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Why are humanlike aliens considered unlikely under the principles of evolution?

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artist's conception of Kepler-186f — extraterrestrial intelligence An artist's conception depicts Kepler-186f, the first Earth-sized extrasolar planet found in its star's habitable zone.

Humanlike aliens are considered highly unlikely because the evolution of life on Earth has followed a unique, circuitous, and non-repeatable path.1

The principles of evolution dictate that organisms are shaped by specific environmental pressures and natural selection over billions of years. Because the history of life on Earth is defined by an unbroken chain of descent from common ancestors—each adapting to the unique and varied conditions of our planet—it is statistically improbable that the exact same sequence of biological events would occur on another world.21

While convergent evolution can lead to similar functional traits in different species, the specific anatomical and cognitive features that define humans are the result of Earth's particular evolutionary trajectory. Consequently, while other intelligent life may exist elsewhere in the universe, the emergence of beings identical or even closely similar to humans is not expected.1