What If Humanity Is Among The First Spacefaring Civilizations?

A recent astrophysics-style model ties humanity’s “early” arrival in cosmic history to a possible future in which the universe becomes colonized by fast-expanding “grabby” civilizations—leaving little room for new intelligent life to emerge. The central claim is not that aliens are imminent, but that the combination of (1) how quickly habitable conditions arise, (2) how long complex evolution typically takes, and (3) the fact that telescopes have found no clear signs of extraterrestrial expansion could make our position in time look less like coincidence and more like evidence of a looming cosmic cutoff.

The reasoning begins with the Copernican principle—humans shouldn’t expect to be in a special time—and weighs it against the anthropic principle, which allows observers to exist only where conditions permit them. If life can take an enormous fraction of the universe’s lifetime to develop, then finding ourselves near the beginning of the era of life would be suspicious. The model quantifies that suspicion by asking how “early” humanity is among all civilizations that will ever arise. Researchers define a civilization’s birth rank as the fraction of all future-and-past civilizations that formed before it: most earlier births implies a late rank; most future births implies an early rank.

To estimate the birth rank, the model needs an appearance rate for intelligent life. It starts with when habitable stars form: rocky planets require heavy elements, which arrive after the early universe has enriched enough, while star formation later declines as interstellar material runs low. Habitable star formation peaks around 15 billion years after the Big Bang—close to today. Then comes the hard part: evolution. Using a “hard-steps” framework, the model treats the path from simple chemistry to spacefaring intelligence as a sequence of extremely unlikely events. With an assumed typical number of hard steps—roughly 4 to 8—the time needed for intelligent life can be estimated, though the approach is acknowledged as simplified.

A second clock matters: how long planets stay habitable. Earth’s surface conditions are expected to remain suitable for liquid water for about another 1 billion years, totaling roughly 5 billion habitable years. If most habitable worlds resemble Earth, new civilizations would mostly stop appearing about 5 billion years after the habitable-star peak—making humanity look somewhat early. The picture changes if red dwarfs (M-type stars) are habitable. Their lifetimes can be thousands of times longer than the Sun’s, potentially extending habitable conditions for trillions of years. If life can arise on such long-lived planets, humanity’s early birth rank becomes much more pronounced.

From there, the “grabby aliens” proposal adds a mechanism to prevent too many future civilizations from forming. It hypothesizes a cosmic deadline: advanced civilizations expand rapidly through space at close to light speed, occupying habitable worlds so thoroughly that new intelligent life cannot evolve afterward. The model assumes only a small fraction of civilizations become such “grabby” expanders, and it tunes parameters so that humanity ends up with a typical birth rank rather than an extreme one. Crucially, the lack of observed alien expansion is used to infer that if grabby civilizations exist, their expansion must be very fast—around one-third the speed of light—because slower expansion would likely have produced detectable signatures sooner.

Simulations under these assumptions suggest colonization is already underway, with roughly half the universe’s volume potentially colonized, and only about one in a thousand intelligent civilizations eventually becomes spacefaring. The waiting time to encounter such civilizations is estimated at roughly half a billion years. The conclusion is explicitly conditional: the model’s plausibility depends on uncertain assumptions about red-dwarf habitability, evolutionary “hard steps,” and how often civilizations expand aggressively. Still, the framework offers a striking way to turn an otherwise empty sky into a quantitative constraint on the future of intelligence in the cosmos.