The Isthmus of Panama is a land bridge--and also a dam that divides Atlantic from Pacific. What happened to marine life when the dam was built?
When the Isthmus of Panama emerged from the sea 3.5 million years ago, the history of life on Earth was forever and dramatically changed. North American animals migrated across the thin land bridge into South America and did well enough to supplant their southern counterparts; southern animals moved northward and, except for the opossum and a few others, died out. In the sea the situation was just the reverse: organisms that once swam between the Atlantic and Pacific oceans became permanently confined to one or the other side of the isthmus, isolated from one another. How did this affect them?
There has been vague talk over the years of a mass extinction in the Caribbean, and a vague assumption that the biological isolation of Atlantic from Pacific happened more or less suddenly. Researchers at the Smithsonian Tropical Research Institute in Panama are now replacing the vagueness with hard data. There was no mass extinction in the Caribbean, they say--or rather, none that can be laid directly at the feet of the Panamanian isthmus. And the rise of Panama began separating Atlantic species from Pacific ones well before the land bridge itself was completed.
The story of Panama begins more than 60 million years ago, when a giant tectonic plate carrying the floor of the Pacific Ocean began diving underneath the Caribbean plate to the east. As the plate sank, it melted, and that molten rock began to force its way up to the surface, through the thin crust of the Caribbean plate, emerging as a chain of spectacular volcanoes. The chain stretched from Mexico in the north, down through what is now Nicaragua, and curved out into the Atlantic. At that time a large seaway still separated the chain from the continent of South America; a strong current flowed west from the Atlantic into the Pacific, and marine life moved freely between the two oceans.
While the volcanic arc steadily grew, South America was slowly drifting toward North America. Then, 10 million years ago, South America began to collide with the volcanic arc, says geologist Anthony Coates of the Tropical Research Institute. When that happened, the arc and parts of the continental shelf of South America began to buckle. The ridge that would become an isthmus started to rise out of the sea.
By 6 million years ago, the geologic record shows, the ridge was still a few hundred feet below the sea surface, and the Atlantic and Pacific were still connected. Certain animals, however, had already felt the ridge’s influence. Nancy Knowlton, an evolutionary biologist at the Tropical Research Institute, has studied the genetic makeup of closely related species of snapping shrimp--shrimps with a claw that makes a finger-snapping sound--from either side of the isthmus. She has found that many Atlantic and Pacific shrimps had already become isolated from one another and had started to evolve into distinct species, millions of years before the seafloor ridge became an isthmus. Those species, Knowlton says, normally live in deeper water or clear water, so their path between the seas was blocked by the shallow and murky water above the ridge long before the ridge rose above the waves. The barrier that is obvious to us was to these shrimps--and probably to other marine species as well--a mere aftermath.
By 3 million years ago, the isthmus that is now Panama and Costa Rica had fully emerged, with global repercussions. The strong equatorial current that had flowed from Africa to Asia through the Panamanian seaway now veered northward, strengthening the Gulf Stream; the warm water now flowed into the North Atlantic instead of the Pacific. Since the trade winds continued to push Pacific surface water to the west, that surface water now had to be replaced by cold water welling up from the depths along the west coast of South America. The cold water was and is rich in nutrients--which is why the Pacific off South America now supports huge populations of plankton and rich fisheries. The Caribbean is warm and relatively nutrient-poor--which is why its waters are clear (there are few plankton), full of corals (they thrive in nutrient-poor environments), and also full of scuba divers (they like clear water and corals).
Another difference between the Pacific and the Caribbean today is that the Pacific is still home to a group of conspicuous snails, known as the paciphiles, that went extinct in the Caribbean sometime after the rise of the isthmus. Based on this observation and little else, paleontologists had constructed a picture of a mass extinction that had supposedly wiped out a large percentage of Caribbean mollusks. Basically, says Jeremy Jackson, a paleontologist at the Tropical Research Institute, that picture was based on crummy data.
Jackson has compiled better data. He has counted more than 800 different types of fossil snails and clams of the Caribbean, dating from the past 8 million years. He has found that rather than killing mollusks, the emergence of the isthmus triggered a 40 percent increase in the number of mollusk types in the Caribbean. The diversity increase continued until 1.5 million years ago. At that point, a spate of extinctions did wipe out one-third of the types. But those losses were replaced by new species, Jackson says, so that overall, diversity did not decrease. In any case, he says, the extinctions could not have been caused directly by the emergence of Panama, because they happened 2 million years later.
Yet the isthmus may well have led to the extinctions in some sort of complicated and indirect way that researchers haven’t figured out yet-- just as they hadn’t recognized, until Knowlton’s study, that the evolutionary impact of the isthmus began millions of years before it became land. Therein lies, as Jackson sees it, the lesson of his and Knowlton’s work, a lesson he says may be relevant to our current efforts to foresee the effects of global environmental change.
We shouldn’t expect to see a simple, direct correspondence between changes in the physical world and changes in the biological world, says Jackson. Everybody is wondering about global warming: it’s going to get a little warmer--what’s going to happen? What this research suggests is that maybe nothing is going to happen, until, all of a sudden, something big will happen. You can’t go out and measure a change in temperature and automatically expect to see a change in biology. It’s going to be much more complicated than that.