考古学的伟大谜团：从天空中揭示的古代亚马逊世界

From the air, you see it only through the constant jolt, tilt and shudder of the low-flying Cessna aircraft. The landscape of the Llanos de Moxos, northern Bolivia, appears as a disconnected patchwork of open grassland savannahs, forest islands and lakes.

It feels random, almost unreadable. Only gradually does the pattern resolve itself: raised causeways or paths fanning out to link the forest islands, and a dense, scattered web of canals threading the terrain. Slowly you realise it’s a structured network of intersecting lines, enclosures and roads – the imprint of past human design.

If you stand on the open savannah, there is almost nothing to see of this ancient network. The horizon feels open, with fires in the distance from local people burning pastures and clearing forest as dry season begins. The old geometry is still faintly perceptible, but you have to know how to look.

Step into the patches of forest and the canopy closes in. The earth softens underfoot and mosquitoes descend in relentless swarms. The sweat on your neck thickens into a humid film, carrying the familiar scent of suncream and the sharper, chemical note of DEET.

In the uneven light between the trees, the landscape dissolves into subtle rises and depressions. Against the rhythmic swish of machetes as our guides cut through the vegetation, your mind tries to piece together the fragments of structures into something coherent. Flying overhead doesn’t reveal anything about this forest area in the way that it does with the savannah. But fortunately recent advances in technology have transformed what we are able to see.

Archaeological explorations in this part of the world have been completely changed by lidar in the past couple of decades. Lidar maps an area from a plane or drone by bouncing rapid laser pulses off the Earth’s surface. Some of these pulses penetrate the forest canopy, reach the ground and reflect back to the sensor.

By measuring the return time, the system can generate highly precise three-dimensional models of the terrain. This allows you to strip away the camouflage of vegetation, making it possible to see what lies below the Amazonian forest for the first time.

It reveals the ancient Llanos de Moxos as not simply a collection of settlements, but an entire urbanised landscape. A large part in the south-east of this region belonged to the Casarabe culture, which dominated between around AD500 and 1400. It extends across 20,000km², which is roughly the size of New Jersey in the US.

The Casarabe organised into a hierarchy of four different sizes of settlements (those forest islands mentioned above) . The biggest ones – the primary settlements – were as large as 3km² or 300 hectares. That’s enough space for over 400 football pitches, suggesting that they could have accommodated substantial numbers of people.

Welcome to our series on the great mysteries of archaeology. Viking explorers, Amazonian cities, artefacts from before civilisation. Archaeology may be all about the past, but it’s constantly shifting with every scientific discovery. This series will dig into some of the most fascinating debates in the field today.

These settlements connect along the raised causeways to smaller secondary and tertiary sites a number of kilometres away, all of which were permanently inhabited as opposed to empty ceremonial hubs. A fourth tier consists of groups of isolated mounds located out in the pampas, which likely correspond to dwelling areas occupied by farmers who would have worked the fields.

It’s not possible to show a lidar image of these four different types of sites interconnecting because they are too far apart for the resolution available, but the image below of a primary settlement known as Loma Cotoca shows the kinds of things we are now documenting.

It features some very impressive civic-ceremonial architecture: conical pyramids over 20 metres tall and U-shaped structures that may have acted as areas for public gatherings for speeches or ceremonies. These were built on top of man-made platforms rising as much as five metres off the ground and extending over 20 hectares. To be clear, this is all still hiding under the forest, but the lidar data reveals the shape, height and layout of what lies below.

The volume of earth moved to create this architecture would have rivalled – and in some cases exceeded – that of well known Andean monuments such as Akapana a few hundred miles to the south-west on the other side of the Andes. Akapana was the epicentre of the Tiwanaku empire that dominated the southern Andes between about AD600 and 1000.

Yet where monuments like Akapana were surrounded by classic, compact bounded cities with thousands of inhabitants, the Casarabe equivalent was completely different. This was dispersed, low-density living amid extensive green space – a form of tropical urbanism that challenges longstanding assumptions about this area as sparsely populated and only lightly modified. It invites comparison with other low-density tropical urban landscapes such as the Maya in central America and the Angkor in latter day Cambodia.

Equally important is the coherence of the Casarabe system. The settlements are rarely isolated, part of a tightly connected network with shared water-management systems. It was clearly all planned and coordinated, designed not only as living spaces but for integrating the population across the region.

