If other intelligent life-forms exist, how might they develop?

For life to form, a planet must be ideally positioned in a solar system.

Why did intelligent-life form on Earth?  If God allowed it, how might intelligent life forms develop on other planets besides earth? These are questions that were tackled by John Gribbon in his 2000 book  "The Case of the Missing Neutrinos and Other Curious Phenomena of the Universe."  The theories he postulates are quite impressive.

1. The planet must hold water:  This is essential for life to form.

2.  The planet must have volcanic activity:  This is essential for elements needed for an ideal atmosphere to develop.

3.  The planet must endure ice ages:  While the Saurornithoide meets all the above criteria, it lived 65 million years ago, a time when the environment of the planet was rather stable.  There were no environmental changes that forced a speed up of the evolutionary process. Around 10,000 B.C. an ice age forced mankind to go into hiding, and this is what forced it evolve into what it is today.

4.  There must be no environmental catastrophe:  It is possible that the Sauronithoide may have evolved, if given enough time, into an intelligent life form.  However, a meteorite slammed into earth, causing ash and heat that created an environment animals larger than 40 kg in mass size to die off.  Since the Sauronithoide was about 60 kg, it was among the catastrophes.

5.  The planet must have a sun with a solar system like earths:  This would be necessary to keep the planets in line, and to create an earth-like environment ideal for life.

6.  The planet must be the third planet from a star like the sun:  The planet must be in an ideal position in the solar system.  "The nearness of the Earth to the Sun makes the Earth rocky; the exact distance of the Earth's orbit from the Sun has determined the nature of the atmosphere and oceans which cover our rocky planet." Planets too close to the sun are too hot, and planets too far are too cold, for an ideal atmosphere for life to evolve. Distance from the sun also determines the effect of gravity in holding the planet into a solid or gaseous state, withe earth being at the ideal distance, again, for life. Planets close to the sun are too dense, and planets too far from the sun are too gaseous and not earth-like at all. Venus is close to the earth, but has an atmoshpere that is thick and creates a "greenhouse effect" that makes the planet too hot (about 500 °).  Mars is close to the earth, but is too light and cold (about 30 °C) for water to form.  So Venus has too much carbon dioxide, and Mars has no water. The Earth, alone, is at just the right distance from the sun to create an environment just right for life to form.

7.  The planet must have an ozone: Oxygen must build up in the atmosphere, where "chemical reactions," stimulated by radiation from the sun, causes an ozone to develop.  This creates a filter that "blocks out th sterilizing ultraviolet radiation. Under this protective blanket, life could move out of the sea and onto the land, while an abundance of atmospheric oxygen allowed new life forms to invent the trick of respiration, using the oxygen as an energy source."

8.  The atmosphere must include carbon dioxide:  Actually, the atmosphere must include an ideal amount.  Too much would cause a greenhouse effect similar to what occurs on Venus, making the planet too hot to sustain life.  Yet an ideal amount is essential for plants to process it into carbon and oxygen.

9.  The atmosphere must include oxygen:  This allows for metabolic respiration to occur, and for life to exist outside water.

10.  The planet must have a moon:  The moon is essential because its gravitational pull controls the rise and fall of the sea level, exposing life to both air and water in the space of a short time (only a few hours), creating an environment for life-forms to be gradually exposed to a new environment (air), and to gradually make the evolutionary changes necessary to live on land.  (See, how the earth and the moon interact at astronomytoday.com)

Now, there obviously can be some variations of these, although the essential elements must be in place in order for life to form.  Then, once the environment is created, and once lifeforms existed on land, then the environment would be set for a hunan-like, intelligent life form to evolve, thus developing the type of intelligence needed to form civilizations.