Amazonian Forest Islands: Accidental Products of Ancient Human Occupation

The Amazon basin isn’t all just a lush tropical forest.  In large portions of western Amazonia wet seasonally-flooded grasslands, not trees, are the norm.   The upper branches of the Amazon River wind themselves across massive flood plains like snakes writhing across a sandy surface.  One feature of these nearly featureless flat plains that has puzzled visitors  for as long time are the presence of hundreds of small forest islands.  These 50 to 200 foot diameter islands in a sea of grass and marshes are only one to three feet above the ground level of the surrounding area and yet that smallest of elevation change means that the soil there is not saturated all the time and so can support tree growth.  But what is the origin of these forest islands? Rocks are virtually unknown in this part of the world.  Maybe they were the result of some natural process such as ancient termite mounds or animal borrows that then attracted other animals and plant which over thousands of years built up more and more sediments and detritus to the point where trees would grow?   Or could they be the result of native South Americans that intentionally built these mounds much like those in North America?  For some forest islands, the answer is apparently: neither.

An example of forest islands on a flood plane in central Bolivia.  These islands had been thought to have formed from natural (non-human induced) processes but now some have been shown to be the result of human activity.

An example of forest islands on a flood plain in central Bolivia. These islands had been thought to have formed from natural (non-human induced) processes but now some have been shown to be the result of human activity.

A paper just published by PLOS One (Early and Middle Holocene Hunter-Gatherer Occupations in Western Amazonia: The Hidden Shell Middens) reports the results of examinations of three forest islands mounds near Trinidad, Bolivia.  What the investigators found is that when they dug into these mounds they discovered they were composed of massive numbers of snail shells, bones of various mammals including human bones, and fire-blackened soils and bones.  That humans were involved, even if not in deliberate way, in the production of these mounds seems to be of little doubt.  The snail shells were lacking their opercula which would be present if they died there naturally. These shells show the signs of having been disposed of in huge heaps as they were consumed.  Bones of game animals and the evidence of deliberate fire point to these sites as being locations were people brought prey and other food sources and prepared them to eat.   At the bottom of the mounds are more snail shells than anything else. It could be that shells were deposited here for some time and after the accumulated a large number the spot attracted more attention which then brought more activity which then resulted in acceleration of the building process.  A positive feedback loop then developed where the “island” became a place to come in high water times and eventually trees, which probably weren’t originally on these mounds, might have grown here to provide even more reason to be attracted to the site.   The formation of these mounds is then not suggested to be the result of an intentional design, such as some form of landscape engineering that would be done thousands of years layer in the same region where ditches and hills were built to control the wetlands by later pre-Columbian dwellers of this area.  Rather the mounds came about by an accidental, yet human induced, series of events.

Screenshot of Google maps showing the possible identification of forest island SM3 from Bolivia.

I took this screenshot of Google maps showing the possible identification of study site forest island SM3 from Bolivia.   Toward the left side near the road there is another smaller forest island. The remainder of the light and dark greens here are grasses and aquatic plants.  I could identify more than 20 similar looking mounds in my short search of the areas using Google maps/earth.

Forest islands suggest a long chronology of human habitation.

How old are these mounds of shells and bones?  The study took cores from the mounds and then dated contents from top to bottom and found that the oldest material on the bottom dated to around 10,000 years.  I’ve written a series of posts and context is key to understanding chronology of human remains (See The frequently overlooked geological context of hominid fossils   and Implications of artifacts and bones on ancient human butchery practices and Geological context II: Neanderthals and the Italian super-volcano.  Claims of radiometric dating are easily ignored by anyone who might find the 10,000 year claim does not fit their worldview.   But an examination of the evidence from this site reveals that the context of the radiometric dating the mounds themselves speak to a long history of occupation and thus presence of human in this portion of the South America.  Let look more closely at some of the data presented in this paper.

Google maps screenshop of forest island site SM1 from the Lombardo et al. 2013 paper describing the formation of these islands.  On the left is a marsh/lake area and to the left are high grasses. The island itself only sits  about 1 foot above the water level but that is enough for trees to find a home there.

I searched on Google maps and found the location of forest island site SM1 from the Lombardo et al. 2013 paper. This is a screen shot I took of that site.   from the Lombardo et al. 2013 paper describing the formation of these islands. On the left is a marsh/lake area and to the left are high grasses. The island itself only sits about 2 feet above the area around it but that is enough for trees to find a home there.

There is one figure that stood out very clearly to me from this paper. That was figure 3 (see below).  In that figure we see a cross-section of one of

Figure 1 from Lombardo et al. 2013 PLosOne paper. This shows the forest island SM1 which I have take a screenshot from Google maps below.

