Aliens likely resemble humans, a new study suggests, as scientists now believe life on Earth evolved in a standardised way that could repeat on other planets.
This challenges the long-held “hard steps” theory, which posits that intelligent life is an “incredibly improbable” event. Researchers at The Pennsylvania State University argue that humanity’s evolution may be the “natural evolutionary outcome” for planets, increasing the likelihood of intelligent life elsewhere.
Co-author Professor Jennifer Macalady stated: “This is a significant shift in how we think about the history of life. It suggests that the evolution of complex life may be less about luck and more about the interplay between life and its environment.”
The study, published in Science Advances, proposes that Earth’s environment became “permissive” for key evolutionary steps, such as the oxygenation of the atmosphere, enabling complex life to develop.
Lead author Dr. Dan Mills explained: “We’re arguing that intelligent life may not require a series of lucky breaks to exist. Humans didn’t evolve ‘early’ or ‘late’ in Earth’s history, but ‘on time,’ when the conditions were in place.”
The team suggests that other planets might achieve these conditions faster or slower than Earth.
The “hard steps” theory, developed by physicist Brandon Carter in 1983, claims that intelligent life is rare due to the improbability of steps like the origin of life and human intelligence.
However, the Penn State team argues that Earth’s habitability evolved through “windows of habitability” driven by environmental changes, making humanity’s emergence a natural result.