We can see that the Casarabe were sustained by drained-field agriculture: the canals were dug to make the land viable for planting during the wet season. The most prominent crop was maize, but there was a remarkable diversity of other produce. This was all embedded within a landscape that was engineered through reservoirs and farm ponds, which helped the Casarabe sustain cultivation and maintain access to water through the dry season in this extremely seasonal environment.

Also very noticeable is the fact that all the major architectural features and burial sites are oriented north-north-west. This suggests these people may have been led by cosmology, with important celestial bodies or regions of the night sky serving as symbolic reference points – hinting at a world where infrastructure, settlement and belief were inseparable.

Rethinking the Amazon

The Casarabe culture covered much less than 1% of Amazonia, which is the whole tropical interior of South America, spanning close to half of the entire continent. For much of the 20th century, this vast area was viewed by archaeologists as an environment that was limiting for human existence.

Poor soils, scarce game, extreme El Niño floods and droughts, and the challenges of tropical disease were all thought to constrain human populations to small, wandering groups living off the land as best they could. Large, settled societies – let alone towns or cities – were considered unlikely, if not impossible.

This view began to shift in the late 20th century for several reasons. Archaeologists realised that Amazonian people had been domesticating a diversity of plants since the end of the Ice Age. They manufactured some of the earliest ceramics in the Americas, and also devised soils known as Amazonian Dark Earths, which combined charcoal, bone and waste materials with the existing poor-quality soil to make it fertile enough for widespread farming.

It also became apparent that just like the Casarabe people, many other cultures across Amazonia had reclaimed vast expanses of seasonally flooded savannahs over several thousand years to create raised and drained field systems.

These discoveries were evidence of long-term settlement and landscape management far beyond what was previously thought possible. It meant Amazonia was not simply a backdrop to human activity; much of the landscape was shaped over the last 13 millennia by the people who lived there.

Enter lidar

Like lasers in the sky, lidar technology has accelerated this transformation in our understanding. The digital process feels near-magical, a “vegetation removal algorithm” that reveals the secrets below.

In practice, however, working with lidar in Amazonia is anything but straightforward. Running such a project here, as I have done, can feel like one of the greatest emotional rollercoasters in field archaeology. It’s all anticipation, frustration and sudden revelation – only comparable, perhaps, with shipwreck exploration.

Depending on what technology is available and most suitable for exploring a particular area, I’ve worked with lidar attached to drones, aeroplanes and helicopters. I’ve learned through trial and error that the technology is only as effective as the logistics and personalities behind it – above all on one occasion when we were trying to integrate a Hungarian lidar sensor with a Brazilian drone.

Lidar can perform beautifully one day and fail the next, depending on the equipment, weather, terrain, batteries, communications and the sheer difficulty of operating in remote Amazonian conditions.

Flights must be carefully planned in remote areas with limited infrastructure, where convective clouds, smoke from fires, wind and even vultures riding thermals can disrupt data acquisition. You have to arrange fuel in advance and improvise landings wherever a safe clearing can be found. Here’s our team refuelling a lidar helicopter in the football field of a small village in Acre state, western Brazil:

You also have to do constant troubleshooting with the technology, such as making sure it’s calibrated correctly and that the data from different flight paths all aligns. What appears in the final images as a seamless “removal” of the forest is, in reality, the product of improvisation, negotiation and persistence.

But given all these challenges, it makes the first successful images all the more powerful when they finally appear. The reward is that we’re finally finding the “lost civilisation” that explorers like Percy Fawcett were searching for a century ago, but by cajoling a drone rather than battering through jungle.

Incidentally, this technology also has important uses beyond archaeology. It can help people to locate and harvest crops like rubber or açaí palm fruits without having to clear so much rainforest. It is also used by pioneering projects such as Amazonia Revelada, which helps Indigenous and traditional people of the Amazon to prove their historic presence within an area to ward off modern commercial interests like loggers or farmers, while also protecting the living history and nature embedded in these landscapes.

Other lidar discoveries

Lidar surveys by French and Ecuadorian archaeologists have revealed that the Llanos de Moxos was certainly not the only example of large-scale, highly integrated society in Amazonia. The Upano Valley, which covers some 300-600km² on the mountainous forest of the Ecuadorian eastern flanks of the Andes, offers another striking example – this time from between about 500BC and AD600–700.

Lidar discovery areas

In Upano, archaeologists have been able to map a vast network of settlements connected by extensive road systems, with large platforms and clusters of buildings arranged in organised layouts across a broad area.

What stands out is not just the scale – thousands of structures – but the rigour of the planning. The settlements didn’t just grow randomly, but as part of a deliberate design: we see straight lines of flat-topped platforms laid out in repeating rows and connected by straight paths that cut cleanly across the landscape, as you can see below.

