We think about the South American mammals about today as being somewhat distinct from those of North America. But almost half of the southern animals, including such familiar creatures as llamas or tapirs, actually have comparatively recent North American origin. By comparison, only a few southerners, including armadillos, porcupines, and opossums, made it through the Central American tropics and the remainder of North America.
An international team of scientists, led by Juan Carrillo of the French Museum of Natural History, has recently tackled this enigmatic asymmetry by utilizing advanced statistics and a collection of about 20,000 fossils. Their reports, now uploaded in the PNAS journal, are enlightening and surprising.
First of all, a little history of research. North and South America were divided by an ocean for much of the last 200 million years and hosted their own distinct and independently developing faunas. The Isthmus of Panama rose from the sea around 3 million years ago when tectonic plates moved and underwater volcanoes erupted. This created a land bridge that allowed animal movement in both directions: the Great American Biotic Interchange, one of evolutionary history's classic intercontinental trade instances.
Mammals on the two continents were somewhat different before the trade. Not that far from the majority of the Northern Hemisphere were North American mammals, but more distinct were the South American ones.
Several migrating animals are now extinct, such as giant soil sloths, and the settlers on both continents diversified profusely. But finally, the trade benefited the northerners, who migrated south and came to control the fauna of South America.
Why have the Southerners been faltering?
Experts used this asymmetry a century ago or so earlier to indicate "superior northerners" out-competing "inferior southerners," a relic of human colonialism's questionable past.
Two theories consider the problem to be a more remarkable migration of mammals from north to south than vice versa, either a higher dispersal rate or a wider pool of northern mammals to make the ride. Two other theories indicate different alien fates-either the northerners diversified further in the south, or on both continents, the southerners gradually experienced further extinctions.
When their latest data and methods were used by the Carrillo-led team to evaluate the numerous theories, they noticed that only one could be supported: the greater disappearance of the initial South American mammals. Yet they had a startling twist to their performance. Although scientists had historically concentrated on extinctions towards the end of the last ice age, such as the elephant-sized giant ground sloths that just went extinct 10,000 years ago, a much older cycle of extinctions was targeted by the current study, one beginning millions of years older.
For the last 12 million years or so, climate change has influenced mammals worldwide. But the phenomenon greatly affected the South American mammals. Thus, extinctions already limited the pool of accessible migrants to travel from south to north.
This latest research offers a more in-depth explanation of the Great American Biotic Interchange dynamics. It provides us a reason for the predominance in present-day South America of originally northern animals, and it helps our comprehension of ecology and populations of organisms. But it leaves us with more concerns, as any good science: why were the earlier mammal extinctions so catastrophic in South America? Hopefully, Carrillo and co. will turn their abundance of fossil evidence and computational wizardry to this interesting topic next.
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