Paleobiologist Geerat Vermeij is enthralled with mollusks. Their shells line the surfaces and fill the cabinets and drawers in his office on the second floor of the Earth and Planetary Sciences Building at UC Davis. But Vermeij's deep study of these organisms isn't just about the animals themselves, it's an avenue for deeper insights about the principles governing evolution and history.
"There really are principles emanating from biology that give history a direction, and a predictable direction," said Vermeij, a Distinguished Professor in the Department of Earth and Planetary Sciences. "It really emanates from profound principles, which are natural selection, on the one hand, but also, and I think most misunderstood and important, agency. The fact that organisms do things, and in doing so, they also modify their environment. There's fundamental feedback going on between organisms and environment."
In a new paper appearing in the Proceedings of the National Academy of Sciences, Vermeij and his research colleague Tracy Thomson catalogued the features of various mollusks in the fossil record and found that early mollusks evolved a unique physical trait once every 2 million years. That frequency began declining roughly 444 million years ago to about one new feature every 9 million years.
"All these unique traits and first occurrences of repeated traits occurred within the first 96 million years of molluscan history," Vermeij said. "Then the frequency goes down by a factor of at least four or five."
The research shows that the evolution of mollusks has become increasingly predictable over its 540-million-year history despite the increasing diversity of mollusk species.
Counting unique physical traits
In their study, Vermeij and Thomson identified 96 unique physical traits found in mollusks over the course of their evolution. Such traits included the spiral coiling of mollusk shells; the radula, which are tiny teeth-like structures mollusks use to eat; and the ventral foot, the muscular sole that mollusks use for movement.
Of those 96 traits, 46 (or 48%) originated during the first 96 million years of their evolution. The leftover 50 (or 52%) evolved in the succeeding 444 million years.
"They're evolving structures, which we always have to remember were successful when they evolved, because we wouldn't see them otherwise," Vermeij said. "And then many of these traits, of course, evolved again and again, modified, perhaps, from the original, but nevertheless the same thing."
The researchers noted that secondary peaks in the frequency of the development of unique traits occurred following mass extinction events during the Triassic and Cenozoic periods. They hypothesized that the environmental constraints following an extinction event could lead to conditions favorable to the evolution of norm-breaking physical traits.
An evolutionary insight into bigger questions
According to Vermeij, evolutionary innovations are the result of feedback loops created between organisms and their environments.
"The environment itself, thanks to the collective work of other organisms, is making many of these adaptive features that would have been mostly potential into reality," he said. "The system allows it to happen."
While the research paper is specific to mollusks, Vermeij said the idea that phenotypes, or observable characteristics, become more predictable as time goes on can be applied to the analysis of human inventions, social structures and scientific discoveries.
"Almost everything in our nonquantum universe has a beginning," Vermeij said. "But at the very beginning, there isn't a trend, which means that in the earliest stages, events are unique. Whereas later on, that's not the case because things just get repeated over and over again."
