The scale of the universe is amazing. There are more than/kloc-0.00 billion stars in the Milky Way, so there are more than/kloc-0.00 trillion galaxies in the universe. This is only a small part of the universe that we can see. Even though there are few habitable planets, their absolute number (as many planets as stars, or even more) indicates that there may be many lives in the universe. Where are the aliens? This problem is Fermi paradox. The universe is very big and ancient, and there is time and space for other intelligent life to evolve, but there is no evidence to prove their existence.
Is intelligence really almost impossible to evolve? Unfortunately, we cannot answer this question by studying alien life. But we can study the history of the earth for 4.5 billion years to see if evolution has been repeated and where it has been repeated. Sometimes, different species converge to similar evolutionary results independently. If evolution is often repeated, then the probability of human appearance is very high, even inevitable.
We can indeed see some remarkable examples of convergent evolution. The marsupial is an extinct marsupial in Australia. It has a pouch like a kangaroo, but it looks like a wolf in other ways. We can also see marsupials, marsupials, minks and marsupials in Australia. It is worth noting that in the whole evolutionary history of Australia, the differentiation of mammals after the extinction of dinosaurs was parallel to that of other continents.
Other striking cases of convergence include dolphins and extinct ichthyosaurs, both of which have evolved flippers that can "slide" in water; There are birds, bats and pterosaurs, which have evolved convergent flight capabilities.
We can also see the aggregation of individual organs. Eyes evolved not only in vertebrates, but also in arthropods, octopus, worms and jellyfish. Vertebrates, arthropods, octopuses and worms independently evolved jaws. All these convergence occurred in the same pedigree, that is, eumetazoa. Eumetazoa are complex animals with symmetrical mouths, viscera, muscles and nervous system. Different eumetazoans have evolved similar solutions to solve similar problems, but the complex body structure that makes all this possible is unique. Complex animals have only evolved once in their life history, which shows that their appearance is extremely low.
Surprisingly, in the history of human evolution, many key events are unique and probably cannot be reproduced. One is the skeleton of vertebrates, which allows large animals to move to land. All animals and plants are composed of complex eukaryotic cells, including nuclei and mitochondria, and eukaryotic cells have only evolved once. Sex has only evolved once. Photosynthesis has only evolved once, increasing the available energy of life and producing oxygen. In this respect, the same is true of human intelligence. There are marsupials and marsupials in the world, but there are no marsupials.
Some places will repeat the evolution of life, while others will not. If we only look for convergence, there will be confirmation bias. Convergence seems to be a universal law, so our evolution may be repeated. However, when you look for non-convergence, you can find countless evidences; And crucially, complex adaptability seems to be the most unrepeatable, so it is extremely impossible.
More importantly, these events are interdependent. Humans didn't appear until fish evolved bones that could climb on land. Skeletons can't evolve until complex animals appear. Complex animals need complex cells, and complex cells need oxygen produced by photosynthesis. None of this would have happened without the initial evolution of life. All living things come from the same ancestor; As far as we know, the beginning of life only happens once.
Surprisingly, all these processes took a long time. Photosynthesis didn't evolve until 654.38+0.5 billion years after the formation of the earth. Complex cells and complex animals didn't evolve until 2.7 billion and 4 billion years after the birth of the earth, and human wisdom didn't appear until 4.5 billion years after the formation of the earth. These groundbreaking events took so long to develop, which means that they are highly unlikely to happen again.
A series of impossible events.
These pioneering events, which only happen once, seem to be just flukes, and may be used as a bottleneck or filter for a series of evolution. If so, then the evolution of mankind is not like winning the lottery, but like winning the lottery again and again. On other planets, these key adaptive abilities may have evolved so late that intelligent life appeared before their sun became a new star, or did not appear at all.
Imagine that intelligent life depends on seven extremely improbable groundbreaking events-the origin of life, photosynthesis, complex cells, sex, complex animals, bones and wisdom itself-assuming that each one has an evolutionary probability of 10%, then the probability of evolving wisdom becomes one in ten million.
The possibility of complex adaptation may be lower. Photosynthesis requires a series of adaptations of protein, pigment and cell membrane. Eumetazoa need all kinds of new anatomical features (nerves, muscles, mouth, etc.). ). Therefore, perhaps these seven key groundbreaking events are only evolving in 1% of the time. If so, only 65438+ 1000 trillion planets will evolve intelligence. If there are not so many habitable planets, then we may be the only intelligent life in the galaxy and even the visible universe.
But here we are. This must mean something, right? If the probability of intelligent life is 100 trillion, what is the probability that we happen to appear on such a planet? In fact, the possibility of appearing on an impossible planet is 100%, because in a world without photosynthesis and complex cells or animals, it is impossible for us to have such a dialogue. This is the anthropic principle: the history of the earth must allow intelligent life to evolve, otherwise we will not think here.
Intelligent life seems to depend on a series of impossible events. However, considering the number of planets, just like countless monkeys banging on countless typewriters and finally writing a Hamlet, intelligent life will certainly appear somewhere in the universe. This almost impossible result is ourselves.