Those who think that intelligent extraterrestrial life is (nearly) impossible argue that the conditions needed for the evolution of life—or at least the evolution of biological complexity—are rare or even unique to Earth. Under this assumption, called the rare Earth hypothesis, a rejection of the mediocrity principle, complex multicellular life is regarded as exceedingly unusual.

The rare Earth hypothesis argues that the evolution of biological complexity requires a host of fortuitous circumstances, such as a galactic habitable zone, a star and planet(s) having the requisite conditions, such as enough of a continuous habitable zone, the advantage of a giant guardian like Jupiter and a large moon, conditions needed to ensure the planet has a magnetosphere and plate tectonics, the chemistry of the lithosphere, atmosphere, and oceans, the role of "evolutionary pumps" such as massive glaciation and rare bolide impacts. And perhaps most importantly, advanced life needs whatever it was that led to the transition of (some) prokaryotic cells to eukaryotic cells, sexual reproduction and the Cambrian explosion.

In his book Wonderful Life (1989), Stephen Jay Gould suggested that if the "tape of life" were rewound to the time of the Cambrian explosion, and one or two tweaks made, human beings most probably never would have evolved. Other thinkers such as Fontana, Buss, and Kauffman have written about the self-organizing properties of life.
Please give a simple list of reasons that extraterrestrial intelligence may not exist or is very rare
Conditions that evolved intelligent life on planet Earth may be exceedingly rare in the Universe. These reasons for that are speculated to be:
1. The solar system is in a safe location within its galaxy.
2. The location of the earth relative to the sun and other planets in the solar system gives it protection from extreme environmental conditions that would prevent life from occurring.
3. The steps that led to advanced life forms (prokaryotic cells to eukaryotic cells, sexual reproduction and the Cambrian explosion) may be very unlikely to occur.