Let's briefly recall the Fermi Paradox: it holds that the universe has produced so many galaxies, there are so many habitable planets, there should be many civilizations, some of which should be able to meet us, but there are not.
Using a more accurate mathematical formula, the number of communications with us is as follows:
From the data, I am optimistic about both "habitable planets" and "life on those planets", because even on Earth, life can exist in extreme environments.
However, many people overestimate the "possibility that life evolved intelligent beings". It seems that there are creatures on a planet that give enough time to "evolve" highly intelligent creatures. Are you sure? Many people understand that the "theory of evolution" is a process with "directionality", as if from simple to complex, from the beginning to the high is the "purpose" of evolution. "Evolution" is understood by many as "biological progress."
Really?
Let's take a look at George Williams' famous book Adaptation and Natural Selection (this is a classic treatise on evolution, much better than my answer, please have time to read. In this book, he questions whether the (later) "higher animals" are really more advanced than the (primitive) "lower animals."
First of all, in terms of the number of genes, the number of human beings is not even as good as the rice we eat every day. The total number of genes in rice is 1.7 times that of humans. Very simple nematodes also have a genetic number of nearly 2w, compared to 2.7w in humans. There are even more mice than humans. In fact, the number of genes in any vertebrate is not less.
Human and other biological gene and base quantity comparison
In terms of complexity, humans do have the most complex brains, but human skin is simpler than fish, and the skin complexity of fish exceeds that of most land mammals. In terms of "adaptability", the simplest bacteria are not worse than humans, and the small strong adaptability that lives around us every day is also super strong. In the struggle between bacteria and humans, it is difficult to say that humans have won an all-round victory. And a lot of creatures have "degenerated", and you don't believe it, you touch your ass.
To put it bluntly, "evolution" has no direction, and the essence of evolution is only change, those changes that adapt to the environment.
Darwin himself said, "After a long period of reflection, I cannot in any way think that all living beings have a natural and inevitable evolutionary tendency."
Change is actually a by-product of natural selection. As Gould argues, "adaptation" is simply adaptation to the specific environment surrounding the species, and in the process of adaptation, organisms may move toward complexity or simplicity. So how do you explain that some creatures are more complex than others? From simple to complex, although there are many at the simple end, there are also some creatures on the complex side. To explain this, he made an analogy:
There is a wall on the "simplification" side, because simplification cannot develop indefinitely, and to the extent of bacteria, it will come to an end, and it cannot be simpler. There is no "wall" on the "complex" side and can go all the way. He (referring to Gould) made the analogy that a drunkard waddles on the road, with a wall on his right and a ditch on his left. A drunk may step on either side, but there is neither a predetermined law nor a tendency to orient. But the wall restrained him to the right, hitting the wall several times to the right, approaching the ditch several times to the left, and falling into the ditch when he went farther to the right. The drunkards repeatedly fell into the ditch, but the left line was not inevitable, not deliberate, but accidental, negative.
That is to say, the progress of living things to higher complexity is negative and accidental, not some kind of necessity. The drunkard himself did not want to fall into the ditch, it was the environment that forced him to do so, and if the environment was good enough, he could not fall into the ditch. This metaphor expresses that biology is helpless to go complex. By the way, "complex" does not mean "intelligent.". This is human conceit, seeing its own characteristics as some kind of apex and benchmark. Complexity can be the specificity of any adaptability, and intelligence is not an "inevitable choice." Hopefully, you understand that the "emergence of intelligent beings" is not so inevitable. If history were to do it all over again, it was possible that humanity would not appear.
Some people think that "intelligent organisms" are some kind of ecological niche, so there must be organisms that appear and occupy this ecological niche. However, this is probably a bit wishful thinking. For example, in Australia, an isolated continent, they have many "parallel animals" with other continents, but they have not appeared "parallel classes" of bats, elephants and humans. There are no marsupial primates on this continent. From this point of view, let alone intelligence, even "primates" are not "necessary". Even the animals on Australia enjoy a common ancestor with us. Taking our human ancestors as an example, according to modern molecular biology, we know that human ancestors twice walked out of Africa. The first was when those ancestors who came out 1 million years ago evolved into Homo erectus everywhere (including "Pekingese"). The second time, the group of intelligent people who came out of Africa 200,000 years ago are our ancestors. Those relatives who went out first did not develop into high intelligence, and were "wiped out" by relatives from Africa. Understanding the directionless nature of evolution, you can explain a bunch of silly questions, "Can orangutans evolve intelligence later", "Why do low-level bacteria still exist and do not evolve into intellectual bacteria".
Even if there are extraterrestrial intelligent beings, I doubt whether they can have a higher civilization, and because of the objective conditions in which intelligent life is located, they may be destroyed before they develop a higher civilization.
Let's go back to the Drake formula at the beginning, where perhaps the two factors are too small, almost smaller than 0, so that even with a huge "cardinality", we are still temporarily "lonely".