Again, this is not urbanism in the conventional sense of dense, continuous occupation. There would have been none of the vertical stacking of buildings that you’d get in European settlements, and there were also green spaces between platform complexes – much more like a forest city.

Like the Casarabe region, this is a distributed settlement pattern that is both open and highly structured, but the arrangement is much more compact. This reflects the limited flat space available on the upper terraces of the Upano River, which rise up to 100 metres above the surrounding landscape.

Elsewhere in Amazonia, we see more variations. In the Upper Xingu of central Brazil, interconnected settlements were arranged around a shared ceremonial and road network, again suggesting a regionally coordinated social world.

Further north, the Tairona people of the Sierra Nevada de Santa Marta in present-day Colombia built terraced stone towns in the mountains, linked by paved paths. This was a form of urbanism shaped entirely by the demands of steep, high-altitude terrain. Below is a lidar image of one area in this region, with the platforms that would have housed the settlements marked in yellow. Below that, you can see what the platforms look like.

In western Amazonia, Acre adds another important variation. From around AD1–1000, people built large ditched enclosures, or geoglyphs, mainly in the south-eastern part of this region along the upper Purus River. These were square, circular, hexagonal or octagonal mounds, often 1-3 hectares in size, with ditches up to four metres deep. These were probably used as ceremonial gathering places rather than permanent settlements.

After about AD1000, these were followed by what we call circular mound villages, occupied until around AD 1650–1700. They featured rings of mounds around central plazas and straight roads radiating out like the rays of the Sun, often built to align with the four main compass points. These “Sun villages” were true settlements, and formed interconnected networks across the southern rim of Amazonia. You can see an example in the lidar image below.

Taken together, these discoveries fundamentally reshape our understanding of Amazonia. We now see a mosaic of managed landscapes, engineered environments and, in some cases, city-scale societies. What unites them is not a shared blueprint but a shared impulse: the organisation of people, space and movement across large landscapes in ways that were deliberate, durable and distinctly their own.

To stress, Amazonia was not uniformly dense or urban. It supported a diversity of types of settlements, from dispersed networks like Moxos to tighter grids like Upano, each of them adapted to local ecological conditions. They shared a low-density urbanism, in the sense of large, interconnected populations without the density of classic cities.

What we still don’t know

How were these societies organised politically and socially? How did they interact with variations in the climate and environment, ranging from the heavy rainfalls and droughts caused by El Niño to rivers forging new routes that could move them away from a settlement within a few generations?

What, if any, connections existed with mountain societies in the Andes? And perhaps most importantly, since both the Casarabe and Upano ceased to build monuments after 1492, what led to their transformation or decline before the arrival of Europeans?

There is active debate between archaeologists over whether these societies transformed because of environmental stress, internal political change, or shifts in things like trade routes or migration.

In the Llanos de Moxos, one possibility is that a prolonged period of climate change affected the Casarabe water-management systems that were so critical to feeding this thriving society. In the Upano Valley, volcanic eruptions and earthquakes may have disrupted settlements and agriculture, although it’s unclear whether that could have led to the area being abandoned.

It seems likely that as we uncover new things, it will reveal more and more integration between different societies. What we are seeing now in Amazonia is much like looking at a satellite image of a country at night: bright, isolated clusters of light – cities that appear disconnected. But as we continue to expand our coverage and fill in the gaps, I think this will change.

What now appear as isolated clusters may also resolve into extensive networks. For example a study across the southern rim of Amazonia has predicted that the kinds of settlement mounds that have been identified so far are likely to occur across about 400,000km², supporting an estimated regional population of roughly 500,000 to 1 million people in the era before the Europeans arrived.

Entire regions may emerge as previously unrecognised centres of population and landscape management. This could be particularly so for the Llanos de Moxos. The whole area covers as much as 200,000km², depending on where you draw the boundaries, stretching into Brazil and even Peru. It is often divided into several apparently distinct cultural regions — the Casarabe (aka the monumental mound region) , and then two others called the platform ridge and zanjas (ditches) regions.

As lidar coverage expands and more archaeological work is conducted, we may begin to understand how these societies were economically specialised. We know, for example, that the fortified villages of the zanjas region had fish weirs spanning hundreds of miles that were capable of capturing vast quantities of migratory fish. The platform ridge region consisted of large drained fields, which could potentially produce surpluses of maize. It is conceivable that these belonged to a broader network that supported the more complex Casarabe centres.

