Unraveling the mystery of how deep Earth processes created evolutionary pathways in the Caribbean islands
The Caribbean islands represent one of the world's most spectacular natural laboratories for studying evolutionary processes. With their remarkable biodiversity and high levels of endemic species (found nowhere else on Earth), these tropical islands have puzzled scientists since Darwin's time. How did non-flying land mammals like sloths and rodents—poor swimmers all—reach these isolated islands? This question has sparked a centuries-old debate between two competing explanations: did animals walk across a now-vanished land bridge, or did they raft across open water on floating vegetation? 3
"The Caribbean islands are a living laboratory of evolution, where deep Earth processes meet biological diversification in fascinating ways."
At the heart of this mystery lies GAARlandia (Greater Antilles Aves Ridge), a hypothesized land bridge that might have connected South America with the Caribbean islands approximately 33-35 million years ago. This popular science article explores how an innovative multidisciplinary research project called GAARAnti is combining geology and biology to solve this enduring mystery of Caribbean biogeography. By coupling subduction dynamics with evolutionary processes, scientists are finally uncovering how deep Earth processes shaped the extraordinary biodiversity of the Caribbean islands. 1 6
The Caribbean region's complex geology originates from the interplay of several tectonic plates. The Caribbean Plate, primarily composed of Jurassic to Lower Cretaceous oceanic crust, has been migrating northeastward between the North and South American plates for tens of millions of years. This movement created a dynamic environment of volcanic arcs, subduction zones, and changing sea levels that periodically connected and isolated islands. 5
Around 50-40 million years ago, a major plate reorganization occurred when North America's absolute plate motion changed to westwards. This caused highly oblique subduction below the Lesser Antilles to become trench-normal subduction at much higher rates. The resulting compression and crustal thickening may have raised the Aves Ridge and other regions above sea level, potentially creating temporary land connections. 5
In 1994, scientists Ross MacPhee and Manuel Iturralde-Vinent proposed the GAARlandia hypothesis, suggesting that during the Eocene-Oligocene boundary (approximately 33 million years ago), plate compression combined with globally lowered sea levels (due to Antarctic ice sheet expansion) exposed a quasi-continuous land bridge. This landspan would have connected South America with Puerto Rico via the Aves Ridge, with further connections to Hispaniola, Cuba, and eastern Jamaica. 6
According to this hypothesis, the ancestors of many terrestrial vertebrates—including sloths, rodents, frogs, and numerous invertebrates—could have walked to the Caribbean islands rather than undergoing unlikely oceanic dispersals. The GAARlandia land bridge was likely short-lived, perhaps existing for only 1-2 million years before subsiding beneath the waves again. 3 6
The biological evidence for GAARlandia comes from both fossil records and molecular dating studies. Paleontological findings have revealed South American mammals in Greater Antilles deposits, including the oldest known Caribbean rodents—two distinct species of chinchilloid caviomorphs (a strictly South American group) found in lower Oligocene sediments in Puerto Rico dating to approximately 29.5 million years ago. 3
Molecular phylogenetics studies examining DNA from both recent and fossil specimens have provided additional support. Research on Antillean sloths (Megalocnoidea clade) indicates they diverged into different species around the time of the Eocene-Oligocene boundary (~34 million years ago), coinciding with the proposed GAARlandia emergence. Similarly, studies of toads, cichlid fish, spiders, and various insects have identified divergence times clustering around this period. 3 7
Not all evidence aligns neatly with the GAARlandia hypothesis. Studies of other species suggest multiple colonization events at different times. For example, the molecular phylogeny of spiny rats (Echimyidae) indicates they colonized the Lesser Antilles much later, during the middle Miocene (approximately 16-11.6 million years ago). This suggests at least two distinct dispersal events: possibly an initial migration via GAARlandia followed by later over-water dispersal. 3
Critics of the GAARlandia hypothesis point to the lack of definitive geological evidence for a continuous land bridge and emphasize that many native species are tolerant of saltwater, making oceanic dispersal plausible. Some researchers argue that the available evidence better supports multiple over-water dispersals rather than a single vicariance event. 6
To resolve this longstanding debate, the GAARAnti project (GAARlandia vs Petites Antilles) was initiated in 2017 as an ambitious multidisciplinary collaboration. Unlike previous studies that addressed the question through either earth sciences or life sciences separately, GAARAnti brings together geologists, marine geophysicists, biologists, paleontologists, and biogeographers in an integrated approach. 1
The project's goal is to reconcile biological and geological clocks through combined use of radiochronological methods, biostratigraphy, and phylogenetic inferences to constrain the Cenozoic paleo-biogeography of the Antillean arc. By generating a more complete picture of when and where areas emerged above sea level, researchers aim to test whether biotic dispersal events correlate with geological opportunities for migration. 1
Quantifying past emergent areas through Cenozoic times
