The emergence and disappearance of species of mammals could be due to wobbles in the Earth's orbit, suggests a new study.
Surveying 22 million years of rodent fossil records, researchers found that peak species turnover corresponded to variations in the shape of the planet's orbit around the Sun, which oscillates between a circle and an ellipse.
"The lack of warm high-latitude summers (normally associated with extreme eccentricity and obliquity) results in ice expansion on Antarctica and later the northern high latitudes as well, which affects the rest of world's climate resulting in cooling and changes in precipitation patterns," lead author Jan A. van Dam, a researcher from Utrecht University, the Netherlands, told LiveScience in an e-mail interview. "In turn, these climate changes may destroy the habitats of the rodents, or give new rodent forms a chance."
Van Dam and colleagues collected rodent teeth from sediments deposited at lake margins and in small streams during a period of 22 million years in Spain. They combined their results with other published data which provided them with records on 132 species of rodents.
"We looked at rodents because small mammals have a higher density of fossils than large mammals, and we concentrated on Spain, because the sediment series are very complete, contain many mammal fossils, and because much work has been done already," van Dam said.
The researchers found that the disappearances of these species were not random and that they corresponded to astronomical cycles, a lengthier cycle that peaks nearly every 2.5 million years and a shorter one that peaks every 1 million years.
The Earth's orbit is closer to a perfect circle nearly every 2.5 million years, whereas the 1-million-year peaks come around when the Earth is shifting its degree of tilt on its axis. Both of these cycles result in global cooling and ice expansions which could explain why mammals survive for an average of 2.5 million years before they disappear, the researchers note in the October 12 issue of the journal Nature.
The pattern of disappearance for the rodents could also explain other mammalian loss.
"Although rodents are especially sensitive to seasonal changes, we suspect that larger mammals are also affected," van Dam said. "Their mean species life span is not much different than that of rodents (in the order of 2.5 million years)."
