Thanks to the Moon, Earth precesses every 26,000 years, much more rapidly than any of the rhythms in which its orbit wobbles -- and so it doesn't undergo any of the rhythmic resonance effects that would drastically change the amount of its spin-axis tilt.

Its tilt does change back and forth over a 2.5-degree arc every 41,000 years -- and even that slight change is probably enough to cause the Ice Ages.

Laskar calculated that, without the existence of the Moon, Earth's obliquity would slew even more wildly than Mars' -- its axial tilt would slew back and forth between 0 degrees and 85 degrees every few tens of millions of years!

It is now thought that there was about a one-in-three chance that Earth, during the Solar System's formation, would undergo a collision that would form a large moon.

But if that Moon-creating collision had happened at another time, when Earth's axis was keeled over more extremely, then the Moon's tuggings would have permanently stabilized the Earth at that more extreme tilt. So there is only about a one-in-twelve chance that Earth would permanently have a tilt as mild as it actually does.

Axial Heat
What would have happened if Earth had an extreme spin-axis tilt relative to the Sun? Very strange -- and very bad -- things. Twice during each orbit, it would have been side-on to the Sun just as it now is -- but at other points during each year, either the North or South Poles would have pointed straight toward the Sun.

It's been known for a long time that this would do utterly grotesque things to its climate. In 1997, climatologist James Kasting carried out detailed computer analyses and discovered just how bad it could get.

If Earth were tilted 85 degrees today, each of its hemispheres would be permanently shrouded in night for six months at a time -- but the other pole would undergo a six-month-long day, during most of which the Sun would be blazing down on it from as high an angle as it blazes down on our own tropics for the few hours around noon each day.

The natural result would be that the temperature at that pole would climb to very high levels. The temperature at the North Pole might climb as high as 50 deg C (over 120 deg Fahrenheit). And because the South Pole is located in the middle of Antarctica, away from the temperature-moderating effects of the ocean, its temperature could climb as high as 80 deg C (176 deg F).

If Earth's continents were all lumped together into a single big continent (as may very well be the case on many other Earth-type worlds), and that continent were centered around one of the poles, its temperatures inland would peak at even more savage levels -- possibly approaching the boiling point of water. And the only known living things that can endure temperatures over about 60 deg C are some kinds of bacteria.

The strangest thing of all would be the fate of Earth's equatorial region. Instead of being the steaming tropics that we know, it would be below freezing -- and covered in ice -- all year round.

For two points during each year, when Earth was "side-on" to the Sun, it would receive as much heat from the Sun each day as our own Equator does -- but during most of each year, the Sun would be shining down on it from much shallower angles, as the Sun shines down on our own high-latitude and polar regions.

In fact, even during the six-month-long periods when one of Earth's poles was shrouded in permanent night, that pole wouldn't get quite as cold as the equator would always be, because it would still be cooling down from the huge amount of solar heat it soaked up during its long broiling day -- and it might not ever get below freezing.

As a matter of plain geometry, if a planet is keeled over more than 54 degrees, the total amount of sunlight energy that a point at its north or south pole soaks up over a year is actually greater than the total amount of sunlight that a point on its equator soaks up over the same year. Instead of having polar ice caps, Earth would have a permanent equatorial "ice belt"!

At moderate latitudes, of course, Earth's temperature extremes would be much less. Nova Scotia would oscillate from freezing to about 13 deg C -- 55 deg F -- each year.

And, of course, if Earth's axial tilt was less than 85 degrees, the extremes would be less - and Kasting did ignore the effects of clouds, which might somewhat reduce the hot temperatures. But it is clear that, if Earth had an axial tilt even modestly greater than the one it has today, it would be a much nastier place.

Kasting calculated that if Earth had a tilt of only 35 degrees but had a single huge continent centered around one pole, daytime temperatures at that pole would still hit up to 91 deg C -- 196 deg F!

In such an environment, life could almost certainly still appear, but it would have much more difficulty evolving into forms that could survive such grotesque temperature extremes -- which would greatly slow down its evolution into more complex forms, maybe by billions of years.

This interference would happen to a large extent even if Earth had no Moon so that its axial tilt swung back and forth from a destructive extreme to a more moderate level over cycles of tens of millions of years. And, as I said earlier, such crazily tilted worlds are probably more common in the universe than modestly tilted worlds like our own.