anton (1984) speculated that the environment was hyperarid during MIS 6, given that monsoon intensity roughly tracks with the global marine isotope curve. The emerging picture in Arabia indicates the situation was more varied than this initial assessment; a smattering of chronometric dates suggests there were brief pulses in precipitation. Compelling evidence for increased moisture at the MIS 7-MIS 6 interface comes from speleothems in Hoti Cave, northern Oman, where U/Th measurements exhibit an increase in growth rate between 200–180 ka (Burns et al. 2001). The prospect of stage 6 sub-pluvials are corroborated by optical dates on fluvial silts at Sabkha Matti (147,000 ± 12,000 BP) (Goodall 1995), two U/ Th measurements from freshwater mollusca within lacustrine sediments at Mudawwara (170,000 ± 14,000 BP and 152,000 ± 8,000 BP) (Petit-Maire et al. 1999), optically dated fluvial silts at Falaj al-Moalla in the Wadi Dhaid, UAE (193,110 ± 30,750 BP), OSL measurements on fluvial silts recorded at the Camel Pit Site, Umm al-Qawain, UAE (174,300 ± 24,110 BP), and optical measurements on evaporitic lacustrine sediments sampled from a relict interdunal sabkha in the Liwa region of the Rub’ al-Khali, UAE (160,000 ± 8000 BP) (Wood, Rizk & Alsharhan 2003). The onset of the Last Interglacial period around 130,000 years ago was punctuated by an abrupt and drastic increase in rainfall over South Arabia that lasted 32 A.G. Parker &. J.I. Rose 3 that featured a series of small lakes spread across the interior (McClure 1984). Radiocarbon measurements on mollusc shells and marls indicate the lakes reached their highest levels around 37,000 BP (McClure 1976). These playas ranged from ephemeral puddles to pools up to 10 m deep, and numbered well over 1000. They are primarily distributed along an east–west axis across the centre of the Rub’ al-Khali basin, covering a distance of some 1200 km (McClure 1984). Similar lake basins have been reported from the Ramlat as-Sabatayn desert in Yemen (Lézine et al. 1998; 2007), as well as the Nafud in northern Arabia (Garrard & Harvey 1981; Schulz & Whitney 1986). Researchers speculate that lake-filling episodes were short-lived; these poorly drained basins would have been recharged by occasional torrential stormflow runoff and disappeared within a few years. Due to the region’s wide catchment area, the Mundafan depression was the exception to this model. Arabia’s thickest lacustrine deposits were recorded here, where single lake periods are estimated to have lasted at least 800 years. Fossilized faunal remains excavated within the Mundafan sediments yielded a menagerie of large vertebrates including oryx, gazelle, aurochs, wild ass, hartebeest, water buffalo, tahr, goat, hippopotamus, wild camel, and ostrich (McClure 1984). Most of these species belong to the family Bovidae, whose survival required expansive grasslands produced by light to medium rainfall distributed evenly over the Rub’ al-Khali. Ostracoda and freshwater mollusca indicative of low salinity were present at Mundafan, as well as species of foraminifera that attest to highly brackish conditions (McClure & Swain 1974). Evidence of grasses, shrubs, and herbs are indicated by both phytoliths and dikaka — thin, tubular fragments of fossilized material scattered in the aeolian sediments around the basins. These floral fossils were formed when dissolved calcium carbonate in the water precipitated onto plants as the lake evaporated. Evidence of fish remains are conspicuously absent from the Rub’ al-Khali lakes, because lakes were rarely refilled and became too alkaline too quickly to develop a population (McClure 1984). In addition to interior palaeolakes, other signals for an MIS 3 wet phase include depositional terraces in the Wadi Dhaid; although undated, their stratigraphic position suggests an age between 35 and 22 ka BP (Sanlaville 1992). Interdunal sibakh recorded in the Liwa region of the UAE have produced thirty-one dates (both uncalibrated 14C as well as OSL) that cluster between 46,500 and 21,500 BP (Wood & Imes 1995; until approximately 120 ka, followed by a second peak in precipitation corresponding with MIS 5a (82–74 ka). U/ Th dates on palaeosols from the western piedmont of the Hajar Mountains in Oman that are correlated with isotopic stages 5e and 5a (Sanlaville 1992). Soils were also noted in the ad-Dahna Desert of northern Arabia, where late Pleistocene dunes overlie two separate pedogenic strata that could only have formed on stabilized dunes with a dense cover of vegetation (Anton 1984). There is a network of Plio-Pleistocene bas-relief gravel channels west of the Wahiba desert that is superimposed by