Concerning the evolution of intelligence
The Global Community of Sentience categorizes life based on what exists on most planets. Bacteria have no nucleus and one cell. Eukaryotes have a nucleus and sometimes have more than one cell. Plants produce their own energy and do not move. Fungi do not produce their own energy and do not move. Animals move. The Biomass provides the galaxy with the first incidence of animals that can photosynthesize. No animals can chemosynthesize.
Among animals, there are categorizations of development. The first step is intelligence, which is associated with the development of complex language and tools. When that species and its subservient species dominate a majority of the biomass of the planet, the are considered the dominant species. By definition, only one species can become dominant on a planet. That species then develops interstellar travel. Through meeting other species, their own existence is contextualized and they begin to think more deeply. Through incompletely examined mechanisms, this realization leads them to be more compassionate to their own species and allows them to develop sentience. Sentience is when more than 50% of a species is deeply empathetic, able to resist "cheep" pleasurable stimuli, and intensely curious about the universe. Sometimes sentience develops before interstellar travel, but travel develops quickly afterwards because of that intense curiosity.
So then, we have a progression from animal to intelligent to dominant to traveling to sentient. Except of course for the Aba, who are in fact colonies of single-cell organisms and not animals. And of course for the Pathrons and Gaspids, who were first sentient then interstellar travelers.
Because the ability to empathize with other animals deeply arises after intelligence, competition limits each planet to only one dominant species. One is reminded of the epic poem by Cristiya the Gaspid. In her powerful words, she laments that she cannot socialize with members of a now-extinct once-intelligent quadruped mammal species because her primitive ancestors killed them all in a violent war of xenophobia.
A planet that contains a species that we expect may become sentient given enough time is called a "developing" planet and that species is called a "developing" species. How, then, do we predict whether a species will grow closer to sentience?
The first sign is quite obvious. The cells of an organism must be able to communicate with each other and there must be a center of processing. For most sentient species, that center is a collection of nervous cells, called a brain. For the Aba and the dominant species in the fifth planet in the solar system BM2-15-84, that center of processing is molecular rather than geographic. Memories and ideas are recorded as novel peptides rather than as neural networks.
The second requirement is a challenging environment. Intelligence is an energy intensive activity, so there must be some reproductive benefit for it encouraged by natural selection. On planets with plentiful water, easy access to dissolved particles, uniform geography, and plentiful sunlight, no intelligent life develops.
The third requirement is social structures. Being able to survive in a society requires a great deal of intelligence. To compete for resources, individuals must be able to understand things from the perspectives of others. No hermit-type species become intelligent.
Social structures lead, somewhat naturally, to language. Usually there are multiple sections in the central processing organ specifically for the processing of language. These sections can than find other uses in other areas of cognition.
The fifth requirement is fine motor control. If an organism can learn to use tools, the environment will further select for more intelligent individuals, because the use of tools is a huge biological advantage. Tools can only be modified if a species has fine motor control. The Krogs, Phrugs, and Siliconoids have fingers on their upper extremities. The Trins of tenticals and suction pads, the Pathrons have three fingers on their tails, the Yogs have many small appendages that end in points, the Gaspids have talons and beaks, Bru/we'em have long and flexible tongues, and the Aba can mold any part of their colony into an appendage.
The final requirement is sex. Sexual reproduction simply speeds up the recombination of genes and thereby the progress of evolution. This is perhaps the most profound example of convergent evolution. All developing species have two sexes. One sex, the males, produces small gametes and the other, the females, produce large gametes. Certainly these species occasionally produce members that don't fit this structure, but they are ultimately not part of the species's evolution because they cannot reproduce. Some species have opted to use technology to aid non-traditional reproduction, but that technology can only develop until after the species becomes dominant. Species with three sexes can be produced in the lab, but they generally don't develop naturally because although they can adapt to environmental change faster then double sexed species, they reproduce much slower because of the probability involved with the meeting of three organisms. Ultimately the benefits don't outweigh the cost in nature, so triple sexed species do not evolve.
So then, we have six requirements that help us pinpoint developing species: central nervous systems, challenging environments, social structures, language, fine motor control, and sex.
We can imagine sentient species that are drastically different, but we find that they are more similar than our ancestors imagined. That is because they all have the same requirements.
Some more notes:
Plants cannot become sentient because their inability to move leads to an inability to decide; intelligence is of no benefit. Intelligence is energy intensive, so it doesn't evolve unless it provides reproductive benefit.
Sex has the benefit of accelerating evolution, but it also provides the advantage of sexual selection. Once intelligence is seen as "sexy," intelligent members of a species have better reproductive success.
As previously mentioned, intelligence is energy intensive. Developing species must have adequate access to nutrition, oxygen, and other resources.