Scientists think they may have solved the puzzle of the mysterious disappearance and re-emergence of certain kinds of mammals every few million years. Paleontologists studying prehistoric fossil data think it has to do with the Earth wobbling on its axis during its orbit around the sun.

Paleontologists at the University of Utrecht in the Netherlands, who studied 80,000 rodent teeth representing 22 million years worth of fossil data in Spain, made an interesting observation: small rodent species, including mice and rats, seemed to disappear and reappear with regularity as Earth's orbit around the sun changed.

Earth

The mammalian turnover, according to expert Jan van Dam, occurred in cycles of 2.5 million years and one million years. The turnover appeared to correspond to adjustments in the Earth's solar orbit.

Scientists say the 2.5-million-year cycle relates to the Earth's orbit around the sun. The one-million year phase relates to wobbles as the Earth tilts on its axis, affecting the amount of warmth parts of the Earth get from the sun.

Van Dam, who led a team of Dutch and Spanish researchers, says the result was an ice sheet expansion and global cooling at the height of each cycle, which may explain why the fossil record uncovered in Spain reveals the extinction and emergence of heartier rodents every 2.5 million years.

Van Dam says it is likely larger mammals had a similar fate. Van Dam says it is likely cosmic changes had a similar effect on non-mammalian species.

"All terrestrial animals and even plants are affected strongly by climate, and once climate gets extreme, all organisms will have to cope with more extreme climates, which occur at specific points in time," said Jan van Dam.

Van Dam made his comments in an interview with the journal, Nature, which published the research.