thinner fluviatile gravels tentatively associated with particularly humid episodes during MIS 5e and MIS 5a (Maizels 1987). Stokes and Bray (2005) obtained over fifty optical dates from megabarchan dunes in the Liwa region, which lies along the eastern margin of the Rub’ al-Khali. Their findings suggest a prolonged period of dune accumulation from 130 to 75 ka. This deposition was attributed to a unique combination of factors such as reduced sea levels in the Arabo-Persian Gulf that produced an abundance of sedimentary material available for transport, a rise in regional groundwater levels, and vegetation cover that stabilized the dunes. Multiple MIS 5 pluvial episodes are signalled by the aforementioned Hoti Cave speleothems, which yield U/Th dates indicating rapid growth between 135–120 ka and 82–78 ka. Researchers noted that speleothem growth was most pronounced during MIS 5e, more so than all subsequent pluvials (Burns et al. 1998; 2001). Most recently, we have dated a series of buried alluvial fans interstratified with fluvial sands along the western edge of the Hajar Mountains in Ras al-Khaimah, UAE. OSL ages obtained from these sediments indicate they were deposited at 117,030 ± 15,080 BP and 107,970 ± 9660 BP. There are meagre climatic data from MIS 4 and early MIS 3 in southern Arabia. Indirect evidence from the HOPE ENV summed probability curve as well as the index of Indian Ocean Monsoon activity (Fleitmann et al. 2007) suggests this time frame was characterized by increasingly hyperarid conditions culminating around 70 ka, followed by a return to a more humid regime by 50,000 years ago. The only physical evidence for aridification during MIS 4 can be inferred from stratigraphic profiles in the Rub’ al-Khali, which attest to a stage of aeolian accumulation sometime before 37,000 years ago. This deposition, however, is relatively minor as compared to the immense aeolian structures that accumulated during the terminal Pleistocene (McClure 1978). Geological investigations in the heart of the Rub’ alKhali sand sea have revealed a landscape during MIS Climate change and human origins in southern Arabia Juyal, Singhvi & Glennie 1998; Glennie & Singhvi 2002). Palaeosols have been recorded in the ad-Dahna desert, which are stratigraphically positioned between MIS 4 and MIS 2 aeolian deposits (Anton 1984). Clark and Fontes (1990) dated calcite formations from ancient hyperalkaline springs in northern Oman, producing radiocarbon ages between 33 and 19 ka. Two soil horizons were discovered around the central plateau of the Yemeni highlands, characterized as molissols — soils that form on landscapes covered by savannah vegetation. Uncalibrated radiocarbon measurements were 26,150 ± 350 BP for the lower stratum, and 19,290 ± 350 BP for the upper horizon (Brinkmann & Ghaleb 1997). Researchers speculate that the Terminal Pleistocene dry phase was more arid than the peninsula had experienced since the Penultimate Glaciation, if not earlier (Anton 1984). Ages obtained from dune formations in the Rub’ al-Khali (McClure 1984; Goudie et al. 2000; Parker & Goudie, 2007), an-Nafud (Anton 1984), and the Wahiba Sands (Gardner 1988; Glennie & Singhvi 2002) all signal a major phase of aeolian accumulation between 17,000 and 9000 BP. Calcite fractures in northern Oman corroborate the evidence for increasing aridity, indicating there was considerably less moisture in the environment starting around 19,000 BP (Clark 1990). The Terminal Pleistocene hyperarid phase ended with yet another pronounced oscillation back to humid conditions. This pluvial phase period lasted until c. 5000 BP, at which time the present climatic regime was established (Overstreet & Grolier 1988; Cleuziou, Inizan & Marcolongo 1992; Sanlaville 1992; Brunner 1997; Wilkinson 1997; Stokes & Bray 2005; Parker et al. 2004; 2006a; 2006b; 2006c). Shifting shorelines and human refugia As profound as these climatic oscillations have been over the course of the Quaternary, the rise and fall of sea level has had an even greater effect on the configuration of the Arabian subcontinent. Taking into account the shallow bathymetry of the Arabo-Persian Gulf and Red Sea basins, nearly 1 million km2 of contiguous land have been repeatedly exposed and submerged by glacioeustatic cycles of marine regression and transgression (Fig. 5). The emergence of the continental shelf around Arabia had direct implications for prehistoric occupation, since the exposed landmass provided abundant sources of fresh water amidst a generally desiccated landscape. The role of littoral zones in the dispersal of modern humans is the crux of ongoing discussion; recent models of human emergence