Around A.D. 600, migrants from across the southern Andes were drawn to a city just south of Bolivia’s Lake Titicaca, the world’s highest large lake, at 13,000 feet above sea level. Founded in A.D. 150, Tiwanaku—a name possibly based on the local Aymara people’s term meaning “stone in the center”—was one of the earliest cities in the Andes. It eventually stretched across 1.5 square miles of harsh altiplano, or high plain landscape, between the Cordillera Occidental and Oriental ranges of the Andes Mountains. Amid monumental mudbrick and stone buildings were adobe homes where, between 10,000 and 20,000 people once lived, and around which they buried their dead in underground tombs. These new arrivals were likely drawn to this seemingly inhospitable landscape by extravagant sacred festivals held in and around Tiwanaku’s monuments.
Evidence that Tiwanaku became a major center for feasting and ceremonies comes in the form of decorated redware pottery, including drinking vessels called keros used for ritual consumption of the maize beer chicha, as well as pits filled with food waste, which archaeologists have found throughout the site. Some of Tiwanaku’s monuments would have been vibrantly painted. Today, they are celebrated for their finely carved stone blocks, some of which can weigh more than 100 tons. The largest of these structures, a 55-foot-tall step pyramid called the Akapana, towered over the city. Monoliths carved in the likenesses of supernatural beings are found throughout the site, some of which stand in the sunken courtyard of a grand temple known as the Kalasasaya. Quinoa and tubers grown in nearby fields, colorful woven cloth, bright feathers collected from rainforest birds, and redware pottery were all exchanged during seasonal gatherings that made Tiwanaku the region’s greatest trading center. Among the city’s many monuments, the temple known as the Pumapunku—or “doorway of the puma” in Aymara—about a half mile southwest of the Akapana, stands out for its precise construction. It is faced with finely cut blocks of andesite and red sandstone into which artisans carved images of its namesake feline, the region’s most fearsome predator and a potent spiritual force.
At its peak around A.D. 700, Tiwanaku’s economic and political influence—based on where archaeologists have found its redware pottery and other trade goods—connected communities spread out over an area the size of California. It reached north to the borders of the Wari people, who were Tiwanaku’s economic and possibly religious rivals. But, by the early second millennium A.D., the city was abandoned, except for a few seasonal residents. Why people quit this metropolis is a mystery that archaeologists have been exploring since the 1800s. Researchers have long been divided over how the collapse of Tiwanaku played out. Some earlier scholars thought that the city must have been conquered by an outside force, perhaps the ancestors of the Aymara people who live in the area today. Others believed that Tiwanaku lost influence slowly over decades or centuries and that regional drought played a role. More recently, archaeologists who believe the city was governed by a noble class have suggested that a popular uprising put a swift end to Tiwanaku’s ruling elite and led to the abandonment of the site.
Recent advances have made dating archaeological remains far more accurate than in the past. These more precise methods of dating, new statistical models based on Bayesian interpretation of probability, and improved atmospheric models have given scholars new tools with which to explore the possible reasons behind Tiwanaku’s demise. Previously, radiocarbon dates from Tiwanaku, which was occupied for about 900 years, were only accurate to within 200 or 300 years. Now, Erik Marsh, an archaeologist at the National University of Cuyo, along with an international group of scholars, has analyzed dozens of radiocarbon dates of organic material unearthed at residences and monuments across the site. Using the new techniques, they have increased the accuracy of the dates to within a few decades. This refined chronology has brought the entire history of the city into much sharper focus. “If you get to a generational scale,” says Marsh, “asking questions about people is completely different.”
Marsh and the team have used this new chronology to investigate when key events in Tiwanaku occurred. First, they sought to pin down when people constructed the city’s major monuments, such as the Akapana and the Pumapunku. They wondered how burial practices at the site changed over time, and for how long potters made Tiwanaku’s distinctive redware ceramics. Finally, they sought to resolve when people left the city. “Tiwanaku seems like an idea that spreads with ideology and ritual, people subscribe to it, and it lasts 400 to 500 years,” says archaeologist Alexei Vranich of the University of Warsaw. “But over time people say, ‘This isn’t working.’ We just don’t know why it wasn’t working.”
