The whole point of understanding evolution is to deduce, somehow, the various steps that lead bit by bit up to the existing attributes or capabilities. Intelligence, defined as the ability to solve problems, depends on some traits already having been evolved, at least to a small amount, first. Perhaps the most critical one, even more important than tool-using, is communication.
Almost all animals on Earth communicate. There are solitary creatures, which communicate to find mates. There are pack, herd, pod, or whatever animals which communicate to warn the others about predators. There are insects which communicate the location of food. Communication can be by audible sounds, gestures, expressions, motions, and probably other means. It is diverse, and that means it has evolved repeatedly in different organisms for different purposes. This little observation has some special implications.
Communication can be used in diverse ways, and therefore it should be capable of shifting from use to use without too much evolutionary delay or impediment. So, as some alien creatures develop communication concerning one area of their lives, it can shift to another. And when there are two or more uses both adding to the fitness competition from one function, that function gets a double dose of selection, and evolves faster.
How might the sequence go? Climbing in a certain environment, a forest or jungle, develops grasping capability for at least upper limbs. Play, in creatures living in groups, develops that capability to tool use, or toy use, which then migrates to tool use, specifically weapons and tools for hunting and food gathering. The known earliest uses are projectiles, meaning found rocks and stones, spears, meaning pointed sticks, and wedged stones in sticks, used as a tomahawk. Once these uses are discovered, evolution will work to make the creature better able to find and use them, and this involves some brain development in different areas than the prior non-tool-using situation did.
Once tool-assisted hunting and gathering gets under way, a whole raft of needs for communication arise. In a group, if the entire group goes out together to look for where game is, or where edible fruits and vegetables, or the alien equivalents, are, that is a very inefficient use of time, and time is important in survival in some situations, scarcity ones rather than abundance ones. Having individuals go out as scouts, and able to communicate findings, such as what animals or birds or fish or fruit or whatever was found, how hard it will be to follow through on the find, how far, and so on, would make the survival of this group much easier.
In humans, communication with this much detail required the evolution of vocal cords to modulate the sound produced during exhalation. Muscles and the nerves to control them are standard equipment all over animals on Earth, so developing the initial musculature there would be only a few mutations. What took many mutations is the development of the brain capacity to utilize them to make more and more complicated sounds. This went simultaneously with improvements in the vocal tract itself. The diversity expressed in our gene pool, even in the gene pool of a small hunting and gathering group, is sufficient to lead to this. The eventual result was a pool of words and finally combinations of words and meanings.
There is a giant change underlying this. It is conceptualization. Conceptualization as used here means the ability to categorize complex scenes and situations into abstract nouns and verbs. An animal might see a mountain, but it does not think the word, 'mountain'. It relies on the visual scene for its decision-making. An alien in a primitive hunter-gatherer group can refer to a mountain as a single word, perhaps a proper noun naming the mountain, or perhaps a more abstract word meaning any mountain. This is the beginning of thinking. Thinking like this is the beginning of intelligence.
Thus it is completely possible to create a list of reasonably simple mutations and resulting physiological changes, each resulting in an improvement in fitness, that leads to intelligence. Perhaps there are multiple paths. But there is not no path. A path can by hypothesized. This means there exists the possibility that, given the correct conditions, intelligence can arise on an alien planet without any extreme requirements.
One requirement, fairly mild, is that there be group living, a clan, of the alien creatures that might eventually develop intelligence. Among Earth primates, there are those whole live solitary lives and those who live in groups. Group living makes sense, from a survival standpoint, if food sources are large and congregated. If food sources are small and dispersed, solitary living allows each creature to seek his/her/its own, and live on that basis. With concentrated food sources, the group can live together feeding off the single source for a time, before moving to the next one. Predator-prey relations can also affect the utility of living in groups. If the predator type which exists can better be defeated by a group of creatures, rather than evaded by an individual, then the group has this survival push as well.
Perhaps something can be discussed further here. It is possible for some improvements by genetic mutation to disappear if there is too much noise in the evolutionary process. One source of noise is the selection of mating preference genes. If mating preferences dominate the evolutionary fitness competition, no intelligence genes can evolve, or at least not fast enough to make any difference. So mating selection has to have either reached its culmination, or must be bypassed in favor of factors that increase survival probability, which may include intelligence and communication. The bypass can happen if there is some finality to a mating selection, for example, by the selection of a dominant individual who mates with multiple companions, where the non-dominant do not. If the dominance lasts for many years in the group, there is time for other evolutionary factors to develop their role and serve as fitness competitors. There may even be a feedback effect here, in that if dominance of this type develops, then the groups which have it, say those in one geographic locale, the most will evolve other aspects faster and then this characteristic will be carried along as the most populous one in the gene pool, not because it is being selected for directly, but because it enables other ones to be selected which actually do promote improvements in fitness competitions.
This idea, that certain fitness characteristics might become more common in gene pools because they enable other genes to express themselves and win the fitness competition, is a general one, and is certainly not restricted to dominance of one individual in a clan. It does imply some linkage, perhaps having genes closely located on a single chromosome, which is again a simple job for evolution to pull off. It may be that a hundred years from now, the location of different genes on chromosomes will be well understood to be groups of enablers acting together.
So the conclusion is, that on a diverse alien planet, with many types of habitats, with different extents, communication should eventually develop in creatures that live in groups and are hunter-gatherers, as long as there is some mechanism to suppress other fitness characteristics which would overwhelm it. To summarize, alien planets might typically evolve intelligent aliens. This isn't the obstacle it might have been thought to have been.
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