from Africa envision rapid coastal migration 33 across the southern route of dispersal, facilitated by a new adaptation to aquatic resources (e.g. Stringer 2000; Mithen & Reed 2002; Mellars 2006). Evidence suggesting the importance of low-lying coastal habitats during the early and middle Holocene has been discovered by Bailey et al. (2007a; 2007b) around the Farasan Islands in the southern Red Sea, who report a nearly continuous line of over 1000 shell mounds along the beachfront. Faure, Walter and Grant (2002) have identified a process of littoral freshwater upwelling they refer to as the “coastal oasis” hypothesis that may help explain the importance of coastal habitats for early humans groups. Depressed sea levels during glacial maxima caused an increase of hydraulic pressure within submarine rivers; consequently, more freshwater flowed through these aquifers. Eventually, this process led to the creation of springs in favourable loci on the emerged shelf with lithology, faults, bathymetry, and/or topography conducive to upwelling. Such a phenomenon can be observed today in the abundant submerged seeps at the bottom of the Arabo-Persian Gulf. The area around modern Qatar is the terminus of several submarine rivers that flow beneath Arabia, creating a mass of upwelling plumes once used by ancient seafarers for restocking their freshwater stores (Church 1996). Indeed, a god known as Enki, the “Lord of the Sweet Waters”, was revered by the Gulf’s earliest inhabitants — a group which originated in the basin prior to its final inundation in the fourth millennium BC. Enki was believed to dwell within the “Abzu”, the freshwater of the deep; in the “Myth of Enki and Ninhursag” he is credited with building canals to bring freshwater to Dilmun (Jacobsen 1987). This example taken from Sumerian mythology serves as a useful frame of reference to highlight both the availability and importance of freshwater within the Arabo-Persian Gulf basin throughout the Pleistocene and Early Holocene. The Gulf is the shallowest inland sea in the world, averaging just 40 m in depth and covering some 225,000 km2 (Sarnthein 1972). Therefore, between 115,000 and 6,000 years ago, when sea levels were depressed below current levels, we speculate that this region served as a large refugium for local biomass (including hominins) constricted by arid conditions (for further discussion of the role of the Gulf basin in protohistory see Sayce et al. 1912; Barton 1929; Cooke 1987; Teller et al. 2000; Kennett & Kennett 2006; Sanford 2006). Throughout the Upper Pleistocene and Early/Middle Holocene, nearly all excess runoff in south-west Asia was funnelled into the Gulf basin via submarine aquifers 34 A.G. Parker &. J.I. Rose figure 5. Map of Arabia showing drainage channels and significant Pleistocene geomorphic features. flowing beneath Arabia, the Karun drainage originating in the Zagros Mountains, and the Tigris and Euphrates Rivers flowing from the Anatolian Plateau. All of these drainage systems converged within the centre of the basin, forming the Ur-Schatt River, which once traversed the entire length of the basin through a deeply incised canyon that is still evident in the extant bathymetry (Seibold & Vollbrecht 1969; Sarnthein 1972; Uchupi, Swift & Ross 1999). The most recent phase of Ur-Schatt River downcutting culminated during the Last Glacial Maximum, when global sea levels were reduced by 120 m and the basin was exposed in its entirety. Sometime around 14,000 BP, the Straits of Hormuz were breached by the Gulf of Oman, and by 12,500 BP marine incursion reached the central basin of the Gulf. This process of infilling has been relatively gradual; the waters only reached the present shoreline as recently as 6000 years ago, at which time the sea exceeded its current levels by 2 m (Bernier et al. 1995; Lambeck 1996; Williams & Walkden 2002). There is freshly unearthed evidence hinting at the importance of the Gulf basin refugium throughout prehistory. One such example is the unexpectedly early presence of Ubaid-related (late Neolithic) sites clustering along the southern coastline and located on islands within the Gulf itself (Haerinck 1991; 1994; Hermansen 1993; Jasim 1996; Beech & Elders 1999; Beech, Elders & Shepherd 2000; Beech et al. 2005; Carter 2006). The sudden appearance of a fully-sedentary, agrarian society within this previously uninhabited niche is incongruous with the pace of local Neolithic development. Therefore, we suggest the pattern of Ubaid-related settlements in south-eastern Arabia is reason to revive the age old, Climate change and human origins in southern Arabia unanswered debate: “from whence came the Sumerians?” (Barton 1929). This question becomes even more intriguing in light of new genetic evidence obtained from modern populations around the Gulf, who carry signature lineages from Africa, Asia, and Europe (Reguiero et al. 2006; Shepard & Herrera 2006). Based on these findings, Reguiero et al. (2006) describe the area as a “tricontinental nexus” for human dissemination. 