Biological evidence supports an early and complex emergence of the Isthmus of Panama

Abstract

The linking of North and South America by the Isthmus of Panama had major impacts on global climate, oceanic and atmospheric currents, and biodiversity, yet the timing of this critical event remains contentious. The Isthmus is traditionally understood to have fully closed by ca. 3.5 million years ago (Ma), and this date has been used as a benchmark for oceanographic, climatic, and evolutionary research, but recent evidence suggests a more complex geological formation. Here, we analyze both molecular and fossil data to evaluate the tempo of biotic exchange across the Americas in light of geological evidence. We demonstrate significant waves of dispersal of terrestrial organisms at approximately ca. 20 and 6 Ma and corresponding events separating marine organisms in the Atlantic and Pacific oceans at ca. 23 and 7 Ma. The direction of dispersal and their rates were symmetrical until the last ca. 6 Ma, when northern migration of South American lineages increased significantly. Variability among taxa in their timing of dispersal or vicariance across the Isthmus is not explained by the ecological factors tested in these analyses, including biome type, dispersal ability, and elevation preference. Migration was therefore not generally regulated by intrinsic traits but more likely reflects the presence of emergent terrain several millions of years earlier than commonly assumed. These results indicate that the dramatic biotic turnover associated with the Great American Biotic Interchange was a long and complex process that began as early as the Oligocene-Miocene transition.

Keywords: biogeography; evolution; fossil; migration; neotropics.

Fig. 1.

MRTT estimated from molecular data. The plots show rescaled migration rate for dispersal (A–C) and vicariance (D) events across the Isthmus of Panama. The rates and their temporal dynamics were estimated using maximum likelihood and 1,000 randomizations to infer 95% confidence intervals of MRTT (shaded areas) (see also SI Appendix, Fig. S1 for a sensitivity analysis) and the time of each rate shift with their confidence intervals (circles and bars at the bottom of each panel). Dispersal between North and South America was calculated for (A) the entire dataset, (B) major taxonomic groups, and (C) direction of dispersal. (D) Comparison between vicariant events separating marine organisms in the Caribbean and the Pacific and the dispersal of terrestrial vertebrates (mammals, amphibians, nonavian reptiles, and birds) between North and South America. The number of rate shifts was selected by model testing via AICc (Table 1 and SI Appendix, Table S10). The dashed line indicates the generally accepted emergence of the Isthmus of Panama at 3.5 Ma.

Fig. 2.

Mammal fossil record in the Americas. The bar plots show mammal diversity through time based on fossil occurrences identified to the species level (reduced for clarity to the past 25 Ma, using time bins of 1 Ma) (see SI Appendix, 1.4 and Table S2 for more details). (A) Total number of fossil occurrences. (B) Diversity trajectories based on the first and last appearances of species (the thickness of curves reflects 95% confidence intervals of ages) (see also SI Appendix, Table S6) for North and South America. (C) Mean fraction of immigrant lineages—that is, North American lineages found in South America and vice versa (see SI Appendix, 1.4 for their definition)—plotted through time as a proportion of the total diversity sampled within each 1 Ma bin on each continent (confidence intervals are given in SI Appendix, Table S2). Approximate curves for global mean temperature (38) and eustatic sea level down-sampled to 0.1 Ma resolution (39) are plotted for reference.