Beneath the seemingly unbroken canopy of the southwestern Amazon rainforest lies evidence of one of the largest monumental landscapes ever discovered in the ancient Americas.

According to new research published in Nature, what archaeologists once believed were scattered ceremonial sites are instead part of an enormous precolonial civilization that may have changed vast stretches of the Amazon nearly 2,000 years ago.

Using airborne LiDAR capable of penetrating forest cover, an international team of researchers says they have identified hundreds of previously unknown earthworks, estimating that more than 20,000, and perhaps as many as 30,000, remain spread across southwestern Amazonia.

The findings dramatically expand the known scale of the so-called Aquiry civilization and suggest that millions may have inhabited the region during its peak nearly 2,000 years ago, forcing a reconsideration of how densely populated and intensively managed the ancient Amazon may have been.

“We also propose that the total human population of the area in 100–300 AD was 1.25–3 million people,” researchers write. “If similar results are found elsewhere in Amazonia, the total precolonial population density and its effect on the current Amazonian soil and forest structure, biodiversity, and even some modeling of the global climate should be re-evaluated.”

The study represents one of the most ambitious archaeological surveys ever conducted in the Amazon. Researchers flew more than 2,700 miles across the forests of Brazil’s Acre and Amazonas states using airborne LiDAR, a laser-mapping technology able to see through gaps in the dense treetops to produce detailed maps of the landscape below. Those maps revealed hundreds of geometric earthworks that had remained hidden for centuries.

The newly discovered structures include squares, circles, rectangles, and multilateral enclosures connected by roads and clustered into monumental complexes.

Some feature ditches up to 43 feet wide and more than 16 feet deep, with towering earthen embankments rising alongside them. Although a handful of these earthworks had previously been identified in deforested areas using satellite imagery, LiDAR revealed that those earlier surveys had dramatically underestimated just how many remained obscured beneath the forest.

One of the study’s most striking findings was that LiDAR scanning uncovered roughly five times more earthworks than satellite imagery had detected across the same landscapes.

In one survey block, researchers had previously identified only 30 structures using conventional imagery. Meanwhile, LiDAR revealed 156. Assuming similar results across the wider region suggests that between roughly 24,000 and 30,000 earthworks may exist within the estimated boundaries of the Aquiry civilization.

Those numbers alone would rank the region among the most monument-rich archaeological landscapes in the world.

For decades, archaeologists have debated whether much of Amazonia could have maintained large, complex societies before European contact. Prevailing interpretations portrayed much of the rainforest as environmentally limiting, capable of sustaining only relatively small and dispersed populations. However, more recent discoveries, including ancient cities in Bolivia and Ecuador, have steadily challenged that assumption.

Now, these latest findings suggest the region may have supported an even larger and more sophisticated society than researchers once imagined.

Rather than representing isolated ceremonial sites scattered through an otherwise untouched forest, the researchers argue the earthworks belonged to a civilization extending about 71,000 square miles, representing about three times larger than previous estimates of the Aquiry cultural area.

Many of the geometric enclosures appear not to have been permanent settlements themselves. Instead, researchers believe they served as ceremonial, political, and public gathering places where dispersed communities periodically assembled.

Similar patterns of temporary gathering centers are known from the Inca Empire, where populations converged for rituals, governance, and communal events despite living across broad landscapes.

Constructing these massive earthworks would have demanded sophisticated planning, organized labor, mathematical knowledge, and long-term social coordination. According to the study, some of these ceremonial centers remained in use for nearly 1,500 years, indicating remarkable cultural continuity across generations.

Estimating how many people actually lived within this landscape presented a separate challenge.

Rather than simply multiplying earthworks by a fixed population estimate, the researchers combined several independent lines of evidence. They examined radiocarbon dates to determine how many sites were occupied simultaneously, compared the earthworks with later mound villages built in the same region, incorporated ethnographic information from Indigenous communities, and evaluated the labor required to construct and maintain the monumental enclosures.

Their study indicates the civilization reached its greatest extent between approximately A.D. 100 and 300. During that period, somewhere between one-quarter and roughly 60 percent of the known ceremonial centers may have been active simultaneously.