Figure 1 from Lombardo et al. 2013 PLosOne paper. This shows the forest island SM1 which I have taken a screenshot from Google maps below.

the forest islands that was closely examined (SM1 which is pictured above in my screenshot from Google maps) and from Figure 1 from the paper to the right.  The investigators dug a pit near the center, making close observations of what they found and they dug down to groundwater level (about 6 feet). They also took core sections from the mound and then out from the mound into the grassy plain. You can see what they found. This massive mound about 200 feet in diameter is composed of massive numbers of freshwater shells that extend more than 6 feet below the current ground surface and has its base on what was a previous flat plain.  Sometime after the formation of most of this mound, floods in the area must have deposited around 6 feet of new soil around the mound.

Figure 3 Lombardo

Figure 3 from Lombardo et al. 2013. This presents a cross section of the island (shell midden as they call it) that is pictured in the figures above.

How does this progression of the mound and the new layers of soil impact our understanding of the chronology of human occupation? Well, the other thing we learn from this paper is that in the very top layers of the mound there are pieces of pottery and other cultural items that speak to the presence of more recent occupation. How recent, well the radiocarbon dating suggests 4000 years toward the present but these first cultural evidences are still the types of things that archaeologists and anthropologists would consider signs of very early developed societies in South America. What are in the lower layers of the mounds are evidence of a very simple hunter-gatherer groups which probably used these sites as seasonal locations to hunt from and collect any other food sources.  Just the size of the mounds and number of shells suggests a very long period of occupation (thousands of years).  The types of animal bones and shells tells us that the environment was considerably drier than it is today.  At some point probably after the hunters and gatherers had disappeared from this are other peoples moved into the area and these mounds would have been natural places to gather, cook food and settle.  Even this was probably thousands of years ago as evidenced by the dating and the fact that the climate was different from today. Then floods brought in many feet of new soil and the area changed into the marshy wetlands that they are today and the mounds became the only sanctuary where trees could live making this forest islands that we see today.  The current “modern” soil that envelops these mounds is probably many hundreds of years at the top to thousands of years old at the base.

Once again the facts and the context of this archaeological site defy typical creationists explanations.   Creationists propose to compress all of these human occupation sites into a chronology that spans only 4000 years.  Explaining these developments of these mounds is a tremendous challenge to that chronology.  And it is made even more difficult by the fact that the ground that lies below these mounds is likely composed of hundreds and hundreds of feet of sediments from floods of earlier times that must still be compressed into a post-Noahic flood world. In the creationist’ chronology there was an ice age 400 years after the flood and it was only then that people were first migrating from the tower of babel across the globe. They traveled up across Alaska and then all the way down into South America.  They then populated this region and ate millions and millions of snails forming these large mounds which then became partially buried by flood of new sediments on which the “newer” civilization of ancient South Americans then lived thousands of years to present.

For anthropologists these landscapes were thought to have been the last to have been occupied by human immigrants. With so many other environments more suitable for human occupation, one would not expect that the very first peoples to arrive in South America would immediately take up residence in this area.  There are many other location in South America that have been identified as being inhabited much earlier than 10,000 years ago but that this areas could have had a sizable population this early suggests the population of South America was even larger in the past than has been previously recognized.

Summing up…

Finally, these are just three easily accessible forest islands that were investigated. The paper does not indicate if other mounds have been tested but satellite imagery suggests there may be hundreds of similar mounds in the region.  If most should prove to be composed of mounds of disposed shells and bones then the sheer number of shells alone would either suggests a population of hundreds of thousands,  which is very unlikely as  the technology is nothing but very simple bone tools, or the presence of small bands of nomadic hunter-gathers that lived in this region for thousands of years moving from place to place (mound to mound).

As I have said many times before, when specific locations are examined in more detail the sites tell of story that is consistent with long periods of occupation in which the occupants developed technologies slowly over time or were replaced by different people groups with new technologies.  Creationist’s explanations must compress the development of these occupation sites into a very short time frame and typically ignore the evidence of changing technology and other cultural advancements (art, writing, etc…).


Early and Middle Holocene Hunter-Gatherer Occupations in Western Amazonia: The Hidden Shell Middens.   Umberto Lombardo et al. PLOS One 2013

Humans explored ‘Treasure Island’ much earlier than thought.  By Charles Chol


  1. This dovetails nicely with a talk I watched Dennis Stanford give at the Adolphus Gustavus Nobel Conference about the Solutrean People in North America. Were their any tools found in these excavations?


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