Heat over ice
The theory that polar regions once experienced summers while the tropics froze under ice has recently been challenged by Darren Williams of Pennsylvania State University in usa. According to him, many organisms which survived this period would have died off if this had happened. Instead, Williams believes that the Earth's polar axis once tilted down towards the sun at an angle 54 degrees or more. Polar regions would then have received more solar energy than the equator, and the glaciers could have grown in the tropics, while the poles would have stayed ice-free.