Changing the size of something doesn't seem to be to hard for evolution to accomplish. Brains are masses of neurons and related cells, and they seem to be able to grow to whatever size is necessary to accomplish whatever tasks the animal has to do. On Earth, whales have the largest brains. It has to be large to manage the senses and motions of the large bodies of these mammals. One speculation is that whales returned to living in oceans about 90 million years ago, and transited there from some herbivore predecessor, perhaps something like a hippopotamus. Perhaps it happened twice, once for baleen whales, which live on plankton and krill, and once for other whales, which live on squid and fish.
It is not hard to imagine a series of simple steps, single mutations, that would lead a creature that lived in river deltas to venture farther and farther out with successive generations, numbering in the thousands, losing the ability to live on land and developing the attributes needed for deep sea living. Did the large brains of these creatures exist in the predecessor land creature, or did it develop after the transition to sea life?
Brains are useful for muscular control, and neurons don't necessarily get more complicated individually, but rather bunch in masses for the control of larger muscles. Thus as the creature evolves to larger size, its brain evolves to control the larger size. Going along with that are all the nerves needed for sensing the position of the muscles, tactile pressure on the skin, along with temperature there, damage sensing, plus control of organs which also have to become larger.
Hunting in those whales which do it requires a special sense: echolocation. Because the speed of sound is much higher, five times, in water than in air, something more impressive than the ability of bats is required. Impulsive sound, clicks, are needed, plus some large mass of neurons in the brain to process these sounds and relate them to the coordinate system that the whale uses. A whale operates in three dimensions, and therefore has a higher requirement for orientation than something like a bipedal human, who does pretty well just standing up and turning around. A whale essentially flies through the water, and that takes much more control, and therefore more neurons and processing ability, than a land animal might need.
Another problem whales faced while evolving is navigation. So far, there is no conclusive understanding of how whales navigate. They may use magnetic sensing, sun position, star patterns at night, echolocation, sound recognition, and likely a combination of several. It is known that they do it quite accurately. Each of these senses would require more neural processing, both on the front end to translate the sensor inputs into something useful, and then more processing to turn it into a location. Further cross-connections would be necessary to correlate two or more senses. Thus there is good reason to suspect that the brains of the predecessor animals which eventually evolved into whales were not nearly so complex. Whales also communicate with each other, which requires more processing, but this is a common phenomena among Earth animals, so it is nothing special for whales, except for the large distances involved.
The point to be taken from this is that, given enough time, there is no problem evolving a brain to match a function, whether it be proprioceptive or navigational, sensory or orientational. Once the basic idea of the layering of associative functions evolves, it can be used for almost anything. Much like the versatility of a computer processor, a wetware processor can be evolved into doing any type of computational task. This means that brainpower is not an obstacle to the development of intelligent alien species, but that the mechanism providing that direction to evolution lies elsewhere.
Dolphins, which may have evolved back into the oceans later than whales, possess large brains as well, and have the same set of situational requirements as whales, and have evolved similar skills, such as echo-location and complex navigation, accordingly. Evolution responds to the environment, and in situations where there is only one functional way to accomplish a task which is crucial to survival and reproduction, that is where evolution will go. Dolphins and whales may be something of a special condition, in that the ocean is a more stable environment than any land area. There is nothing like drought, fires, heat waves, floods, and the other land hazards for sea mammals, so they have a longer, more leisurely time to evolve their capabilities. But most likely, the longer times are not necessary.
Elephants are another example of creatures which have evolved large brains. The same argument about needed neural networks to control all the mass of the animal, skin sensors, muscles, organs, and so on apply here. Elephants have a versatile trunk, reputed to have forty thousand muscles embedded in it, which all require neural processing to control. This may be a very important lesson. It is possible to evolve a tentacle-like appendage which has the ability to hold objects and move them and more, but it takes much more processing ability to do that than something like a hand and arm. This may explain why octopi are not smart, and never developed tool-using ability. Too much processing would be needed.
This seems to be propelling the argument toward interstellar convergence. If the only appendage which can be used for tool-handling and therefore lead to the evolution of human-like intelligence, is the hand with an opposable thumb or two, then aliens would have to evolve this on their own, and so should at least have this in common with us and with all the other intelligent aliens who have formed civilizations throughout the galaxy. It would also be possible to make the argument, but somewhat weaker, that two arms are the most that can be processed efficiently, so we should see aliens with two upper limbs, consisting of arms, not tentacles, and hands with thumbs.
That also means that if we understand how arms with hands and thumbs have to evolve, we can back-track the conditions to determine yet more things that an alien planet would have to have in order to give rise to an alien civilization. With a lot of luck, it will be something detectable that can be seen from very, very far away.
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