Last night I attended a lecture at the Harvard Museum of Natural History on “From Supercontinents to Islands—Evolution in Motion.” It amazes me that a supercontinent like Gondwana existed as recently as about 200 million years ago. That’s a small fraction of earth’s history—less than 5%--and it means that plate tectonics are awfully fast. At least in geological timeframes.
The speaker, Gonzalo Giribet, showed a slide of the age of the ocean floor. Very little was between 200 and 400 million years old and virtually nothing older than that. Almost all of it is younger than 180 million years. In contrast, continental surfaces feature rocks ranging all the way close to the 4.65By age of the earth. That’s amazing.
The islands he was discussing were those that had formed by breaking away from the big continents during the split up of Gondwana into the current continents. For example, New Zealand, Madagascar, Sri Lanka, and many parts of Southeast Asia have been identified as having once been part of Gondwana and then were isolated. By tracing the evolutionary history of species on these islands, information can be inferred of the time and history of that isolation.
One controversy, I learned, is whether or not New Zealand was entirely submerged under water after its isolation. Indications are that it may have been submerged about 25 million years ago. The author showed evidence that he thinks is increasing in the last few years that the island was not entirely submerged after all. This stems from the diversity of species of invertebrates. He really loves those daddy long-legs.
Giribet has also spent a good deal of time in Antarctica, deep sea diving in those icy waters to retrieve countless specimens of invertebrates. I learned that part of what influenced Antarctica to have a very cold climate after a relatively warm climate was that its isolation from Gondwana opened up a circumpolar ocean current that in effect isolated the island and became a refrigerator. What an amazing history.