Was malaria present in the Amazon before the European conquest? Available evidence.

Introduction

Whether or not malaria was present in the New World before 1492 is a question that has been debated multiple times in a myriad of disciplines. Anthropologists, archaeologists, epidemiologists, historians, and biomedical scientists each provide evidence against the presence of malaria before the conquest. The lines of evidence are based on recently acquired knowledge about the evolution of the malaria parasite in humans and primates, the limited occurrence of genetic mutations caused by malaria, connections between selection for human ABO blood groups and malaria, the unsuitable environmental conditions for malaria transmission during migratory movements through Siberia and Alaska by the first occupants of the American continent, and historical records of pre-Columbian civilizations. Opposite views have also been proposed, primarily based on the fact that Aztecs knew the therapeutic properties of the cinchona tree (from which quinine is derived) for fevers. No record, however, suggests that malaria occurred among this civilization. Despite the preponderance of available evidence indicating that malaria was not present in the New World prior to the European conquest, the jury is still out on a definitive resolution of the question.

The purpose of this paper is to synthesize the available evidence and identify the final steps that are still needed to obtain a clear-cut resolution to the timing of the introduction of malaria to the New World. We focus attention on the Brazilian Amazon, where the ecological conditions are, and have been for centuries, most conducive to malaria transmission. We review the available evidence in favor of the introduction of malaria after the conquest, and introduce additional facts based on historical documents and recent archaeological discoveries. Finally, we argue that current biomolecular technology applied to skeletal remains collected in different Amazonian locations has the potential to more firmly establish that there was no malaria in the Amazon prior to the European conquest.

Historical documents

Much prior research indicates that the Brazilian Amazon was not just a pristine forest prior to the conquest. In addition, the idea that the Indians were a population free of diseases has been refuted, although diseases such as smallpox, measles, and yellow fever were not present in pre-colonial times.

The use of historical documents to address the presence of malaria in the ancient Amazon is compromised by the fact that the native indigenous populations had no formal written language. Therefore, all available historical documents were written by Europeans (colonizers, Jesuits, and scientists). The later they were written, the more they tended to reflect the situation after the first contact between natives and conquerors. However, several of the early writings offer valuable evidence for the discussion.

A careful reading of the very first document ever written about Brazil—Pero Vaz de Caminha's letter to D. Manuel, king of Portugal, dated May 1,1500, 9 days after Brazil's ‘discovery’—reveals a very promising land, with good air and climate, and a very healthy indigenous population. Father Manoel da Nóbrega, who led the first Jesuit mission sent to Brazil in 1549 (in fact, the first mission ever sent to the New World), stated during his early years of stay that he did not have knowledge of people dying of fever but only of old age. Father José de Anchieta, another Jesuit sent to Brazil in 1553, also described the land as having very good and healthy air; a place where people used to live up to their 90s.

Two additional facts are worth mentioning at this point. First, it took almost half a century for Portugal to effectively start occupying Brazil, so very little contact was made with Indians until the first Jesuit mission arrived. Second, at the time of discovery malaria was believed to be caused by bad air, and was already a problem in Portugal. Therefore, its presence in the newly discovered land would not pass unnoticed by the Portuguese. One can argue that all these events took place along the coast of Brazil, and thus did not constitute evidence regarding the Amazon. However, in this case an important analogy can be made. First, the physical and cultural characteristics of the natives along the coast and in the Amazon were most likely similar. Although different tribes were spread throughout what is now Brazil, they were all part of the Tupí peoples. Second, both the coast of Brazil and the Amazon offered favorable ecological conditions for malaria transmission. In fact, the coast of Brazil became a very malarious area after the intensification of European occupation. Finally, archaeological evidence, as detailed later, corroborates these hypotheses.

Another important piece of historical evidence is the reported association between malaria (among other diseases) in the indigenous populations and contact with slave traders and African slaves coming mainly from Angola, Mozambique, and Cape Verde.

Archaeology

Archaeological findings in the Amazon also provide significant evidence against the presence of malaria before the conquest. Soil analysis revealed that highly fertile dark soils, called Indian black earth, occur mainly under the influence of man throughout time. These soils are often observed at archaeological sites in the Amazon. The depth of the Indian black earth at a particular site gives an indication of the time of human occupation—approximately 10 years per each centimeter. This finding has two major implications for the debate about the presence of malaria in the Amazon. The first is related to patterns of settlement, and the second to population density.

