Gondwana genesis: a combination of molecular data, anatomical evidence, and knowledge of ancient geography is providing new answers to the contentious issue of when—and where—modern birds arose

Natural History, Dec, 2001 by Joel Cracraft

About 65 million years ago a giant asteroid, perhaps six miles in diameter, tore through Earth's atmosphere and slammed into the Gulf of Mexico, carving a crater nearly a hundred miles in diameter. The impact released billions of megatons of energy. Smoke and dust from the ensuing firestorms cast a sooty blanket over much of Earth, blocking sunlight and widely disrupting terrestrial and marine ecosystems. With this cataclysm came the mass extinction event that marks the boundary between the Cretaceous and Tertiary Periods--the K-T, as it is called by geologists.

The K-T mass extinction has captured our imagination because of claims that it brought about the end of the dinosaurs. Yet as most people know, the descendants of dinosaurs--birds--survived and are with us today. The common ancestor of modern birds, which are known technically as Neornithes, and of bipedal maniraptoran dinosaurs (think Velociraptor) lived sometime during the Jurassic, more than 150 million years ago. Many lineages of premodern birds subsequently branched off the avian tree, including one of the most famous and important fossils, Archaeopteryx, recovered from the Upper Jurassic Solenhofen limestone quarries of Bavaria. After Archaeopteryx, numerous Cretaceous forms evolved. These were less advanced anatomically than modern birds, and all are now extinct. As far as we know, none survived the K-T asteroid event.

Although most of the attention given to bird evolution has focused on birds' relationships to dinosaurs, crucial questions about the origin and early evolution of modern birds have largely gone unanswered. When and where did they arise? How and when did they diversify and spread around the world? And what influence, if any, did the K-T mass extinction have on their history?

In recent years, ornithologists have sought to investigate these questions using very different kinds of evidence. Taking a traditional approach, some paleontologists point to the virtual absence of modern birds in the Late Cretaceous and their abundance in the Early Tertiary in North America and Europe. From this they conclude that the evolutionary history of modern birds began in the Northern Hemisphere after the K-T extinction event.

A contrasting view of modern bird beginnings comes from the expanding science of molecular systematics, which uses comparative DNA data to illuminate genealogical relationships. By correlating the amount of genetic difference between pairs of species with their presumed time of divergence from each other based on fossil evidence, some scientists believe they can calibrate a molecular "clock." The clock allows them to date the divergence of species pairs having no fossil record: the greater the difference in DNA sequence between two organisms, the farther back in time their evolutionary separation is thought to have occurred.

Several studies have independently examined sequence differences for a handful of genes in a small number of bird groups. The various investigators calibrated their clocks using different fossils and somewhat different methods of analysis, yet each concluded that these groups are so different genetically that they must have diverged sometime in the Cretaceous--that is, before the asteroid hit. According to this view, the pattern of the fossil record is biased: the large Tertiary record of birds exists only because sediments of that age are abundant compared with those of the Cretaceous, and the fossils are found mostly in the Northern Hemisphere because that's where paleontologists do most of their work.

Such scientific debates are often frustrating because they are unlikely to be resolved easily. Each side is relying primarily on one kind of evidence (here, either fossils or molecules), and too much time is spent dismissing the views of the opposing side. One way out of the conundrum is to obtain yet another kind of evidence and see how it fits together with the other data.

Research in my laboratory at the American Museum of Natural History has done just that. Employing new data--both molecular and anatomical--we have attempted to reconstruct the genealogy of modern birds and to use those findings to understand their past geographical distributions. This method of mapping reveals that many groups of modern birds were widely distributed on the ancient Southern Hemisphere supercontinent known as Gondwana and that they attained their present-day distributions as that landmass rifted and drifted during the Cretaceous and Early Tertiary. Combining knowledge about the breakup of Earth's early landmasses with our understanding of avian relationships and the fossil record, we have concluded that many lineages of modern birds existed prior to, and therefore survived, the K-T mass extinction event, thus supporting the conclusion derived from molecular clocks.

To understand this story, it helps to envision how Earth's landmasses fit together in the Early to mid-Cretaceous, about 120 to 100 million years ago. North America, Europe, and Asia formed a large supercontinent called Laurasia. Far to the south, Gondwana was beginning to separate into the continents we recognize today. The South Atlantic Ocean was forming and was continuous with another newly created ocean that extended eastward around the southern and eastern coasts of Africa. Yet Africa and South America maintained a connection across their midsections and probably did so until about 100 to 105 million years ago.