Mammal species do not seem to last very long in the grand scheme of things, persisting for an average of 2.5 million years, according to the fossil record. By studying the fossilized teeth of rodents over a span of 22 million years, Jan van Dam of Utrecht University in the Netherlands and his colleagues confirmed this cycle of rodent species rise and fall. But they also found that it closely matched variations in Earth's orbit--and may have definitively linked the two. "If you have seven or eight of these in a row all showing the same pattern, it becomes extremely unlikely that it can be reduced to chance," explains paleoecologist Paul Olsen of Columbia University.

By looking at more than 80,000 teeth personally collected or published in others work, van Dam and his colleagues were able to create a database of 132 rodent species that lived on the banks of ancient rivers, streams and lakes in what would become Spain. They compiled a timeline, checking in every 100,000 years, to determine when these species arose and died out as well as when the greatest number flourished. Species that disappeared for more than one million years were counted as extinct. Analyzing this timeline, they found that the birth and death of these species closely overlapped in time and clustered around specific cycles at one million and 2.4 million years.

These cycles of origination and extinction correlate closely with known astronomical cycles: a 2.37-million-year cycle between an elliptical and circular orbit and a 970,000-year change in the tilt of Earth's axis. The two combine to create periods in which our planet did not tilt very much as it revolved around the sun, thereby eliminating seasons and resulting in less climatic variability. "The lack of warm high-latitude summers--normally associated with extreme eccentricity and obliquity--results in ice expansion on Antarctica, and later the high northern latitudes as well, which will affect the rest of the world's climate resulting in cooling and changes in precipitation patterns," van Dam says. "These climate changes may destroy the habitat of rodents or give new rodent forms a chance."

The paper presenting this finding, published in Nature on October 12, is the first detailed evidence of such a linkage, although it has been suspected in the past. The fossil record for other types of animals is more difficult to work with; for example, lizards lack the clear differences in tooth structure that help delineate rodent species, and insects notoriously fail to fossilize regularly. "We're left with just a few groups," Olsen says. "My own research has focused on fish and fossil pollen spores, and they show exactly these kinds of patterns." Van Dam and his team plan to expand their search into other animal groups to look for the same patterns. "Paleoceanographers found indications for climate change--cooling--at these types of configurations in deep sea cores," he notes.

At present, Earth is in a period of axial tilt and an elliptical orbit, a recipe for climate shifts but relatively stable mammal populations. But such periods in the past have not included such high levels of greenhouse gases in the atmosphere, and that is driving an unprecedented--an unpredictable--climate change. "There is no precedent for it in any of the other previous cycles that are well documented," Olsen adds. That may shorten the life span of mammal species presently on the planet.