The prehistory of neotropical lowland forests

Although they have persisted for tens of millions of years, neotropical lowland forests have changed greatly in extent and composition due to climatic variation and to human impacts. In a symposium at the 2002 meetings of the Association for Tropical Biology, hosted by the Smithsonian Tropical Research Institute (STRI), Panama, scientists presented the latest results of research on neotropical forests and their transformations up to the time of Columbus.

Bruce MacFadden, of the University of Florida, has re-examined a mammal fossils of mid-Miocene age (18-16 million years ago) collected in central Panama in the 1960s, but not previously described in detail. At the time a deep oceanic channel across eastern Panama separated North and South America. Although the species are closely related to those of North America of the same age (instead of geographically closer South America), most appear to have been adapted to forested habitats, rather than being grasslands species like those that dominated North America. In particular, the low-crowned teeth of herbivores reveal that they fed on soft-leaved forest vegetation rather than grasses, and future analysis of carbon isotope ratios in enamel may further confirm this.

Lake cores are increasingly being used to obtain records of pollen, phytoliths, charcoal, and other evidence of forest distribution and composition from throughout the neotropics over the past 20,000 years. Dolores Piperno, of STRI, described how this data shows colder, drier conditions than those of the present prevailed in many areas in the late Pleistocene. In some areas forests persisted, although they often contained a mix of lowland species with those found only at higher elevations today, creating tree communities quite different from modern ones. In others, grassland replaced forest. Later, after the arrival of humans, forests gradually retreated with the spread of slash-and-burn agriculture, only to return again when indigenous populations were decimated after colonization by Europeans. Barbara Leyden of the University of South Florida presented data from lake cores from the Yucatan Peninsula. Although stable isotopes identified a severe drought at the end of the Mayan Classic Period, the pollen record showed the vegetation was not greatly affected except for maize and associated weedy species.

Calcite deposited in caves over thousands of years preserves information on changes in rainfall over time. Matt Lachniet of STRI reported on work (with collaborators Y. Asmeron, S. Burns, W. Patterson, G. Seltzer, C. Wurster, and D. Piperno) on oxygen isotope ratios from cave deposits in Panama and Costa Rica, which shows the late Pleistocene to have been cooler and/or drier than the early and mid-Holocene, with an abrupt transition at about 11,000 years ago. During the Holocene, smaller variations in precipitation took place over cycles of about 15 years.

The periodic climatic phenomenon known as the El Niño/Southern Oscillation (ENSO) has had drastic effects on human economies and societies both today and in the recent past, and undoubtedly affected ancient peoples as well. Along the coast of central Peru, where ENSO has its strongest effects, Daniel Sandweiss of the University of Maine has found that the archeological record provides evidence of changes in this phenomenon over time. Marine organisms preserved in archeological sites show that there was little or no ENSO activity from about 9000 to 6000 years before the present; a low frequency of events from 6000 to 3000 years ago; and a frequency since then similar to that known historically.

Vertebrate remains from archeological sites can provide important clues to local environments and to how human societies interacted with them. Richard Cooke of STRI summarized archeological data on pre-Columbian vertebrate faunas from throughout Central America, extending as far back as 7000 years ago in central Panama. Animals used for food were usually acquired in habitats within a few kilometers of the sites in which they were found, although items such as dried fish could be traded some distance inland. In contrast, products obtained from animals that were important for ceremonial purposes often came from much farther away, and from habitats such as forest that were no longer present near settlements due to clearing for agriculture.

The localized dark fertile soils known as terra preta are anomalous among the generally infertile soils of the Amazon Basin. Rich in potsherds and other archeological remains indicating generation by past human occupations, the practices that created them have been unknown. Eduardo Neves reported on research (with collaborators Robert Bartone, James B. Petersen, and Michael Heckenberger) in Brazil indicating that these soils resulted from processes of local population growth, sedentarization, and the development of complex societies.

Until recently few archeologists accepted that the first humans in the Americas might have arrived much before 11,000 years ago. Tom Dillehay of the University of Kentucky summarized information from South Americas that has helped to promote the acceptance of much earlier dates. Although linguistic and genetic evidence still links American populations to Asia, there are also hints of an early contact from Europe. The exceptionally well preserved site of Monteverde in Chile has provided some of the best evidence for early human occupation in southern South America. Plant remains show the inhabitants had an extensive knowledge of the resources their environment had to offer: they not only used more than 50 plant species for food, they also exploited more than 20 species of medicinal plants, and some of these came from considerable distances away.

The Association for Tropical Biology was founded in 1963 to promote research and encourage interchange of ideas about the biology of tropical environments, and publishes the quarterly bulletin, Biotrópica.

The Smithsonian Tropical Research Institute (STRI), with headquarters in Panama City, Panama, is one of the world’s leading centers for basic research on the ecology, behavior and evolution of tropical organisms.