Searching for fossil pollen of Theobroma cacao, the wild chocolate tree
To see the research paper that this story relates to, go here
What's your chocolate bar of choice? Are you a milk chocolate kind of person? Prefer something riven with caramel and nuts? Or do you like your chocolate as pure and dark as two colliding black holes? Whatever variety you might lust for, the all-important ingredient which makes it chocolate, the cocoa power, is derived from the beans of a small, evergreen tree, native to lowland tropical regions of Central and South America: Theobroma cacao.
Our modern obsession is not the start of the story between this little tree and humans. Theobroma (meaning in Greek "food of the Gods") was popular amongst native societies in Central and South America long before Europeans ever arrived in the New World. Native people harvested not just the beans, but also the fruits of Theobroma cacao, alongside a group of its sister species in the same genus, Theobroma. They enjoyed it so much in fact that, as with many other fruiting trees, archaeologists suspect that native people planted new seeds and cultivated entirely human-made stands of chocolate tree, thus helping Theobroma to spread across the lowland tropics.
And this is where I and other researchers that study past vegetation become interested. With all the archaeological discoveries that have come out of the lowland Neotropics in recent decades, one now looks at seemingly 'natural' bits of forest and has to wonder "how much has the species composition of that forest been influenced by human management through time"? A tantalising question and a tricky one to address.
The creation of 'chocolate forest islands'.
Some of the most impressive and extensive pre-European archaeological sites in Amazonia have been discovered in the Llanos de Moxos, a patchwork mosaic region of seasonally-flooded savannah grasslands and evergreen rainforest in the eastern lowlands of Bolivia. Although this region is now sparsely populated and mostly given over to low-maintenance cattle ranching, the presence of numerous earthworks (large habitation mounds, raised agricultural fields, canals and causeways and huge ring ditches) suggest that larger, pre-European populations were highly active in their management and shaping of the landscape.
In the north-east province of Iténez, the seasonally-flooded grasslands are interspersed by naturally-formed 'islands' of forest that grow on raised outcrops of resistant rock, above the wet season flood level. However, beneath the dense forest canopy, many of the islands are covered by ancient, man-made ditches. The savannah is also criss-crossed by raised causeways, which probably served to link-up the islands during times of flood. Another, perhaps more subtle sign of human influence, may be the forests themselves. Local people and botanists have noted the unusual concentration of Theobroma trees in these forests; earning them the name 'chocolate forest islands"' One of the questions that we wanted to ask was: is this the legacy of intentional planting of Theobroma by people in the past? If so, was it the native pre-Europeans or later Jesuit mission communities that did the planting?
Needle in a haystack
In an attempt to answer these questions, and reconstruct more generally how the ring-ditch builders in Iténez interacted with the forest island vegetation, we took a sediment core from a lake (Laguna La Luna) located right next to what we later dubbed the La Luna forest island. The forest island is c. 8 km2 and surrounded along its edges by a c. 3m deep and 4m wide boundary ditch. Near to the centre of La Luna island, there is a (suspiciously) circular bog, from which we also took a surface sediment sample.
Our analyses of fossil pollen and charcoal form the lake sediment core produced a c. 6000 year record of vegetation change and fire history on the La Luna island. We discovered that people first inhabited this site as early as 2000 years ago; growing maize and probably engaging in some prescribed burning of the landscape. At this time of early occupation, the vegetation on the island was grassland, rather than forest, which grew under a drier past climate. Later on in the site's history, as the regional climate became wetter, forest expanded to cover the site, as it does today. Whilst people continued to occupy the island once it had become forested, there is no evidence that at any point they cleared large areas of the forest to build the ring ditch that circumscribes the island. So, whatever the land-use practices of these people were, there was apparently no need for labour-intensive and destructive forest clearance.
But what of the chocolate? I can tell you that this was not the palynologists day (or series of many days at the microscope, more accurately). The hunt for Theobroma pollen did not turn up any positive IDs, even when extended counts up into the thousands were conducted on the surface samples from both the lake and the bog. The fact that no Theobroma was encountered form the bog (which stands amongst a plethora of chocolate trees and, we assume, has a very localised catchment area) indicates that Theobroma is one of those elusively silent taxa that produces little pollen and is clearly made to frustrate palynologists. Many other "economically useful" plant taxa, such as palms, are similarly rare or absent from the pollen record and present a challenge to investigating the legacy of past human land use. However, while there are challenges, some recent papers have demonstrated that palynological techniques can be used to detect ancient agroforestry or compare the natural history of an "economic taxon" to records of historical human land use and past climate change (see Rushton et al. and McMichael et al.).
As for the La Luna work, although the search for chocolate pollen was a bust, some useful insights came out of the study. Not least intriguing is the indication that humans were present and active in this landscape before the forest. What's more, the islands were occupied during and after the climatically-driven rainforest expansion. Can we assume, therefore, that the forest in Iténez has never been without a human influence? What might humans have done to shape this forest, and the soils that in grew on, as it edged its way southward and encroached around their settlements?
In this region at least then, the distinction between natural and anthropogenic forest perhaps falls apart. However, these forests in the south are far more dynamic than and very young (c. <2000 years) compared to the million years old core of the Amazon rainforest. The relative importance of human land use since the last ice age in the development of the wider Amazon is therefore still hotly contested. Many exciting avenues lie ready to be explored, which incorporate palaeoecology with archaeological data, genetic evidence and perhaps some in-depth, widespread ecological statistical analyses of modern forest composition.
In the meantime, if you find yourself in some quite part of that continental jungle, surrounded by magisterial trees, a living floor of insects and the peaceful noise of birds, you may be pleased to ponder whether you are walking in the garden of Eden or through the unkempt garden of an ancient Amazonian horticulturalist.
