I don't usually review non-fiction books, but this one is a very relevant exception. The author is a zoologist used to applying Darwin's theory of evolution to the development of life on Earth, and he applies the same analysis to explore how and why life might develop on other planets. The Contents list gives a good idea of the author's approach:
Form vs Function: What is Common Across Worlds?
What are Animals and What are Aliens?
Movement: Scuttling and Gliding Across Space
Intelligence (Whatever That Is)
Sociality: Cooperation, Competition and Teatime
Information: A Very Ancient Commodity
Language: The Unique Skill
Artificial Intelligence: A Universe Full of Bots?
Humanity, As We Know It
Each of the characteristics of life are explored in detail, and the author's summary of how life logically has to have developed is worth quoting in full:
Early life was simple, gaining energy from non-living sources, perhaps mostly from the star around which the planet orbits, but also directly from the heat of of the planet and maybe from other sources, like radiation.
The first innovation was that some life forms (I'll call them 'predators') began to get their energy from others ('prey'), exploiting the work of others in harnessing energy from nature. Freeloading is always an option, and game theory would seem to indicate that the evolution of this kind of 'cheating' is inevitable.
Both predators and prey are competing to achieve their goals of eating, and avoiding being eaten. Movement would then evolve.
Once organisms can move, social behaviour follows. Prey animals can reduce their chances of being eaten by aggregating, and this opens the possibility of more active defence strategies: sentinel behaviour, building structures etc.
If any two organisms are to associate together, communication is necessary, at the very least so that they can find each other.
At this point (if not before) the complex interactions between organisms, both those that are helping each other and those that are competing (either with similar organisms or with predators/prey), lead to the evolution of intelligence: the ability to predict the world and to make decisions that are beneficial to you.
The combination of communication, social behaviour and intelligence leads to the evolution of communication system that can contain large amounts of information, leading to an ecosystem that would be very familiar to us. Alien creatures will be singing like birds, roaring like lions and whistling like dolphins, even if their precise forms, and even the chemical makeup of their bodies, will be entirely unexpected.
How long such an ecosystem continues like this, we don't know. Perhaps the next step is incredibly unlikely. We know that it occurred at least once in the universe, but it took at least 3 billion years from the first step in our story. Whatever the reasons and whatever the mechanism, at some point, complex communication evolves into language.
Finally, possibly inevitably, a social and intelligent organism, with the skill of language, develops complex technology. It is hard to see how any other outcome is possible. Soon, they will be building spaceships and exploring the universe - if they manage to avoid destroying themselves first.
Of course this summary doesn't do justice to the author's case; the book contains a mass of evidence to support his argument that Darwinian evolution seems inevitable, regardless of the setting, and that this is likely to result in intelligent life. Well worth reading.