Four-and-a-half billion years ago, an asteroid slammed into earth, knocking it onto its side and touching off a crazy climate that nurtured the first living things but later triggered giant ice sheets in the tropics.
Or so suggests Gregory Jenkins, a meteorologist at Pennsylvania State University. In Earth's infancy, 3.8 million to 2.5 billion years ago, its surface was warm enough for life even though the young sun was much fainter than it is today. Jenkins found he could explain the balmy temperatures if Earth had been rotating sideways at the time. Recently, geologists have found evidence of glaciers at Earth's equator between 800 million and 540 million years ago. When Jenkins modeled what the climate would have been like during that time if Earth's axis were tilted at a steep, 70-degree angle, ice appeared at the equator.
According to the theory, Earth remained off kilter until around the beginning of the Cambrian Period, 540 million years ago, when a buildup of continents near the south pole flipped our planet to its present 23-degree tilt. The switch to a less extreme climate could explain the remarkable diversification of life at that time. "This is radically different than what most of us think about Earth," Jenkins says.