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Thought Experiment: How would life on Earth have developed if...

  1. Jan 5, 2016 #1
    What would have happened to to the evolutionary process had the Earth's axis of rotation had no tilt? What comes to me first would be a total lack of anything based on the year calendar. There would be no deciduous trees or dormant grasses, certain animal species would have no particular mating seasons (since there would be no seasons), and other animals would be prey to predatory life forms all year round. All species would have little tolerance for changes in temperature, so biological diversity would vary greatly with the latitude. The next question: would any place on the surface of the Earth be inhabitable if the tilt were 90 degrees?
  2. jcsd
  3. Jan 5, 2016 #2


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  4. Jan 6, 2016 #3
  5. Jan 6, 2016 #4
    Dr. Courtney, Good read, but I disagree with the premise raised in the first article. If the Earth (or any other planet) had no tilt, Would receive the full brunt of the Sun only at the equator. The incidence of solar energy on a horizontal surface varies approximately (thanks to the atmosphere and the fact that the sun is not a point source) with the cosine of the angle off the perpendicular. Not only that, but the full brunt would happen only once a day. In essence, the earth would be in a permanent state of March 21 (or September 21). I see no threat to life there. With no seasons, however, I believe life would have evolved in spectacularly different ways.
  6. Jan 6, 2016 #5


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    @Dan Allred
    I would have thought the "bulge" would have given the spinning tilted planet a wobble or precession which they do not mention.

    You can also consider an elliptical orbit to give seasons, which will then be of asymmetrical time length - summer shorter than the winter depending upon the frame of reference - ie distance to the sun or dividing the time of orbit into two separate halves of equal length.
    The whole planet would then be in a winter or summer state.

    In addition, the tilt of the earth is in relation to a plane "parallel" to its orbit, or perpendicular to the axis of revolution of the earth about the sun.
    If you consider a plane instead perpendicular to the axis of rotation of the earth, the earth is seen to orbit about the sun moving above and below this plane in its orbit around the sun. It may be easier to see then, or comprehend, if other planets, with orbits around the sun perpendicular to the axis of the plane had or would have had, and how much of an affect on a planet that had started out with a no tilt condition to its own orbital plane in the beginning and thus commence seasons where once there were none billions of years

    Its not so much science fiction as orbital physics then.
  7. Jan 6, 2016 #6
    I'm not sure if I agree or not, but here are some of the more scholarly (peer reviewed) papers that make and build on the case. The idea seems to be well accepted.



  8. Jan 25, 2016 #7


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    FYI, Mercury has an axial tilt of only 0.03 degrees.
    Unfortunately, Mercury doesn't have enough atmosphere to regulate temperature, so you broil during the day and freeze at night, so seasons are the least of your troubles.

    We'll probably find some habitable exoplanets with very low tilt at some point. Maybe at that point it will become a more important scientific question.
  9. Jan 27, 2016 #8
    Most life in the deep oceans couldn't care less about the seasons, so I don't see it having any negative effects on evolution, like you said, it'd just be different. The rain forests don't really experience much seasonal changes either and life absolutely thrives there. The stability would probably cause life to specialize a little faster, but in the long term, I think it'd still be very resilient. Seasonal changes wouldn't affect it's evolution, but climate changes would. It wouldn't be adapted for cyclical changes, but I think it would still be fairly resilient. The ability to survive in multiple niches at once would be a huge benefit to a species.
  10. Jan 27, 2016 #9
    Underwater and surface currents are very much influenced by seasonal warming and cooling. I suspect the change to ocean life would be about as drastic as it would for land dwelling creatures.
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