Marsh did not conduct any new excavations at Tiwanaku or take any new radiocarbon samples. He simply asked archaeologists who had worked at the site since the 1980s to share the dates they had collected during the course of their research. In all, these archaeologists supplied him with 102 dates, including 45 that had never been published. Radiocarbon dating is based on the predictable rate of decay of the radioactive isotope carbon 14. Living organisms absorb carbon 14 from the atmosphere, but the absorption stops when they die. The remaining carbon 14 decays at a constant rate while the amount of stable carbon 12 remains fixed. Thus, the proportion of carbon 14 to carbon 12 allows researchers to estimate the age of a sample of organic material. Because it is impossible to get an exact measure of the different carbon isotopes in a sample, radiocarbon dates are always given an error range of plus or minus a number of years. Those error ranges can span decades or even millennia.
In 2022, Marsh ran the 102 dates from Tiwanaku through a computer program called OxCal, which is maintained by Oxford University. Using OxCal, researchers can apply Bayesian statistical modeling to a set of radiocarbon dates. These Bayesian models account for the ways that each radiocarbon date from a site relates to other dates from the same site. “The standard method of radiocarbon dating makes no assumptions about where or when a radiocarbon sample may have come from,” says Alex Bayliss, an archaeological statistician with Historic England. “Bayesian statistical methods allow archaeologists to factor in the context of where the dating sample came from.” For instance, a sample from near the surface will generally have been deposited more recently than one that is found deeper in the excavation. Another example of a relationship that can be factored into a Bayesian model is when a dating sample is found in association with an artifact that has a known date range, such as a piece of pottery made in a distinctive style. Factoring in contextual information allows archaeologists to narrow a radiocarbon date’s error range to decades, or sometimes even less.
To further refine the dates from Tiwanaku, Marsh called upon Santiago Ancapichún, a climate scientist at the University of Magallanes, to model levels of atmospheric carbon at Tiwanaku. The concentration of carbon in the air affects how much carbon ends up in a sample of wood or bone. Atmospheric carbon levels are different in the northern and southern hemispheres and change over time, so calibration curves—graphs of these levels—are factored into programs such as OxCal to refine dates. However, the issue is more complex at sites where air from the two hemispheres mixes. This happens at Tiwanaku in the summer, even though it is well south of the equator. Ancapichún’s atmospheric models show how much air from each hemisphere is present at a specific place. He was able to provide Marsh with a site-specific calibration curve for Tiwanaku, further reducing the error ranges of the radiocarbon dates in his study. By using both the calibration curve and Bayesian modeling, Marsh reduced error ranges from centuries to decades, in some cases less than the span of a human life.
The new chronology of Tiwanaku surprised Marsh and his colleagues in several ways. The first was their finding that most of Tiwanaku’s major monuments were built within a few decades of each other rather than across the city’s entire history. Radiocarbon dates associated with the construction phases of the Akapana and the Pumapunku, for example, cluster between A.D. 600 and 700. “The original construction of the big monuments and the really fancy stonework was within a few generations,” says Marsh.
Another surprising finding revealed by the new chronology was that so few radiocarbon dates fell between A.D. 800 and 900. “For at least a few generations, almost nobody lived here,” says Marsh, “although I’m sure people were visiting and there was a small resident population.” The population rebounded in the A.D. 900s, but at this time, the people of Tiwanaku stopped burying their deceased relatives at the site. “If you build a tomb, you’re putting a very important person into the ground,” says Marsh. “At the very least, you’re anticipating the family is going to live here for a while.” While more people were living at Tiwanaku, it seems they could no longer consider it somewhere they had a stable future. “People decided this isn’t a good place to leave your grandparents,” says Marsh. “Either you don’t know when you’re coming back, or this place isn’t holy anymore.”
The new chronology shows that around this time the people of Tiwanaku did start a macabre new tradition—ritualized murder. A mass grave at the site dated from the mid-tenth century to the early eleventh century A.D.. contained skeletons buried face down, some with bound limbs, showing signs of blunt force trauma and head wounds. The people of Tiwanaku may have treated these bodies so carelessly because some of the victims were not originally from the area. Genetic analysis by a team from the University of Warsaw revealed that one woman found with her hands tied behind her back came from the Amazon, at least 100 miles to the east. This suggests that residents of Tiwanaku may have raided the region and taken captives to sacrifice.
