It's plausible that humans are not unique but rather a natural evolutionary outcome, with human-like life being common in the universe, especially if conditions that favor life are similar elsewhere, according to the principle of mediocrity. Newer research suggests intelligent life may form even under specific planetary conditions, challenging older theories that deemed it an incredibly rare event. 

Why humans might be common:

• Convergent Evolution: 

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  The theory of convergent evolution suggests that similar environments can lead to similar traits developing independently, even in different species. This could mean that intelligent, tool-using beings might emerge on other planets with Earth-like conditions. 
• Principle of Mediocrity: 

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  This concept extends the Copernican principle, suggesting that our position and experiences on Earth are representative of the universe as a whole. This implies that the evolution of complex life, including humans, is not exceptional but typical. 
• Universal Laws: 

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  The chemical and physical laws governing the universe are universal. If these laws allow for the development of life, and if conditions on other planets are similar to Earth's, then complex life could arise elsewhere as well. 

Why it's unlikely that humans are unique:

• Vastness of the Universe: 

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  The universe is incredibly large and contains a vast number of stars and potential planets. Given this immensity, even with low probabilities of complex life arising on any single planet, the numbers suggest life has likely appeared many times over cosmic history. 
• Challenging "Hard Steps" Theory: 

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  A recent model challenges the decades-old idea that intelligent life is a highly improbable event requiring a rare sequence of "hard steps". Instead, this new model suggests that intelligence may be a more probable outcome of planetary and biological evolution. 

The Implication:

The idea that human-like life might be common implies that when we eventually make contact with extraterrestrial intelligence, they may not be so alien after all. 

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In a universe of 2+ trillion galaxies, how likely is it that humans are truly unique?

https://www.reddit.com/r/universe/comments/1mao0fe/in_a_universe_of_2_trillion_galaxies_how_likely/

Geocentrism is a modern belief that was thought to be true not too long ago. 1500's - 1600's. What other assumptions are wrong?