The Amazon may seem homogeneous at first glance. Yet it only takes a little bit of time in this lush ecosystem to realize that the only constant throughout this basin is its dynamism: a tree falls, and that patch gives space for dormant seeds and seedlings to grow; a river meanders, and the abandoned bank becomes a beach. Only a few meters of elevation make the difference between terra firme forest and varzea wetlands. There are innumerable noises and textures in the Amazon and the animals within it show significant genetic differences. Today, scientists are using the historical sciences of geology and biogeography (the study of distributions of species) to paint a clearer picture of how this landscape and its inhabitants have developed and the role water has played in it.
In the mid-Miocene epoch, within the Tertiary period about 12 million years ago, South America as a whole looked very different. Its exact composition and appearance is an active and debated area of research, but all would agree that the uplifting Andes were exerting new forces on the regions to the east of these mountains and water played a key role in influencing the landscape. Some scholars believe that middle Amazonia was one large lake, and there were seas to the north through the llanos to the east, where the Amazonian delta now is, and to the south, where the Paranaense sea lay. What would eventually become the Amazon river were smaller tributaries that flowed westward to the Pacific or to one of these seas. The rest of the Tertiary period saw the Andes continue to rise, and this extensive lake formation transformed into a more fluvial-wetland system that eventually broke through to the east and formed the modern channel of the Amazon or main stem flowing into the Atlantic.
“Tentative middle Miocene palaeogeography of the Caribbean realm and South America (based on Iturralde-Vinent & MacPhee 1999; Del R ıo 2000; Hern andez et al. 2005; Hoorn et al. 2010b; Candela et al. 2012” via ResearchGate
Dating of these changes is highly contested, as different sets of researchers have argued for different constructions of the Pleistocene epoch (the last 2.5 million years), but it seems clear that there was significant dynamism in the wetlands throughout this period as well. Some researchers have found evidence for the Lago Amazonas hypothesis: Western Amazonia was also covered in this period by a large lake from about 40,000 to 15,000 years ago, when it started to drain (Frailey, et al., 1998; Campbell, 1990). This area was met with catastrophic floods between 10,000 years ago to 2,000 years ago (Campbell & Frailey, 1984; Campbell, et al., 1985). Other researchers suggest that the Amazon started flowing eastward in the Late Pleistocene (~1 million years ago), indicating an earlier clear fluvial system instead of a lake (Aleixo & Rossetti, 2007), and others argue that we simply haven’t studied this geographic area enough to make any conclusions (Tuomisto, et al., 1992).
However, all of these changes were influenced by tectonism, the process by which tectonic plates shift and form plateaus and mountains, and the related uplift of and subsequent burying of geologic arches which created elevation in lowland valleys and defined the location of terra firme forest and varzea wetlands (Rossetti & de Toledo, 2007). We see the influence of this dynamism in the diversity of species found in the region today. From the very beginning, literature on these prehistoric lake systems has included biogeographic evidence for this watery past. For example, osteoglossids (or fish in the same family as the arowana) are unable to move up rapids yet are found upriver of them in the Orinoco basin–a possible remnant of an ancient lake system (Goulding, 1980, in Frailey, et al., 1988).
Ardea alba (c) Walter Wust.
Today, advances in technology have made phylogenetic data from natural history collections and field research more accessible, so researchers have been able to characterize biogeographical patterns left behind by the development of today’s extensive mosaic of wetlands from the Miocene lake system. For instance, Lontra longicaudis, the Neotropical river otter, appears to have diversified between 2.9 and 0.9 million years ago, around when the large lake system started becoming a river system and the thick forest started to expand (Ruiz-Garcia, et al., 2018). The same researchers also looked at mitochondrial diversification in river dolphins and theorize that Atlantic Ocean dolphins came to the Amazon basin through the Paranaense sea, and were then separated when the Amazon and Paraná basins split (Ruiz-Garcia, et al., 2018). Birds, an extremely diverse group in the Amazon, show patterns suggesting more recent diversification than these mammals. Upland forest species and riverine species have different histories: upland forest species seem to have diversified into the Amazon only in the early-middle Pleistocene era, while riverine species diversified earlier in the Pliocene (Claramunt, S., 2014; Aleixo & Rossetti, 2007). This makes the birds that live along water older residents, yet more evidence that the Amazon has always been a dynamic waterscape.
We still have much to piece together and resolve about the history of this incredible landscape and the communities of plants and animals that inhabit it, but it is clear that water has played a critical role in shaping the Amazon we know today.
Aleixo, A., & Rossetti, D. de F. (2007). Avian gene trees, landscape evolution, and geology: towards a modern synthesis of Amazonian historical biogeography? Journal of Ornithology 148(S2): S443-S453.
Claramunt, S. (2014). Phylogenetic relationships among Synallaxini spinetails (Aves: Furnariidae) reveal a new biogeographic pattern across the Amazon and Parana river basins. Molecular Phylogenetics and Evolution 78: 223-231.
Crouch, N., Capurucho, J., Hackett, S., & Bates, J. (2018). Evaluating the contribution of dispersal to community structure in Neotropical passerine birds. Ecography 41: 1-10.
Frailey, C., Lavina, E., Rancy, A., & de Souza Filho, J. (1988). A proposed Pleistocene/Holocene lake in the Amazon basin and its significance to Amazonian geology and biogeography. Acta Amazonica 18(3-4): 119-143.
Jaramillo, C., Romero, I., Apolito, C., Bayona, G., Duarte, E., Louwye, S., Escobar, J., Luque, J., Carrillo-briceño, J., Zapata, V., Mora, A., Schouten, S., Zavada, M., Harrington, G., Ortiz, J., & Wesselingh, F. (2017). Miocene flooding events of western Amazonia. Science Advances 3: e1601693.
Mori, S., Kiernan, E., Smith, N., Kelly, L., Huang, Y., Prance, G., & Thiers, B. (2016). Observations on the Phytogeography of the Lecythidaceae Clade (Brazil Nut Family). Phytoneuron 30(April): 1-85.
Rasanen, M., Salo, J., Jungnert, H., & Romero, L. (1990). Evolution of the western Amazon lowland relief: impact of Andean foreland dynamics. Terra Nova 2(4): 320-332.
Rossetti, D. de F., & de Toledo, P. M. (2007). Environmental changes in Amazonia as evidenced by geological and paleontological data. Revista Brasileira de Ornitologia 15(2): 2251-2264.
Ruiz-Garcia, M., Escobar-Armel, P., de Thoisy, B., Martínez-Agüero, M., Pinedo-Castro, M.,
Shostell, J. (2018). Biodiversity in the Amazon: Origin Hypotheses, Intrinsic Capacity of Species Colonization, and Comparative Phylogeography of River Otters (Lontra longicaudis and Pteronura brasiliensis, Mustelidae, Carnivora) and Pink River Dolphin (Inia sp., Iniidae, Cetacea). Journal of Mammalian Evolution 25(2): 213-240.
Tuomist, H., Ruokolainen, K., & Salo, J.. (1992). Lago Amazonas: Fact or Fancy? Acta Amazonica 22(3): 353-361.
Written by Natalia Piland.