Christophe Pottier

A comprehensive archaeological map of the world's largest preindustrial settlement complex at Angkor, Cambodia

The great medieval settlement of Angkor in Cambodia [9th–16th centuries Common Era (CE)] has for many years been understood as a “hydraulic city,” an urban complex defined, sustained, and ultimately overwhelmed by a complex water management network. Since the 1980s that view has been disputed, but the debate has remained unresolved because of insufficient data on the landscape beyond the great temples: the broader context of the monumental remains was only partially understood and had not been adequately mapped. Since the 1990s, French, Australian, and Cambodian teams have sought to address this empirical deficit through archaeological mapping projects by using traditional methods such as ground survey in conjunction with advanced radar remote-sensing applications in partnership with the National Aeronautics and Space Administration (NASA)/Jet Propulsion Laboratory (JPL). Here we present a major outcome of that research: a comprehensive archaeological map of greater Angkor, covering nearly 3,000 km2, prepared by the Greater Angkor Project (GAP). The map reveals a vast, low-density settlement landscape integrated by an elaborate water management network covering >1,000 km2, the most extensive urban complex of the preindustrial world. It is now clear that anthropogenic changes to the landscape were both extensive and substantial enough to have created grave challenges to the long-term viability of the settlement.

Uncovering archaeological landscapes at Angkor using lidar

Previous archaeological mapping work on the successive medieval capitals of the Khmer Empire located at Angkor, in northwest Cambodia (∼9th to 15th centuries in the Common Era, C.E.), has identified it as the largest settlement complex of the preindustrial world, and yet crucial areas have remained unmapped, in particular the ceremonial centers and their surroundings, where dense forest obscures the traces of the civilization that typically remain in evidence in surface topography. Here we describe the use of airborne laser scanning (lidar) technology to create high-precision digital elevation models of the ground surface beneath the vegetation cover. We identify an entire, previously undocumented, formally planned urban landscape into which the major temples such as Angkor Wat were integrated. Beyond these newly identified urban landscapes, the lidar data reveal anthropogenic changes to the landscape on a vast scale and lend further weight to an emerging consensus that infrastructural complexity, unsustainable modes of subsistence, and climate variation were crucial factors in the decline of the classical Khmer civilization.

The water management network of Angkor, Cambodia

Meticulous survey of the banks, channels and reservoirs at Angkor shows them to have been part of a large scale water management network instigated in the ninth century AD. Water collected from the hills was stored and could have been distributed for a wide variety of purposes including flood control, agriculture and ritual while a system of overflows and bypasses carried surplus water away to the lake, the Tonle Sap, to the south. The network had a history of numerous additions and modifications. Earlier channels both distributed and disposed of water. From the twelfth century onwards the large new channels primarily disposed of water to the lake. The authors here present and document the latest definitive map of the water network of Angkor.

Vegetation and land-use at Angkor, Cambodia : a dated pollen sequence from the Bakong temple moat

Investigating the use of land during the medieval period at the celebrated ceremonial area of Angkor, the authors took a soil column over 2.5m deep from the inner moat of the Bakong temple. The dated pollen sequence showed that the temple moat was dug in the eighth century AD and that the agriculture of the immediate area subsequently flourished. In the tenth century AD agriculture declined and the moat became choked with water-plants. It was at this time, according to historical documents, that a new centre at Phnom Bakeng was founded by Yasovarman I.

Angkor Wat: an introduction

Abstract The temple of Angkor Wat, visited annually by tens of thousands of tourists, is without question one of the great archaeological sites of mainland Southeast Asia. Less obvious to the casual visitor is that it was but a single element in a large dispersed city. The papers in this special section demonstrate how recent research using LiDAR, ground-penetrating radar and targeted excavation have transformed our understanding of Angkor Wat and its surroundings.

The buried "towers" of Angkor Wat

Abstract The impressive architecture of Angkor Wat conveys a sense of harmonious design, but recent survey using ground-penetrating radar (GPR), coupled with targeted excavation, reveals a more complex picture. Fragmentary traces of a quincunx of earlier towers have been detected, which were partially demolished when the outer enclosure and western gateway were completed. Are these the remains of a shrine used during the construction period? If so, they throw new light on associated ritual activity during the building of Angkor Wat.

