Why Did The Amazonian Upano Civilization Collapse?

The Upano civilization in the foothills of the Andes Mountains in Eastern Ecuador had a city the size of Roman era London at about the same time as Roman era London, the discovery of which was just published yesterday, in the journal Science.

This archaeological civilization collapsed in approximately 300-600 CE, and the area was only repopulated by the Huapula culture, ca. 800 CE, two centuries or more later.

Why did it collapse?

Several possibilities suggest themselves.

1. The Roman empire was brought down, in significant part, by a major climate event that was global in scope. It could be that the same climate event caused the Upano civilization to collapse halfway around the world. As explained at the link:

Work by dendro-chronologists and ice-core experts points to an enormous spasm of volcanic activity in the 530s and 540s CE, unlike anything else in the past few thousand years. This violent sequence of eruptions triggered what is now called the ‘Late Antique Little Ice Age,’ when much colder temperatures endured for at least 150 years.

But we don't know much about the climate impact of this event in Ecuador or its vicinity. We do know, however, that an extreme climate event did cause a massive population collapse in South America, including the Amazon region, around 6200 BCE. We also know that the Medieval Climate Anomaly, a warm period of the Northern Hemisphere from ca. 1000 CE -1200 CE was matched by similar effects in South America:

The vast majority of all South American land sites suggest a warm MCA. Andean vegetation zones moved upslope, glaciers retreated, biological productivity in high altitude lakes increased, the duration of cold season ice cover on Andean lakes shortened, and trees produced thicker annual rings.

Similar comparisons have been made to South America for Europe's Little Ice Age from 1500 CE to 1850 CE. In that time period, "the climate of southwestern Brazil was wetter than it is now, for example, while that of the country's Northeast region was drier."

The correlations seem to be driven, in part, by ocean temperatures.

2. After a thousand years, soil exhaustion similar to the soil exhaustion that was an important cause of the late Neolithic population crash in Europe could have made the civilization unviable. As the abstract of the 2019 paper linked in this paragraph explains that event:

The focus of this paper is the Neolithic of northwest Europe, where a rapid growth in population between ~5950 and ~5550 cal yr BP is followed by a decline that lasted until ~4950 cal yr BP. The timing of the increase in population density correlates with the local appearance of farming and is attributed to the advantageous effects of agriculture. However, the subsequent population decline has yet to be satisfactorily explained. One possible explanation is the reduction in yields in Neolithic cereal-based agriculture due to worsening climatic conditions. The suggestion of a correlation between Neolithic climate deterioration, agricultural productivity, and a decrease in population requires testing for northwestern Europe. Data for our analyses were collected during the Cultural Evolution of Neolithic Europe project. We assess the correlation between agricultural productivity and population densities in the Neolithic of northwest Europe by examining the changing frequencies of crop and weed taxa before, during and after the population “boom and bust.” We show that the period of population decline is coincidental with a decrease in cereal production linked to a shift towards less fertile soils.

3. They could have been defeated militarily by a neighboring civilization such as the Muisca confederation which was well established in the Northern Andes by the 8th century CE, the Tairona civilization thrived in the Sierra Nevada de Santa Marta mountain range in northern Colombia from the 1st century CE until the Spanish arrival in the 16th century, the Moche civilization that thrived on the north coast of Peru from about 100 to 800 CE, the Wari Empire was located in the western portion of Peru and existed from the 6th century to the 11th century, or the Tiwanaku empire was based in western Bolivia and extended into present-day Peru and Chile from 300 to 1000. See generally here.

4. They could have fallen to a wave of Old World diseases brought to South America through pre-Columbian first contact with Polynesian seafarers, albeit, a wave of diseases less severe (or overcome and recovered from with more time to do so) than in the Columbian European first contact case. The timing is a few centuries too early compared to other data about possible Polynesian-South American contacts, however.

5. They could have not actually collapsed, but relocated. Precedents for this kind of mass migration include the Na-Dene people of Canada who migrated to the American Southwest (around 1000 CE), and the Ancestral Puebloans (around 800-900 CE). For example, the geographically nearby Cañari civilization, whose capital is reputed to be the source of the mythical city of El Dorado, that existed contemporaneous with the 13th to 16th century Inca civilization, could have been derived from the descendants of the Upano people.