Aerial view of Llanos de Maxos, showing forest islands
surrounded by grassy floodplain. The forest islands are about
300 feet long, and their ground level is raised about 3 feet
above the surrounding plains.Credit: Lombardo et al.
Pomacea snails Note the flap,
or opercula, blocking the shell opening
to protect the snail from drying out.
All of these concepts came together in an unexpected way during a study of several forest islands in Llanos de Moxos published in the August, 2013 issue of PLoS One. Forest islands serve as refuge areas even today during floods, so the authors of the study had begun excavations in three of these hillocks to look for evidence of the Earth Mover culture. On all three hillocks, they did find a layer with artifacts that resembled those of the Earth Mover culture. However, as they dug deeper, they found that the entire centers of these 300-foot mounds were made up of middens full of snail shells. The shell-rich layer extended downward for more than 2 meters, below the present day water table. Radiocarbon dating showed that the deepest part of these shell middens were more than 10,000 years old! These shells were therefore laid down long before the Earth Mover culture was present.
Lombardo et al. make several arguments that these ancient shell-rich deposits are the result of human activity. First, the layer also contained animal bones and charcoal, which are commonly found in human middens elsewhere. The archaeologists also detected chemical compounds associated with human feces within the layers. Furthermore, the snail shells themselves were overwhelmingly specimens of large edible Pomocea snails, and the vast majority of these had had their opercula removed. Thus something had eaten all those snails. Although many animals in this region do feed on apple snails, no known predator creates middens of this size. The authors thus conclude that only human activity could have led to such a large deposit.
Lombardo et al speculate that the hillocks formed over centuries of use. Once a midden became large enough, silts would begin to accumulate around its margins, its crest would no longer be flooded, and trees could grow, further increasing the attractiveness of that spot for the inhabitants. Much later, the Earth Mover culture would find the same spots to be useful for their seasonal activities.
A last item is the extreme age of these midden deposits, with the deepest layers dating to 10,5000 years ago. The very first humans are thought to have arrived in the Americas around 14,000 years ago. This does not leave a lot of time for the culture which created these shell middens to have established itself at this remote location of the upper Amazon. As Lombardo et al comment elsewhere, either these groups were extremely adept at exploiting this harsh ecosystem or they arrived extremely soon after the Americas were populated. Alternatively, perhaps the 14,000 year date for first peopling of the Americas needs re-examination.