Fossils of single-celled organisms have been recovered from rocks as old as 3.5 billion years. Animal fossils first appear in rocks dating back about 1 billion years. The occurrence of fossils in unusual places, such as dinosaur fossils in Antarctica and fish fossils on the Siberian steppes, is due to the shifting of the plates that make up Earth's crust and environmental changes (such as ice ages) over time. The best explanation for dinosaurs on Antarctica is not that they evolved there, but that Antarctica was once part of a much larger landmass with which it shared many lifeforms.