Oxygen-rich Atmospheres of Ancient World May Have Spawned Gigantic Insects

Extract from journal "Nature" Thursday, May 11, 1995

During trips to museums as a young boy, Jeff Graham was fascinated by replicas of giant dragonflies that are believed to have inhabited the Earth 300 million years ago. Now a researcher in the Marine Biology Research Division at UCSD's Scripps Institution of Oceanography, Graham believes he and his colleagues have solved the mystery of what allowed dragonflies and other insects of the era to become so large.

Writing in the May 11 issue of the journal Nature, Graham, director of the Scripps Center for Marine Biotechnology and Biomedicine, theorizes that a jump in the oxygen concentration of the Earth's atmosphere during the late Paleozoic era, which ended about 250 million years ago, may have allowed many species of insects to attain a large size and facilitated a number of other biological changes, including the vertebrate invasion of land. The history of macroscopic life on Earth is divided into three great eras: the Paleozoic, Mesozoic and Cenozoic. Each era is then divided into periods. The latter half of the Paleozoic era, includes the Devonian period, which ended about 360 million years ago, the Carboniferous period, which ended about 280 million years ago, and the Permian period, which ended about 250 million years ago.

According to recently developed geochemical models, oxygen levels are believed to have climbed to a maximum of 35 percent and then dropped to a low of 15 percent during a 120-million-year period that ended in a mass extinction at the end of the Permian. Graham, Nancy Aguilar, a Scripps graduate student, and colleagues propose that such a jump in oxygen would have had dramatic biological consequences by enhancing diffusion-dependent processes such as respiration, allowing insects such as dragonflies, centipedes, scorpions and spiders to grow to very large sizes. Fossil records indicate, for example, that one species of dragonfly had a wing span of 2 1/2 feet. Insects are extremely dependent on diffusion for respiration because they do not use their circulatory system for oxygen transport. Instead, they rely on a tracheal gas-exchange system consisting of a branching network of tubes that extend from the body surface to small tracheoles within respiring cells.

"In order for insects to support flight, which requires a tremendous amount of energy, they probably would have had to live in an atmosphere where there was more oxygen present," Graham said. Indeed, Graham argues that the greater density of the oxygen-rich atmosphere during the late Paleozoic era could have also facilitated the evolution of insect flight by increasing the lift achieved with the earliest proto-wings, which may have originally served a respiratory function in addition to a means of locomotion and predator escape.

Graham and colleagues also propose that an increase in atmospheric oxygen could have assisted in the invasion of land by the first terrestrial vertebrates. "To go from water to land is a tremendous change because suddenly an animal has to bear its own weight," Graham said. "When you think about the oxygen requirements for an animal that goes from basically floating or sitting still to one that is walking on land, it represents an incredible increase. And we think that this enhanced oxygen atmosphere would be a crucial factor in this evolutionary step." Because respiration in plants is also dependent upon diffusion, high levels of oxygen in the atmosphere would have enabled plants to achieve considerable height and many of the Carboniferous plants were tree-like. In addition to allowing diffusion through thicker support elements, an atmosphere rich in oxygen would have enhanced the oxygen-dependent biosynthesis of lignin, a structural material of many plants during the Carboniferous period.

"We know there were some giant tree-like lycopods at this time," Graham said. "Some of them got to be 90 feet tall. The lycopods (horsetails) living today are quite small." Increased oxygen concentrations in the ocean and in fresh waters and a greater oxygen penetration into sediments may have allowed a number of aquatic species to flourish, Graham said. Because the majority of marine invertebrates lack well-developed respiratory systems, a higher concentration of atmospheric oxygen also would have enabled them to increase in body size. The largest known brachiopod, Gigantoproductus, for example, which lived in the early Carboniferous period, had a shell about one-foot wide. Geochemical models indicate that near the close of the Paleozoic era, during the Permian period, global atmospheric oxygen levels dropped to about 15 percent, lower that the current atmospheric level of 21 percent. The Permian period is marked by one of the greatest extinctions of both land and aquatic animals, including the giant dragonflies. But Graham and colleagues do not believe the drop in oxygen played a significant role in causing the extinction. "There were many factors involved in the Permian extinction and oxygen may have been one of those factors, but it was not the major factor," Graham said. "There were lots of other things happening -- the continental shapes were changing, there was volcanism and severe changes in local weather patterns, including rainfall and temperature." Some creatures that became specially adapted to living in an oxygen-rich environment, such as the large flying insects and other giant arthropods, however, may have been unable to survive when the oxygen atmosphere underwent dramatic change, he said. But many adaptations that occurred as a result of changes in oxygen were not lost.

"We view the oxygen increase as enabling special kinds of evolution to occur and some of those changes have stayed with us," Graham said. "For example, insect flight, the invasion of the terrestrial environment, and the establishment of life on land. We think all those things were helped by an increase in oxygen, but, obviously, when the oxygen levels fell, the animals sustained themselves." The marked evolutionary changes occurring in the Carboniferous-Permian vertebrates may have been brought about by variations in atmospheric oxygen and carbon dioxide, he said.

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