There are several pieces of evidence and breakthroughs that scientists would need to determine if there is other forms of intelligent and complex extraterrestrial life out there:
Detection of biosignatures: Scientists would need to detect specific chemical or physical markers of life, such as the presence of certain elements or molecules that are known to be associated with life, such as water, oxygen, or methane.
Detection of radio signals: Scientists would need to detect radio signals that are clearly of artificial origin and that display characteristics of intelligence. This would require the development of more powerful telescopes and other instruments to scan the cosmos for such signals.
Direct observation of exoplanets: Scientists would need to directly observe exoplanets, either by imaging them directly or by measuring their spectra, and determine if they have conditions that could support life, such as the presence of water or an atmosphere.
Discovery of microbial life: Scientists would need to discover microbial life, such as bacteria or simple organisms, on other planets or moons in our solar system, which would provide strong evidence that life could exist elsewhere in the universe.
Breakthroughs in understanding of the origin of life: Scientists would need to make breakthroughs in understanding of the origin of life and the conditions under which life can arise, in order to understand the likelihood of life existing elsewhere in the universe.
These are some examples, but it's important to note that finding evidence of extraterrestrial life would likely involve multiple lines of evidence and a combination of different techniques and approaches.
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