The prehistoric peopling of Southeast Asia

Ancient migrations in Southeast Asia The past movements and peopling of Southeast Asia have been poorly represented in ancient DNA studies (see the Perspective by Bellwood). Lipson et al. generated sequences from people inhabiting Southeast Asia from about 1700 to 4100 years ago. Screening of more than a hundred individuals from five sites yielded ancient DNA from 18 individuals. Comparisons with present-day populations suggest two waves of mixing between resident populations. The first mix was between local hunter-gatherers and incoming farmers associated with the Neolithic spreading from South China. A second event resulted in an additional pulse of genetic material from China to Southeast Asia associated with a Bronze Age migration. McColl et al. sequenced 26 ancient genomes from Southeast Asia and Japan spanning from the late Neolithic to the Iron Age. They found that present-day populations are the result of mixing among four ancient populations, including multiple waves of genetic material from more northern East Asian populations. Science, this issue p. 92, p. 88; see also p. 31 Ancient genomes reveal four layers of human migration into Southeast Asia. The human occupation history of Southeast Asia (SEA) remains heavily debated. Current evidence suggests that SEA was occupied by Hòabìnhian hunter-gatherers until ~4000 years ago, when farming economies developed and expanded, restricting foraging groups to remote habitats. Some argue that agricultural development was indigenous; others favor the “two-layer” hypothesis that posits a southward expansion of farmers giving rise to present-day Southeast Asian genetic diversity. By sequencing 26 ancient human genomes (25 from SEA, 1 Japanese Jōmon), we show that neither interpretation fits the complexity of Southeast Asian history: Both Hòabìnhian hunter-gatherers and East Asian farmers contributed to current Southeast Asian diversity, with further migrations affecting island SEA and Vietnam. Our results help resolve one of the long-standing controversies in Southeast Asian prehistory.

Ancient Genomics Reveals Four Prehistoric Migration Waves into Southeast Asia

Two distinct population models have been put forward to explain present-day human diversity in Southeast Asia. The first model proposes long-term continuity (Regional Continuity model) while the other suggests two waves of dispersal (Two Layer model). Here, we use whole-genome capture in combination with shotgun sequencing to generate 25 ancient human genome sequences from mainland and island Southeast Asia, and directly test the two competing hypotheses. We find that early genomes from Hoabinhian hunter-gatherer contexts in Laos and Malaysia have genetic affinities with the Onge hunter-gatherers from the Andaman Islands, while Southeast Asian Neolithic farmers have a distinct East Asian genomic ancestry related to present-day Austroasiatic-speaking populations. We also identify two further migratory events, consistent with the expansion of speakers of Austronesian languages into Island Southeast Asia ca. 4 kya, and the expansion by East Asians into northern Vietnam ca. 2 kya. These findings support the Two Layer model for the early peopling of Southeast Asia and highlight the complexities of dispersal patterns from East Asia.

Glass artifacts at Angkor: evidence for exchange

Although glass beads were found in large quantities in Southeast Asia during the Iron Age and into the first millennium CE, glass artifacts from the Angkorian period (ninth–fifteenth centuries CE) are less common and have not been as well-studied. This paper presents the results of an analysis of 81 glass beads and artifacts from the ninth-century royal capital of Hariharālaya and later (twelfth–fourteenth centuries CE) contexts from the walled city of Angkor Thom. Compositional analyses using laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) have identified glass belonging to three broad compositional groups. The earlier Hariharālaya sites have numerous glass beads and vessel fragments made from vegetal soda glass, associated with Middle Eastern production, as well as high-alumina mineral soda glass of a sub-type frequently found at Iron Age sites in Southeast Asia and likely produced in South Asia. Beads from the later-period sites within Angkor Thom are primarily lead glass, associated with Chinese glass production, and different sub-types of high-alumina mineral soda glass that have also been found at sites in Southeast Asia dating from the fourteenth to the nineteenth centuries CE. A small number of beads from Angkor Thom also have a vegetal soda composition distinct from beads at Hariharālaya. The results of this study provide a new type of evidence for elite participation in broader regional exchange networks during the Angkorian period.

The demise of Angkor: Systemic vulnerability of urban infrastructure to climatic variations

The historic urban collapse of Angkor is linked to infrastructural complexity and climatic variability. Complex infrastructural networks provide critical services to cities but can be vulnerable to external stresses, including climatic variability. This vulnerability has also challenged past urban settlements, but its role in cases of historic urban demise has not been precisely documented. We transform archeological data from the medieval Cambodian city of Angkor into a numerical model that allows us to quantify topological damage to critical urban infrastructure resulting from climatic variability. Our model reveals unstable behavior in which extensive and cascading damage to infrastructure occurs in response to flooding within Angkor’s urban water management system. The likelihood and extent of the cascading failure abruptly grow with the magnitude of flooding relative to normal flows in the system. Our results support the hypothesis that systemic infrastructural vulnerability, coupled with abrupt climatic variation, contributed to the demise of the city. The factors behind Angkor’s demise are analogous to challenges faced by modern urban communities struggling with complex critical infrastructure.