Indian black earth sites vary in extent depending on their location. Those along the rivers are usually larger than interfluvial ones. They have an elongated shape following the river, and extend several hundred meters inland. This is a pattern similar to current planned settlement areas in the Amazon, which use roads, and not rivers, as the reference point. Riverine villages were common in the Amazon, given the importance of rivers for both transportation and food, before roads were cut through the rain forest in the mid-1900s. Studies carried out in the early 1900s revealed that riverine populations frequently suffered from malaria. Wherever the disease was reported, communities were unstable, had low population density, and high mobility. If malaria was present in ancient times and mobility was high, the depth of Indian black earth sites could not be too large, since the pressure of poor health conditions would result in short occupation time.

Regarding population density, Smith estimates that Indian black earth sites were occupied by large populations. In one of the sampled sites, measuring 80 ha, an indigenous population as large as 18,000 people is estimated to have inhabited the area. This is a very densely populated area, especially compared with current numbers in the Amazon. Settlement areas promoted by the government in the 1970s had families living on land parcels that averaged 100 ha (one parcel for each family). Later on, the size was reduced to an average of 40 ha to accommodate a large population of surplus migrants.

A recent archaeological discovery corroborates Smith's findings. With the aid of remote sensing analysis, Heckenberger et al. identified 19 large integrated pre-Columbian settlements, located in the Upper Xingú region of the Brazilian Amazon, dated between 1200 and 1600 AD. The discovery suggests that in the pre-colonization era the Amazon had areas with sustainable agricultural production in harmony with the biodiversity. Moreover, the findings show that these areas were densely populated. In one particular settlement the density was 6–12.5 hab/km2. This figure is much higher than the density observed in current settlement projects in the Amazon, especially during years when malaria is epidemic. For example, in the Machadinho settlement project, located in the state of Rondônia, the density was only 1.6 and 3.2 hab/km2 in 1985 and 1986, respectively. These were years of severe malaria epidemics in the area, that promoted high human mobility and abandonment of land. In 1995, when malaria was endemic in Machadinho, the population density was 6.1 hab/km2. Finally, large settlements with high population density in ancient times were also reported in the archaeological work of Roosevelt.

Genetics and evolutionary biology

The relevance of population density becomes clearer with a brief analysis of the evolution of the malaria parasite. Genetic analyses suggest that P. vivax is older than Plasmodium falciparum. It has a lower virulence than P. falciparum, and can produce relapses for 3–5 years. As a consequence, it survives even in adverse environments for malaria transmission, with small and isolated populations. P. falciparum, however, probably became adapted to humans after the advent of agriculture, and the resulting higher population density.

Therefore, it is unlikely that large, stable and densely populated communities, in which sustainable agricultural practices were observed, could survive in the ancient Amazon if malaria was already a major threat. Most likely the people would have suffered mainly from P. falciparum, and would have been both experiencing high mobility and living at lower population densities. Interestingly enough, Heckenberger et al. estimated that the areas in the Upper Xingú region were abandoned after 1600–1700 AD, resulting in reforestation. That region was only reached by the Portuguese in the 1700s, when Portugal decided to expand its frontiers to the interior of Brazil, mainly in search of gold and diamonds. After that time, several historical documents report the occurrence of the disease. One example is the study made between 1817 and 1820, when two naturalists traveled in Brazil and observed enlarged spleens among people living along the margins of all rivers from the Southeast region to the Amazon, clear evidence of the presence of malaria.

Epidemiological barriers

Cunha argues that there was no epidemiological barrier to Europeans in America, in sharp contrast to sub-Saharan Africa. In the former case, Indians died in massive numbers after contact with Europeans, while in the latter case Europeans died in large numbers after contact with Africans. If malaria had been present in the Amazon before the conquest, it is quite likely that they would have experienced an epidemiological barrier, analogous to what they encountered in Africa. At present, there is an epidemiological barrier in the Amazon that is unfavorable to migrants who move there in search of land. However, the barrier is not so much associated with mortality anymore, but to morbidity. Malaria is one of its primary components.