People at Tiwanaku began to build new structures in the tenth century A.D. using stones from older monuments, including the Akapana and the Pumapunku. One project involved renovating a residential area called the Putuni. “It’s almost like we’re looking at two Tiwanakus,” says Marsh. “The earlier one says, ‘Let’s have big community projects, nice fancy adobe architecture with intricate stonework, and tombs for the ancestors.’ Later, a second group of people comes in and says, ‘Let’s reuse all the old blocks and start sacrificing people.’”
One thing that did persist through the different phases of Tiwanaku’s history was the city’s iconic redware pottery. The earliest dates for this type of pottery fall around A.D. 620 and coincide with the building of the Akapana and the Pumapunku, while the latest dates indicate the pottery was still being used by the city’s last residents around 1040. This 400 years of continuity reveals that some aspects of Tiwanaku’s culture endured; no other pottery style in the Andes lasted for this long. The distinctive kero vessels made during this final period indicate that ceremonies involving consumption of chicha continued to be important. Decorations on pots also provided the people of Tiwanaku with a form of visual communication, a way of telling stories that may have reinforced the sacred ideologies that made the city such a vital ritual center.
Within a few decades of 1010, according to the new chronology, people left their permanent residences at Tiwanaku and ritualized murder came to an end. A team led by archaeologist Alan Kolata of the University of Chicago in the 1990s found carbonized grass roofs in residential areas. Some scholars have suggested that this means homes were burned down in a violent uprising. Marsh, however, points out that there are only a few clear cases of burned roofs, and for now, archaeologists are uncertain whether the burning was a citywide phenomenon.
Small hearths and pits that were cut into the foundations of earlier buildings show that people continued to live in temporary structures at the site for another generation, until about 1050. Archaeologists have found no definitive evidence that people resided at Tiwanaku in the following centuries. Farmers continued to grow crops in the fields around the city long after it was abandoned, however, suggesting that a small population living nearby may have carried on some of the city’s traditions. In the fourteenth century, the Inca took over the territory around Lake Titicaca. They reestablished Tiwanaku as an important sacred site, and the long-abandoned city came to play a central role in their mythology.
Archaeologists can now rule out some potential causes for why people left Tiwanaku in the early eleventh century. Even before Marsh and his colleagues developed their new chronology, scholars had largely rejected invasion by a foreign power as a reason for the city’s downfall. Marsh points out that there is no evidence of warfare at the site such as fortifications to defend the city or storehouses that would have been needed to supply an army. In addition, archaeologists haven’t found any skeletons with the types of defensive wounds or unhealed injuries that would suggest they died on the battlefield.
Many have pointed to severe drought as a possible explanation for the city’s collapse, but the new chronology allows researchers to rule that out as well. The climate around Lake Titicaca did, indeed, become extremely dry, but not until a century or two after people stopped living at Tiwanaku. Because the new chronology shows the city was abandoned within a few decades, Marsh believes it is accurate to say that Tiwanaku collapsed. If people were motivated to burn down buildings and leave their homes, circumstances must have become intolerable. In the last few generations, leaders were incapable of or uninterested in organizing large building projects, which had been key to maintaining community. Seasonal gatherings had pumped life into the site for centuries, but at some point, people stopped making the pilgrimage. Whatever the case, residents and visitors must have felt these sudden and drastic changes within their lifetimes. “We can sometimes talk about processes of collapse and societal change in quite abstract, dehumanized ways,” says archaeologist Nicola Sharratt of Georgia State University. “This research clearly reminds us that these are processes that are experienced by human individuals, and that sometimes the experience is deeply unpleasant.”
Archaeologist Charles Stanish of the University of South Florida points out that other urban centers, such as those in the neighboring Wari state, were also depopulated in the early second millennium A.D., and that the timing of Tiwanaku’s abandonment revealed by the new chronology is consistent with what was happening in the rest of the region. “It was a dispersal of the population,” Stanish says. “It was a political collapse, an economic collapse.” Vranich thinks that for some reason people lost faith in the ideology that Tiwanaku represented. “In a place like Tiwanaku,” he says, “which is based on the idea of creating these large festivals that attract a lot of people and a lot of labor, this is a slow death knell.”