Estimating divergence times among living and fossil mammals
Refining knowledge of geological structures in the Aves Ridge
The GAARAnti project is organized into several interconnected scientific tasks: 1
Through Cenozoic times using geological fieldwork to track histories of emergence and submergence
Among living, recently extinct, and fossil mammals from the Lesser Antilles
In the Aves Ridge and Lesser Antilles back-arc domains
Of subduction to simulate topographic variations in response to deep processes
And testing influences of abiotic versus biotic variables
| Time Period | Geological Events | Potential Biological Impact |
|---|---|---|
| Cretaceous-Paleocene (88-55 Ma) | Great Arc of the Caribbean formation | Potential early colonization routes |
| Eocene-Oligocene (40-23 Ma) | Possible GAARlandia emergence | Major vicariance event for terrestrial taxa |
| Late Eocene-Miocene (43-20 Ma) | Second arc formation east of present Lesser Antilles | Additional potential dispersal pathways |
| Pliocene-Present (5-0 Ma) | Current arc formation | Modern distribution patterns established |
Fieldwork on St. Barthélemy Island in the northern Lesser Antilles has revealed crucial geological evidence. Researchers discovered that magmatic events lasted much longer than previously estimated (from 44 to 23 million years ago), with a westward migration of tectonovolcanic activity. Careful geological mapping showed that the island emerged above ocean surface during the Oligocene (40-23 million years ago), indicating terrestrial lands in the Lesser Antilles that hadn't been previously considered part of the GAARlandia landmass. 3
Paleomagnetic studies of carbonate platforms and magmatic rocks demonstrated that St. Barthélemy underwent a counterclockwise rotation of about 15-25° after the end of the Oligocene (23 million years ago). These results helped researchers consider different tectonic scenarios explaining plate deformation in the northeastern Caribbean and the formation of the present-day trench curvature. 3
The May-June 2017 GARANTI marine cruise collected crucial geophysical data aboard the R/V L'Atalante. Researchers acquired three lines of wide-angle seismic refraction data, 3,560 kilometers of multichannel seismic reflection lines, and 12 sample dredges, along with gravity, magnetic, and bathymetry data. 3
The multichannel seismic data provided images of stratigraphy along the Aves Ridge, revealing erosional surfaces that were once exposed to air as well as tectonic structures relating the ridge flank to the Grenada Basin. These findings help reconstruct the pre-opening paleogeography and identify areas that might have been above sea level during the critical period. 3
| Research Method | Application | Key Insights Generated |
|---|---|---|
| Seismic reflection/refraction | Imaging subsurface structures | Identified erosional surfaces indicating past emergence |
| Molecular phylogenetics | Estimating species divergence times | Revealed clusters of diversification events ~34 Ma |
| Biostratigraphy | Dating fossil-bearing sediments | Established ages of oldest rodent fossils in Caribbean |
| Paleomagnetic studies | Determining tectonic rotations | Documented 15-25° counterclockwise rotation of St. Barthélemy |
| Numerical modeling | Simulating subduction dynamics | Predicted surface response to deep Earth processes |
| Research Tool/Method | Function | Application in GAARlandia Research |
|---|---|---|
| Multi-channel seismics | Imaging subsurface structures | Mapping ancient land bridges and erosion surfaces |
| Radioisotopic dating | Determining absolute ages of rocks | Establishing chronology of magmatic events and emergence |
| Phylogenetic inference | Reconstructing evolutionary relationships | Estimating divergence times between island and mainland species |
| Biostratigraphy | Dating sedimentary sequences using fossils | Correlating geological formations across regions |
| Paleomagnetic analysis | Determining past orientations of rocks | Documenting tectonic rotations and latitudinal shifts |
The GAARAnti project represents a pioneering approach to integrating earth and life sciences to address complex biogeographical questions. By reconciling biological and geological timeframes, researchers are generating major advances in understanding Cenozoic Antillean biodiversity dynamics. This work highlights how regional geodynamic evolution may have driven paleogeographic changes that are still reflected in current biology. 1 5
Beyond its scientific contributions, the project promotes preservation of both the geological and paleontological patrimony of the Antilles and disseminates results through local institutions and collaborations with schools. This outreach component helps build scientific capacity and public appreciation for the unique natural history of the Caribbean region. 1
The story of GAARlandia exemplifies how solving complex scientific mysteries often requires collaboration across disciplines. What began as a debate between biologists studying species distributions and geologists studying plate tectonics has evolved into an integrated research program that acknowledges the deep connections between Earth dynamics and biological evolution.
While questions remain about the exact extent and duration of GAARlandia, the evidence suggests this hypothesized land bridge did indeed play a significant role in shaping Caribbean biodiversity. The emerging view is that both vicariance (via land bridges) and dispersal (across water barriers) contributed to the complex biogeographic patterns observed today.
"As research continues, the Caribbean remains a natural laboratory for understanding how geological processes create opportunities for biological diversification—a testament to the dynamic nature of our planet and the life it supports."