July 7, 2009

Study Sheds Light On Earth’s Evolutionary History

Nearly everyone can recall the high school textbook illustrations of the planet's first land-dwelling creatures, ubiquitously represented as comic-looking fish with short, stumpy legs.  A team of paleontologists, however, are challenging these standard depictions, saying that the earth's first tetrapods were for more diverse than previously suspected.

"Some looked like crocodiles, some looked like little lizards, some like moray eels, and some were snake-like," explained Jennifer Clack of the University of Cambridge, who is spearheading the effort to set the evolutionary record straight.

Eons before mammals, birds or even dinosaurs would make their debut on Earth's evolutionary stage, there were rich and diverse communities of these four-legged creatures filling a variety of ecologically distinct niches.

"They occupied all sorts of niches and habitats. And they varied tremendously in size "” from about 10 cm long to 5 meters."

Clack, who has studied the fossil record of these creatures for over twenty years, recently teamed up with prominent ichthyologists, or fish biologists, Charles Kimmel of the University of Oregon and Brian Sidlauskas of the National Evolutionary Synthesis Center in North Carolina. 

Together, the team set out to understand the anatomical modifications that allowed fish to first crawl out of the world's oceans and achieve such remarkable physical diversity.  By focusing their research on 35 different species of tetrapods that crawled around the coastal regions between 385 and 275 million years and tracing changes in the dimensions of interlocking bones in their skulls, the scientists hope to get a clearer picture of their skeletal evolution as a whole.  Clack's team believes that the evolution of the skull, particularly a region of the skull known as the palate, often serves as a bellwether and reference point for the shape and size of other organs in an organism's evolutionary history.

"I tend to think the genetic instructions for making a skeleton come from how you make individual bones first, and then how you fit those bones together as a refinement of that," said developmental biologist Charles Kimmel, who co-authored the group's report.

When the team began constructing a phylogenetic tree for the organisms, tracing out their evolutionary lineage by examining changes in certain body features, they found that the size and rate of change in different bones was by not always coordinated.  According to Sidlauskas, the disproportionate growth of one or several bones in relation to others leads to an eventual reshaping of body structure from one lineage to the next.

"Sometimes a change in size can have indirect consequences for the shape of the animal," he explained. "When different parts of an animal's body change size at different rates over evolutionary time, that can generate changes in body shape from one species to another."

Even more fascinating, several of the changes in body structure observed by the group proved consistent with an evolutionary phenomenon known as paedomorphosis, in which adult organisms begin to retain features that were once characteristic only of juveniles.

"Paedomorphosis is definitely there," said Clack.  "[T]he descendents of some groups are retaining the proportions that their juveniles had in the past."

The scientists explained that the results of their study are a significant puzzle piece in understanding how tetrapods were able to develop such astonishing physical diversity in a relatively short period of time.  They also shed light on one of the major phases in the evolutionary history of life on Earth"”the transformation for fish to amphibians.

"One of the big questions at the moment is: where did modern amphibians come from?" said Clack. "One of the hypotheses is that they have evolved by paedomorphosis and miniaturization from early tetrapods. This study lends weight to that idea."

The team's findings will appear in the July 16 2009 online issue of the Journal of Anatomy.


Image 1: This is a photograph of a museum reconstruction of Acanthostega, an early tetrapod. Acanthostega measured about 2 feet (0.6m) in length. Credit: Photo courtesy of Jennifer Clack

Image 2: This is an artist's depiction of the tree-of-life for early tetrapods, showing 100 million years of palate evolution and diversification. The outer edges of the diagram represent the diversity of palate size and shape. Artwork by Brian Sidlauskas. Credit: Artwork by Brian Sidlauskas


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