Assuming each ceremonial center served communities of roughly 300 people, the researchers estimate the region’s population reached between about 1.25 and 3 million people during its peak. After accounting for earthworks that LiDAR may have missed beneath dense forest, they conclude the total could have been as high as 3.8 million.

Those estimates substantially exceed many previous reconstructions of southwestern Amazonia and imply population densities rivaling or exceeding some later historical agricultural societies.

Equally significant is what these discoveries reveal about how ancient Amazonians altered their environment.

The study points to fertile soils rich in nutrients, evidence of extensive cultivation of maize, squash, manioc, beans, peanuts, sweet potatoes, and chili peppers, as well as widespread management of Brazil nut groves and numerous economically important palm species.

Rather than exhausting the rainforest, researchers contend these communities actively managed forests, soils, and waterways throughout centuries, creating ecological legacies that remain visible today in the composition and biodiversity of Amazonian forests.

Present-day ecological studies have increasingly shown that many Amazonian forests long regarded as pristine wilderness actually bear the imprint of thousands of years of Indigenous landscape management. If similar densities of monumental earthworks are identified elsewhere across Amazonia, researchers argue that assumptions about precolonial population sizes, biodiversity patterns, soil formation, and even climate models may need substantial revision.

The work additionally underscores the key role of LiDAR in modern archaeology. Over the past decade, the technology has revealed hidden Maya cities beneath Central American jungles, extensive urban complexes in the Bolivian Amazon, and extensive settlements in Ecuador’s Upano Valley.

Nevertheless, researchers caution that important questions remain unanswered. The southeastern limits of the Aquiry civilization are still poorly understood because large portions of Rondônia have yet to receive comparable archaeological investigation. Dense vegetation also continues to obscure portions of the landscape, meaning many earthworks almost certainly remain undiscovered even after LiDAR surveys.

“These ancient achievements constitute an important legacy that is still observable in the soil, and the composition and biodiversity of the contemporary tropical forest,” researchers write. “This legacy should be fully considered in future studies of Amazonia, including climate modeling and sustainable land-use planning.”

The recent study, “Over 20,000 precolonial earthworks in the Southwest Amazonia,” by Martti Pärssinen et al., appeared in Nature.

Tim McMillan is a retired law enforcement executive, investigative reporter and co-founder of The Debrief. His writing typically focuses on defense, national security, the Intelligence Community and topics related to psychology. You can follow Tim on Twitter: @LtTimMcMillan.  Tim can be reached by email: tim@thedebrief.org or through encrypted email: LtTimMcMillan@protonmail.com 

For decades, paleoanthropologists have largely agreed that somewhere along the evolutionary course from apelike ancestors to modern humans, our bodies got dramatically larger. However, one lingering mystery has been when this intense growth began.

Did our ancestors gradually grow bigger over millions of years? Did the shift coincide with the emergence of the genus Homo? Or did it occur later, after some of the earliest human species had already appeared?

A new study published in the Proceedings of the National Academy of Sciences (PNAS) argues that the answer is more complex than any single theory has suggested.

Rather than supporting a single long-standing hypothesis, researchers used a sophisticated statistical method to show that human body size likely followed two evolutionary courses simultaneously: a slow, steady increase spanning millions of years, punctuated by a much larger leap in later members of the genus Homo.

The findings help reconcile decades of conflicting research while offering one of the clearest pictures yet of how, and perhaps why, our ancestors evolved into larger-bodied humans.

“Despite its central role in hominin evolution, there is little consensus on how body size increased through time,” researchers write. “We provide strong evidence for a marked size increase in later-occurring Homo, alongside a moderate general increase over time among all hominins.”

Body size is one of the defining characteristics of an organism. In humans and our extinct relatives, it has been linked to everything from brain expansion and walking efficiency to metabolism, hunting behavior, and the ability to travel across increasingly large landscapes. Understanding when body size changed, therefore, helps researchers reconstruct not only what our ancestors looked like, but how they lived.

Yet despite decades of fossil findings, scientists have struggled to agree on the evolutionary pattern behind those changes.

Some previous studies suggested that hominins simply became larger over time. Some contended that body size remained relatively stable until the emergence of Homo. Still others have proposed that the real transformation arose only after early species such as Homo habilis, often regarded as one of the first members of our genus.

