If someone asks the question, are humans alone in the universe? I believe that most people would answer in the negative. In fact, the scientific community also generally agrees that there are a large number of intelligent civilisations in the vast universe, some of which may even be far ahead of our civilisation, and in order to better describe the technological level of intelligent civilisations in the universe, scientists have also proposed the concept of "Cosmic civilisation levels".

The reason we don't see time travelers is actually because they are not in our timeline

Theoretically, life can take many different forms, such as silicon-based life, sulphur-based life, carbon-based life, amino-life, hydrogen-based life, etc. However, so far we only know that life exists on earth, and all known life, including us humans, is carbon-based.

It is proposed that birds originated from dinosaurs, and that "ancestor birds" are a transitional species between birds and dinosaurs.

The solar system is a system of celestial bodies kept in motion by the sun's gravity, which means that all bodies within the effective range of the sun's gravity are part of the solar system, so by this definition, how large is the solar system?

Intelligent life is rare in the universe, or perhaps it is just wishful thinking on our part. As mentioned earlier, just in the galaxy, there are hundreds of millions of planets that have the natural conditions to nurture life, so a reasonable speculation is that life has evolved on many planets, and even a number of planets have evolved intelligent life.

The existence of reproductive isolation, so that these situations can not occur, even if there are some individuals of different species sometimes complete the mating, but they generally can not produce offspring, or produce offspring can not continue to reproduce

Compared to radio messages sent by humans, this method seems to be more suitable for intelligent civilizations to announce their presence in the universe, because this method belongs to the "broadcast" in space, and intelligent civilizations in all directions can be observed

Of all the known rocky planets in the solar system, the one with the most similar natural environment to Earth is Mars, and exploration data show that Mars also has an atmosphere, four seasons of the year, and even a rotation period similar to that of Earth.