The goal of this section is not to debate whether or not we are the only intelligent life in the Universe, but rather to help better understand that the chances of intelligent life existing elsewhere are as low as winning the lottery 5 times in a row. Here is why.
To spread life, a planet needs water, an atmosphere and a favorable temperature. The atmosphere is there to protect the planet from deadly sunrays and to minimize temperature changes between night and day. It isolates life from the extreme conditions of space.
Water protects the planet from the sun, but more importantly, because of its density, being that it is 1000 times denser than air, it provides a perfect environment for elements to gather and create chemical reactions. On most other planets, water has evaporated or is frozen. But when it is liquid, the atoms gather together to form molecules and consequently, those molecules meet to create more complex organisms. Life on Earth appeared in the ocean first.
Life on Earth appeared in the ocean first
The temperature on Earth is ideal, as it is located at a perfect distance from the sun. If it were to be positioned just a little bit further or a little bit closer, it would be impossible for life on Earth to exist. Such is the case for Mars and Venus. Therefore, under favorable temperatures, extremely complex organisms can form such as plants and animals, each containing an intricate blueprint known as DNA.
The Earthís orbit needs to maintain that perfect distance from the sun to avoid extreme temperatures. On the other hand, the sun must provide enough energy to feed life, and must stay alive for a long enough period of time to let evolution occur. Bigger stars posses a shorter lifespan (of a few million years), and smaller ones do not emanate enough light. The sunís lifespan is estimated to be about 11 billion years.
Gravity, which is proportionate to a planetís mass (normaly the biggest in size), must be strong enough to exert a pull on vital gases like carbon and oxygen. However, it cannot be too strong or else it will trap everything, including deadly gases such as hydrogen. A strong gravity will also attract meteorites. The Earth is fortunate to be surrounded by giant planets (Jupiter and Saturn) that act like shields, attracting most of the large meteorites.
As life established itself in the oceans, plants began the act of breathing. This gradually started replacing the abundant carbon dioxide in the atmosphere with oxygen, the same way trees do today. This also produced an ozone layer capable of protect life on the ground from the sunís deadly rays. This process is what made life outside water possible.
Plants made life possible outside water
In conclusion, to maintain life and to ensure a sustained evolution, we need a very favorable environment such as that found on Earth and a lack of natural disasters. Even though our planet presented perfect conditions, it still took more than 4.6 billion years for intelligent life to happen. It is estimated that humans have lived on earth for a period of about 5 million years. This is a short period of time considering the lifespan of our planet. This information helps us realize the value of human life and pushes us to become more aware of the consequences that destabilizing our ecosystem could have.