Or perhaps – who knows – the relationships were more fluid and reciprocal. For now, the question remains open. But it is precisely this possibility of deep regional integration that lidar is beginning to bring into view. In time, we may even begin to identify Casarabe outposts scattered across the Llanos de Moxos.

What happens next

There’s still a huge amount to be done with lidar. Vast areas, particularly in the Ecuadorian and Peruvian Amazon – remain unexplored. One recent study suggested that there could be more than 10,000 more urban structures of the kind I’ve been describing still hidden throughout Amazonia, all of them dating from pre-European times.

Looking ahead 20 years, it is likely that our map of Amazonia will look very different. One promising technology is satellite-based lidar systems, which could provide broader, though less detailed, datasets across large areas. Advances in machine learning are also beginning to help us identify archaeological features within massive datasets, speeding up a labour-intensive process.

Against this, there are time pressures in some places. Llanos de Moxos, for instance, is unfortunately in rapid transition. The very ground that holds the traces of ancient networks is being transformed by mechanised agriculture and large-scale terraforming for rice cultivation and pastures.

We also need to keep reminding ourselves that lidar is only the first step. What really matters is how it’s brought together with other lines of evidence. Most sites discovered by lidar have yet to be excavated, so we’ll have to do much of that, looking for everything from bones and plants to ceramics and weapons.

So far, most excavation has been in the Casarabe area of the Llanos de Moxos. The reason, for instance, that we know the culture lived primarily on maize was through the discovery of over 60 human skeletons, which underwent carbon isotope analysis. The same research paper also analysed excavated duck bones to show that the Casarabe were feeding them maize too, suggesting animal domestication in a continent that was not generally known for it.

Another fascinating Casarabe find is a single buried skeleton who may have been a leader, because he had a collar of jaguar teeth around his neck. He was also wearing ear pieces made of armadillo shell, studded with mottled blue stones called sodalite – it’s not clear what these were for.

We’ll also need to obtain more precise dates for key events using techniques like radiocarbon dating, and more pinpoint accurate environmental data to help support theories about ancient changes to the climate – as opposed to the wider regional information we’ve tended to rely on until now. Lake sediments are great environmental archives, preserving evidence of things like vegetation change and landscape disturbance.

Also important is comparing genetic data from excavated bones with people who live in these areas today – in dialogue and collaboration with local communities whose histories, memories and knowledge are essential to understanding these landscapes.

It’s all a question of how lidar is brought together with all this other evidence. The most convincing reconstructions will come from the convergence of all of these. One further major challenge ahead, however, will be to bridge the gap between scientific reconstructions and how past peoples understood and inhabited their world. Archaeology is increasingly rich in data, but we have to relate it to lived experience.

That is no easy feat, but it is essential if we are to move from mapping past worlds to understanding them. Crucially, Amazonia – with its rich, still-vibrant Indigenous societies and ethnographic record – offers an exceptional opportunity to do this, providing rare continuities through which to anchor and critically engage our interpretations of the past.

Lessons for today

My own sense is that we will move towards a view of Amazonia not as an exception, in line with the old view that the people lived within an untouched paradise, but as part of a broader pattern of human-environment interaction. The rainforest will be understood not only as a biological system, but as a historical one – shaped, in part, by the people who lived within it.

This does not mean the Amazonian people who simply lived “in harmony” with nature; the evidence points to something more interesting. Although Amazonian societies developed complex, and at times intensive, forms of land use, the evidence consistently shows that they often did so while maintaining continuous forest cover. Far from the large-scale deforestation that we might assume was necessary for such elaborate forms of human life, their practices created mosaics of managed forest, gardens, orchards, wetlands and settlement areas.

We know partly from lake sediment data that people enriched the forests with species that provided food, building materials, medicines and other resources, from açaí and cacao to palms, cinchona and copaiba. The fact that some of these species endure today suggests that past land use left lasting ecological legacies.

In the context of today’s climate crisis, the long-term balance that these people achieved offers a powerful lesson: it is possible to sustain complex societies without destroying the forest, if land use is guided by principles that integrate ecological knowledge, cultural values and a commitment to the continuity of the living landscape.

What lies beneath the Amazon is not just a hidden past. It is a reminder that even the most seemingly untouched landscapes can carry deep histories, waiting – sometimes just beneath our feet – to be revealed.

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José Iriarte receives funding from the European Research Council, the Arts and Humanities Research Council, the British Academy, National Geographic and the Wenner-Gren Foundation for Anthropological Research.