Passage IV

NATURAL SCIENCE: This passage is adapted from Stephen Jay Gould's The Panda's Thumb (©1980 by Stephen Jay Gould).

A population of the green turtle, Chelonia mydas, nests and breeds on the small and isolated central Atlantic island of Ascension. London soup chefs and victualing ships of Her Majesty's Navy found and exploited these turtles long ago. But they did not suspect, as Archie Carr discovered by tagging animals at Ascension and recovering them later at their feeding grounds, that Chelonia travels 2,000 miles from the Coast of Brazil to, breed on this "pinpoint of land hundreds of miles from other shores," this "barely exposed spire in mid-ocean".

Turtles feed and breed on separate grounds for good reasons. They feed on sea grasses in protected, shallow-water pastures, but breed on exposed shore where sandy beaches develop—preferably, on islands here predators are rare. But why travel 2,000 miles to the middle of an ocean when other, apparently appropriate breeding grounds are so much nearer? (Another large population of the same species breeds on the Caribbean coast of Costa Rica.)

Perhaps, Carr reasoned, this odyssey is a peculiar extension of something much more sensible, a journey to an island in the middle of the Atlantic, when the Atlantic was little more than a puddle between two continents recently separated. South America and Africa parted company some 80 million years ago, when ancestors of the genus Chelonia were already present in the area. Ascension is an island associated with the Mid-Atlantic Ridge, a linear belt where new sea floor wells up from the earth's interior. This upwelling material often piles itself high enough to form islands.

Iceland is the largest modern island formed by the Mid-Atlantic Ridge; Ascension is a smaller version of the same process. After islands form on one side of a ridge, they are pushed away by new material welling up and spreading out. Thus, islands tend to be older as we move farther and farther from a ridge. But they also tend to get smaller and finally to erode away into underwater seamounts, for their supply of new material dries up once they drift away from an active ridge. Unless preserved and built up by a shield of coral or other organisms, islands will eventually be eroded below sea level by waves.

Carr therefore proposed that the ancestors of Ascension green turtles swam a short distance from Brazil to a "proto-Ascension" on the late Cretaceous Mid-Atlantic Ridge. As this island moved out and sank, a new one formed at the ridge and the turtles ventured a bit farther. This process continued until turtles found themselves locked into a 2,000-mile journey. The hatchlings float to Brazil on the Equatorial Current, but how do they get back? Carr supposes that they begin their journey by celestial cues and finally home in by remembering the character (taste? smell?) of Ascension water when they detect the island's wake.

Carr's hypothesis is an excellent example of using the peculiar to reconstruct history. I wish I could believe it. I am not troubled by the empirical difficulties, for these do not render the theory implausible. Can we be confident, for example, that a new island always arose in time to replace an old one—for the absence of an island for even one generation would disrupt the system. And would the new islands always arise sufficiently "on course" to be found? Ascension itself is less than seven million years old.

I am more bothered by a theoretical difficulty. If the entire species Chelonia mydas migrated to Ascension I would have no objection, for behavior can be as ancient and as heritable as form. But C. mydas lives and breeds throughout the world. The Ascension turtles represent only one among many breeding populations. Although its ancient ancestors may have lived in the Atlantic puddle 200 million years ago, our record of the genus Chelonia does not extend back beyond fifteen million years. In Carrs scheme, the turtles that made the first trips to proto-Ascension were rather distant ancestors of C. mydas (in a different genus at least). Several events of speciation separate this Cretaceous ancestor from the modern green turtle. Now consider what must have happened if Carr is right. The ancestral species must have been divided into several breeding populations, only one of which went to proto-Ascension. This species then evolved to another and another through however many evolutionary steps separated it from C. mydas. At each step, the Ascension population kept its integrity, changing in lock step with other separate populations from species to species.