Saturday, September 07, 2013

Heaps of snail shells: Evidence for human occupation of Llanos de Moxos 10,000 years ago

The Llanos de Moxos region of northern Bolivia is a flat tropical savannah, covering about 90,000 square miles, at the eastern edge of the Andes mountains and pierced by major tributaries of the Amazon. The climate in the Llanos de Moxos is extremely harsh, cycling between extreme dryness and flooding. The ecosystem is similar in appearance to the Florida Everglades, dominated by aquatic vegetation, sedges and grasses. Because of frequent inundation, trees are restricted to occasional hillocks termed forest islands.
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.   
Though the climate in Llanos de Moxos is very harsh, this region has been received a lot of attention because of evidence that it supported a large pre-Colombian population of fishers and farmers called the Earth Mover culture. These inhabitants are thought to have raised massive earthworks to channel the flood waters and to improve agricultural output. But it has been very difficult to find conventional evidence for human habitation, such as rock tools or burials,  in this area. This might be because suitable rock for making tools has always scarce throughout this ecosystem, and because the harsh climate favors rapid decomposition of other types of evidence of human activity.  So archaeologists studying ancient inhabitants of this area have had to rely on other kinds of evidence.

Pomacea snails Note the flap, 
or opercula, blocking the shell opening
to protect the snail from drying out.
Credit: snailbusters
One of the major foodstuffs within the Llanos de Moxos is the Pomacea family of freshwater snails, more commonly known as apple snails. These molluscs thrive in tropical wetlands, and are in fact an invasive species problem in Florida and in Hawaii. The pre-Colombian peoples of the Llanos de Maxos ate huge numbers of these snails, leaving middens (essentially rubbish heaps) containing tons of snail shells alongside their earthworks.

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.

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