35 Discussion: modelling demographic response In considering the early human drama that unfolded amidst the backdrop of the wildly metamorphosing Arabian landscape, it is useful to consider a passage from Abd al-Qadir al-Jilani’s ninth-century text Kitab Sirr alAsrar, later translated into English by Sir Isaac Newton: “that which is below is like that which is above” (Dobbs, 1983). A more down-to-earth rendition of this statement might imply that demographic history in Arabia mirrors the evolution of the landscape. This metaphor is rooted in deep geological time, when the Arabian tectonic plate split from Africa, forming the Red Sea trough. Genetic studies indicate that modern human emergence occurred in a similar manner, when a group derived from mtDNA superhaplogroup L3 branched from the ancestral population sometime prior to 75,000 BP (e.g. Metspalu et al. 2004), perhaps as early as 300,000 years ago (Yotova et al. 2007). Far from a geographic barrier, the Red Sea was a conduit for both plants and animals throughout the Quaternary. While there was no land bridge at any point during this phase (Siddall et al. 2002: 203–206), reduced sea levels rendered the narrow crossing at the southern extent of the Red Sea a negligible barrier. Moreover, the body of water flanking the eastern margin of the Arabian subcontinent did not exist for the majority of the Upper Pleistocene (Lambeck 1996; Uchupi, Swift & Ross 1999); the absence of this waterway was undoubtedly an integral part of the prehistoric plot. Distributions of flora and fauna across eastern Arabia — predominantly Palearctic taxa — demonstrate the region’s natural Eurasian affinities. It is not surprising that Upper Pleistocene lithic industries follow the same geographic patterning (Rose 2007). We have seen that life and death within the interior of South Arabia was dependant upon the intensity of the Indian Ocean monsoon. When global insulation patterns forced the monsoon northwards, the hinterland was transformed into a sub-tropical savannah. Conversely, when rainfall ceased during glacial maxima, the majority of the peninsula became a barren wasteland. Given these extreme fluctuations, in conjunction with its position at the intersection of Africa, Asia, and Europe, it is likely that South Arabia served a unique role in the prehistoric world: a bridge during pluvials and a barrier during arid phases. It is only appropriate to envision these processes in the sense of long term, inter-regional genetic exchange; we do not yet possess a suitable degree of resolution within the palaeoclimatic and archaeological records to assess actual demographic events. Any statement about Upper Pleistocene human migrations out of Africa and into Arabia would be premature and grossly presumptive. There are data suggesting that certain parts of the Arabian subcontinent served as stable refugia during environmental downturns. Thus, to understand the role of the peninsula in the story of human origins, it is more productive to consider populations tethered to refugia, expanding and contracting from such habitats during cycles of amelioration and desiccation. This paper has demonstrated that Quaternary climate change in Arabia was a complex process that produced diverse landscapes across the peninsula. For instance, while glacial events resulted in an entirely inhospitable environment within the interior, portions of the emerged continental shelf were paradisiacal gardens. Rather than a simplistic scenario of Homo sapiens marching across Arabia at the onset of the “human revolution”, we must expect that prehistoric occupation was as complex and varied as the landscapes upon which they dwelt. References Ambrose S. 1998. Late Pleistocene human population bottlenecks, volcanic winter, and differentiation of modern humans. Journal of Human Evolution 34: 623–651. 36 A.G. Parker &. J.I. 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Parker, Human Origins and Palaeo-Environments (HOPE) Research Group, Department of Anthropology and Geography, Oxford Brookes University, Oxford OX3 0BP, UK. e-mail agparker@brookes.ac.uk J.I. Rose, Human Origins and Palaeo-Environments (HOPE) Research Group, Department of Anthropology and Geography, Oxford Brookes University, Oxford OX3 0BP, UK. e-mail jrose@brookes.ac.uk