What's your chocolate bar of choice? Are you a milk chocolate kind of person? Prefer something riven with caramel and nuts? Or do you like your chocolate as pure and dark as two colliding black holes? Whatever variety you might lust for, the all-important ingredient which makes it chocolate, the cocoa power, is derived from the beans of a small, evergreen tree, native to lowland tropical regions of Central and South America: Theobroma cacao.
Our modern obsession is not the start of the story between this little tree and humans. Theobroma (meaning in Greek "food of the Gods") was popular amongst native societies in Central and South America long before Europeans ever arrived in the New World. Native people harvested not just the beans, but also the fruits of Theobroma cacao, alongside a group of its sister species in the same genus, Theobroma. They enjoyed it so much in fact that, as with many other fruiting trees, archaeologists suspect that native people planted new seeds and cultivated entirely human-made stands of chocolate tree, thus helping Theobroma to spread across the lowland tropics.
And this is where I and other researchers that study past vegetation become interested. With all the archaeological discoveries that have come out of the lowland Neotropics in recent decades, one now looks at seemingly 'natural' bits of forest and has to wonder "how much has the species composition of that forest been influenced by human management through time"? A tantalising question and a tricky one to address.
The creation of 'chocolate forest islands'.
Some of the most impressive and extensive pre-European archaeological sites in Amazonia have been discovered in the Llanos de Moxos, a patchwork mosaic region of seasonally-flooded savannah grasslands and evergreen rainforest in the eastern lowlands of Bolivia. Although this region is now sparsely populated and mostly given over to low-maintenance cattle ranching, the presence of numerous earthworks (large habitation mounds, raised agricultural fields, canals and causeways and huge ring ditches) suggest that larger, pre-European populations were highly active in their management and shaping of the landscape.
In the north-east province of Iténez, the seasonally-flooded grasslands are interspersed by naturally-formed 'islands' of forest that grow on raised outcrops of resistant rock, above the wet season flood level. However, beneath the dense forest canopy, many of the islands are covered by ancient, man-made ditches. The savannah is also criss-crossed by raised causeways, which probably served to link-up the islands during times of flood. Another, perhaps more subtle sign of human influence, may be the forests themselves. Local people and botanists have noted the unusual concentration of Theobroma trees in these forests; earning them the name 'chocolate forest islands"' One of the questions that we wanted to ask was: is this the legacy of intentional planting of Theobroma by people in the past? If so, was it the native pre-Europeans or later Jesuit mission communities that did the planting?
Needle in a haystack
In an attempt to answer these questions, and reconstruct more generally how the ring-ditch builders in Iténez interacted with the forest island vegetation, we took a sediment core from a lake (Laguna La Luna) located right next to what we later dubbed the La Luna forest island. The forest island is c. 8 km2 and surrounded along its edges by a c. 3m deep and 4m wide boundary ditch. Near to the centre of La Luna island, there is a (suspiciously) circular bog, from which we also took a surface sediment sample.
Our analyses of fossil pollen and charcoal form the lake sediment core produced a c. 6000 year record of vegetation change and fire history on the La Luna island. We discovered that people first inhabited this site as early as 2000 years ago; growing maize and probably engaging in some prescribed burning of the landscape. At this time of early occupation, the vegetation on the island was grassland, rather than forest, which grew under a drier past climate. Later on in the site's history, as the regional climate became wetter, forest expanded to cover the site, as it does today. Whilst people continued to occupy the island once it had become forested, there is no evidence that at any point they cleared large areas of the forest to build the ring ditch that circumscribes the island. So, whatever the land-use practices of these people were, there was apparently no need for labour-intensive and destructive forest clearance.
But what of the chocolate? I can tell you that this was not the palynologists day (or series of many days at the microscope, more accurately). The hunt for Theobroma pollen did not turn up any positive IDs, even when extended counts up into the thousands were conducted on the surface samples from both the lake and the bog. The fact that no Theobroma was encountered form the bog (which stands amongst a plethora of chocolate trees and, we assume, has a very localised catchment area) indicates that Theobroma is one of those elusively silent taxa that produces little pollen and is clearly made to frustrate palynologists. Many other "economically useful" plant taxa, such as palms, are similarly rare or absent from the pollen record and present a challenge to investigating the legacy of past human land use. However, while there are challenges, some recent papers have demonstrated that palynological techniques can be used to detect ancient agroforestry or compare the natural history of an "economic taxon" to records of historical human land use and past climate change (see Rushton et al. and McMichael et al.).
As for the La Luna work, although the search for chocolate pollen was a bust, some useful insights came out of the study. Not least intriguing is the indication that humans were present and active in this landscape before the forest. What's more, the islands were occupied during and after the climatically-driven rainforest expansion. Can we assume, therefore, that the forest in Iténez has never been without a human influence? What might humans have done to shape this forest, and the soils that in grew on, as it edged its way southward and encroached around their settlements?
In this region at least then, the distinction between natural and anthropogenic forest perhaps falls apart. However, these forests in the south are far more dynamic than and very young (c. <2000 years) compared to the million years old core of the Amazon rainforest. The relative importance of human land use since the last ice age in the development of the wider Amazon is therefore still hotly contested. Many exciting avenues lie ready to be explored, which incorporate palaeoecology with archaeological data, genetic evidence and perhaps some in-depth, widespread ecological statistical analyses of modern forest composition.
In the meantime, if you find yourself in some quite part of that continental jungle, surrounded by magisterial trees, a living floor of insects and the peaceful noise of birds, you may be pleased to ponder whether you are walking in the garden of Eden or through the unkempt garden of an ancient Amazonian horticulturalist.