Our Rare Earth
In the beginning, namely the Big Bang, conditions were so violent that only simple atoms like hydrogen and later helium survived. So where did all the other elements come from?
The simple answer is that they were "cooked up" by stars much larger than ours, stars that then exploded and spread their products into the cosmos. These exploding stars are called supernovas.
Our own star, the Sun, and its planets happily were formed from supernova dust that was rich in heavy elements. Some star and planet systems, especially in earlier universe times, were not so fortunate, and their Periodic Tables had many gaps.
Additionally, had we come along sooner in the universe, both quasars and supernova explosions would have been hazardous and sterilized our Earth. Had we come later, the probability of having plate tectonics would have decreased dramatically. As we shall discuss later, plate tectonics have been vital to the sustained temperatures on Earth over billions of years.
Continuing, it turns out that our spiral galaxy is a perfect shape. Elliptical galaxies often do not have enough heavy elements. Neither do globular clusters, which also have hot and colliding stars.
Our galaxy is also a perfect size, since smaller ones often lack the heavy elements.
Our Sun has a perfect location in our galaxy. In the center there would be too many collisions. On the edges there are frequently not enough heavy elements.
The large planets in our solar system fortunately did not spiral into the Sun or outer space, taking Earth with them, as is often the case. In fact, the planets, including Earth, all have relatively stable orbits.
Additionally, our sister planet, Jupiter, runs interference for us by clearing out many comets and asteroids.
Our Sun is the right size to last a long time, which is great for evolving complex life. At 5 billion years, our Sun is halfway through its life.
Earth is the right distance from the Sun, basking in the rather narrow habitable zone. Mars and Venus, although reasonably close to us, did not fare so well.