If you enjoy leaves turning, pumpkin patches and other rites of autumn, thank the giant impact event early in Earth's history which knocked our planet's axis off kilter, creating the seasons we that know and love today. But was there just one planet-tilting impact?

University of Western Ontario geologist Grant Young thinks there were two, separated by billions of years. He has been studying rocks from the Ediacaran Period, 540 to 635 million years ago and says he sees signs of massive changes to Earth's seasons and climate that can best be explained by a axis-shifting collision of a small planetary body into the ocean about 570 million years ago.

That's long after the famous smash up with a Mars-sized body that is credited with creating the Moon around 4 billion years ago and giving Earth its mild tilt and modern seasons. It was also at the time Earth was seeing the earliest animals, or metazoans, come into being.

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“I think this might have stimulated the evolution of metazoans,” Young told Discovery News.

The scenario he has presented in the October issue of GSA Today is that there was first the collision that created the Moon. But instead of that giving Earth its current tilt (which wobbles a bit, and is currently at 23.5 degrees), that first event knocked the planet over almost on its side. That orientation would give the poles a temperate climate without nights for half the year and the equator much less sunshine all year round.

This could also lead to equatorial glaciers, the deposits of which have been found and attributed instead to a Snowball Earth – an ice-locked planet – rather than any shift in the planet's axis. Then a relatively recently, at 570 million years ago, a second and smaller collision almost righted the Earth and gave us the seasons we now have.

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The evidence Young cites includes what's called the Shuram isotopic anomaly which is found in 580-million-year-old rocks throughout the world. The anomaly is the largest of its kind, signaling a massive worldwide change in the atmosphere and oceans that would seem to be tied to the advent of animal life. But the jury is still out on what it all means.

“There are a whole lot of things that have defied explanation,” said Young. “I think that one possibly is that there was an impact at that time. This would have a tremendous effect.”

Young details his arguments in his paper. But so far some of the more prominent impact experts aren't biting.

“I don't believe at all that we had any 'recent' impacts (over the last couple of billions of years) that were capable of altering the Earth's axis,” said Christian Koeberl who is an authority on early impacts and is now general director the Natural History Museum of Vienna, Austria, and professor at the University of Vienna. “This requires a huge impact event, far beyond anything that would be documented.”

It's doubtful Snowball Earth advocates will favor the hypothesis either. None have responded to inquiries from Discovery News about Young's paper.