"There seems to have been a dramatic improvement in the habitability of the Earth at around the Precambrian–Phanerozoic boundary," Williams said. "I have suggested that reduction of obliquity was the main cause of this major change in habitability." Models of the Earth's climate with a high obliquity by atmospheric physicist Gregory Jenkins at Howard University buttress this idea.
Of course, many other explanations have been offered for the Cambrian explosion, though each has its drawbacks. Ideas include a greater concentration of atmospheric oxygen or calcium or phosphorous in seawater, or even the evolution of eyes jumpstarting biodiversity.
Williams' hypothesis has its own big gap: a mechanism that could have clipped the planet's tilt by about 30 degrees in 100 million years prior to the Earth's oldest confirmed circumpolar glaciation. Research into the history of tectonic processes within the Earth and gravitational interaction with the moon may illuminate the matter.
A "Goldilocks" obliquity?
At this point in exoplanetary research, very little is known about the characteristics of most alien worlds beyond their size, mass and orbital period. Discerning axial tilts and the effect they have on planetary habitability will be an important aspect of the search for alien life in the decades ahead.
It could turn out that Earth's obliquity of 23.5 degrees, like its orbital distance from the Sun, is a "Goldilocks" figure for seasonality – not too extreme in either direction – and therefore ideal for complex life.
"Obliquities of bodies in the solar system have been studied extensively," said Heller. "But with exoplanets we are entering new territory."
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