Other than supercontinent formations, North and South America were unconnected until several million years ago; then a tiny strip of land joined the continents, causing impacts that affected the entire planet. This small stretch of land, known as the Isthmus of Panama, transformed the global climate and launched a great natural experiment in plant and animal migration.
But exactly how many million years ago did the Americas join? The “standard model” dates to about 3 million years ago, while some more recent studies say it is between 6 and 15 million years old, with “an initial land bridge” from 23 million years ago, according to a 2016 report. of the magazine. Progress of science (opens in a new tab).
‘Still controversial’ Camilo Montes (opens in a new tab)a structural geologist at the Universidad de los Andes in Bogotá, Colombia, told LiveScienceKick.com.
Related: How do we know how old the Earth is?
Isthmus of Panama emerges on the map
The Panamanian land bridge arose from tectonic forces. Large chunks of crust called tectonic plates cover Earth’s outer shell, crashing together to form mountain ranges and pulling apart to create ocean basins. The isthmus formed when the Caribbean tectonic plate wedged between the plates carrying the Americas, Montes said. The resulting tectonic activity raised the seafloor while creating volcanoes that broke the ocean’s surface like islands, according to NASA Earth Observatory (opens in a new tab). Over millions of years, sediment from ocean currents filled the spaces between the islands, until the land bridge as we know it was formed. This seemingly small land bridge isolated the North Atlantic Ocean from the North Pacific Ocean, changing ocean currents.
The resulting isthmus proved transformative, forcing a warm equatorial current from the Gulf of Mexico out into the Atlantic and onto the North American coast as the Gulf Stream. Other current-related changes made the Atlantic saltier, driving the global thermohaline (Greek for “heat” plus “salt”) circulation, according to the Science Advances study.
“The reason we have the weather we have today is because the isthmus rose,” Carlos Jaramillo (opens in a new tab), a staff scientist at the Smithsonian Tropical Research Institute, told Live Science. Just 30 to 120 miles (50 to 200 kilometers) wide and around 400 miles (640 km) long, “this little, little piece of land affected the climate of the entire planet.”
The isthmus also provided a very important passage for living things from one continent to another. The resulting Great American Biotic Interchange (GABI) was “a massive biological invasion experiment,” according to a 2020 study published in the journal PNAS (opens in a new tab) by Jaramillo and colleagues.
Existing North American creatures such as the Virginia opossum (Didelphis virginiana) and porcupines arose from South American migrants, while “ancestors of bears, cats, dogs, horses, llamas, and raccoons” traveled the other way, according to the Earth Observatory. Meanwhile, the isthmus isolated previously mixed oceanic populations that later evolved independently.
The standard model originated from research in the 1970s, according to the Science Advances study. That includes a 1978 study in the journal geology (opens in a new tab) who examined deep-sea core fossils; discovered that marine species became isolated on both sides of the isthmus about 3 million years ago. Over the next several decades, subsequent studies reaffirmed this timeline, according to the Science Advances report.
Traditional GABI dating also supported that estimate, according to a 2013 study in the Marine Science Newsletter (opens in a new tab) diary. The scientists mainly used data from animal fossils to establish the timing of GABI, according to a 2010 study in the ultrasound (opens in a new tab) diary. Such fossils were best dated by their location in rock layers, according to a 2008 review in the journal. Paleoceanography and Paleoclimatology (opens in a new tab).
The scientists also linked the closure of the isthmus to the time of ancient global cooling. The researchers noted that the formation of the ice sheet and related cooling coincided with the time the isthmus was thought to have formed (judging from fossil evidence), based on the 2008 review. So they hypothesized that that this cooling was related to changes in currents caused by the isthmus. For example, some researchers said that the Gulf Stream, by bringing more moisture to the Arctic, generated more sea ice, which would reflect more sunlight and cause cooling, according to Woods Hole Oceanographic Institution (opens in a new tab).
Challenges to date of the Isthmus
A 2015 publication by Montes and colleagues in the journal Science (opens in a new tab) challenged that traditional view. crystals called zircons traveled in rivers from present-day Panama to Colombia 15 to 13 million years ago, revealing a terrestrial connection, the study found. Subsequently, a 2015 study of fossil and DNA evidence in the journal PNAS (opens in a new tab) found “significant waves” of terrestrial organisms crossing the continents between 20 million and 6 million years ago.
Soon after, however, the 2016 Science Advances study dismissed those challenges through a “comprehensive review and reanalysis.” He found a mix of genes from surface water and marine animals from both sides of the isthmus until about 3 million years ago. Furthermore, he calculated that the mass migrations of land mammals from GABI would begin around that point.
However, more recent research has continued to challenge the standard model. A review of Jaramillo in the book “Mountains, Climate and Biodiversity (opens in a new tab)(Wiley-Blackwell, 2018) further argues for a broader GABI timeline and challenges the links between the closure of the isthmus approximately 3 million years ago and the thermohaline circulation. Some recent geological finds support an older link, Jaramillo said.
An earlier shutdown could have major consequences for several fields. I would suggest that scientists need a new explanation for the global cooling 3 million years ago. It has also spurred some biologists to re-examine the “molecular clocks” used to estimate evolutionary timelines, and often calibrated to the birth of the isthmus. “They realized that their… phylogenies [branching diagrams of evolutionary relationships] it could be very different and much older,” Montes said.