According to researchers, one reason for the disagreement is that earlier analyses typically examined these competing ideas independently. They also often failed to account for uncertainties inherent in the fossil record, including disagreements over species classification, differences in body mass estimation methods, and uncertainties in evolutionary relationships among fossil species.

To address those limitations, researchers compiled one of the largest hominin body-size datasets assembled to date, incorporating body mass assessments from 386 fossil specimens representing 21 different taxa.

They then analyzed the data using Bayesian phylogenetic generalized linear mixed models—a statistical approach that accounts for evolutionary relationships while encompassing multiple layers of uncertainty.

The team repeated its analyses across 1,000 different phylogenetic trees and 1,000 separate datasets, allowing them to test whether the overall pattern remained consistent despite uncertainties.

The results pointed to two simultaneous trends.

The first was a modest but persistent increase in body size across the hominin family as a whole. On average, body mass increased by roughly 2 pounds (0.9 kilograms) every million years, suggesting that larger bodies gradually became more common throughout human evolution.

However, the second trend proved far more dramatic.

Instead of all members of Homo becoming substantially larger, the researchers found the strongest evidence for a distinct jump in body size among later members of the genus, particularly species that appeared after Homo habilis. That pattern regularly emerged regardless of how the fossil data were analyzed.

In other words, early members of Homo may not have represented the major biological turning point scientists once assumed.

This finding disputes a common assumption that simply belonging to the genus Homo meant possessing a significantly larger body. Instead, the study suggests that some hallmark features traditionally associated with humans, including larger body size, may have evolved after the genus’s origin.

That interpretation also corresponds to broader changes that occurred later in human evolution.

Researchers note that this period coincides with evidence for more efficient upright walking, greater reliance on meat consumption, and expanding home ranges. Larger bodies may have been both a consequence of and a contributor to those behavioral and ecological shifts.

The study likewise reinforces another growing theme that evolution rarely follows a straight line.

Species such as Homo floresiensis, often nicknamed the “Hobbit” because of its diminutive stature, and Homo Naledi frequently appeared as outliers with unusually small body sizes despite belonging to relatively recent branches of the human family tree. Their existence demonstrates that body size did not simply increase in a uniform march toward modern humans.

Instead, evolution repeatedly experimented with different solutions depending on local environments and ecological constraints.

The researchers also argue that part of the longstanding disagreement over body-size evolution stems from the fossil record itself. Different studies have estimated fossil body masses using different skeletal elements, comparison populations, and statistical techniques, sometimes producing substantially different results for the same individual.

By expressly incorporating those uncertainties into their analysis, rather than treating them as fixed values, the new framework seeks to capture the range of plausible evolutionary scenarios rather than relying on a single interpretation.

Even so, researchers caution that important questions remain unresolved.

Improved fossil discoveries, more accurate body-mass estimation methods, and a better understanding of species relationships could all further refine the picture. The study also does not explicitly account for sexual size differences between males and females, which may have been greater in some early hominin species than in modern humans.

Nevertheless, the wider conclusion is intriguing.

Rather than choosing between competing theories, the evidence suggests both contain elements of the truth.

Body size did increase gradually across the hominin lineage. But the most considerable leap appears to have occurred later, after the earliest representatives of Homo had already evolved.

That conclusion may help settle one of paleoanthropology’s longest-running debates while showing how advances in statistical methods are transforming scientists’ ability to extract new insights from fossils that have often been studied for decades.

“Overall, these findings bring clarity to a fundamental question in human evolution and show how the application of rigorous phylogenetic comparative tools can unlock patterns previously hidden in the fossil record while accounting for multiple sources of uncertainty,” researchers write. “By reconciling conflicting findings within a single framework, our results clarify key evolutionary transitions and highlight the complex nature of body size change in human evolution.”

Tim McMillan is a retired law enforcement executive, investigative reporter and co-founder of The Debrief. His writing typically focuses on defense, national security, the Intelligence Community and topics related to psychology. You can follow Tim on Twitter: @LtTimMcMillan.  Tim can be reached by email: tim@thedebrief.org or through encrypted email: LtTimMcMillan@protonmail.com