Could human existence depend on gravitational waves? A research team led by John R. Ellis of Kings College London suggests this might be the case. Key elements in our biological makeup could originate from astrophysical events driven by gravitational waves.

Iodine and bromine, for instance, exist on Earth due to a specific nuclear process triggered by neutron star collisions. These stars, orbiting in pairs, spiral inward and eventually collide because they emit energy as gravitational waves. This chain of events might directly link the existence of gravitational waves to the presence of mammals on Earth.

Human bodies mainly consist of hydrogen, carbon, and oxygen, along with various trace elements. Among the 20 elements essential to human life, those with atomic numbers below 35 are created during supernovae—explosions of stars that scatter atoms across the universe. However, iodine, crucial for thyroid hormones, and bromine, vital for collagen formation, are produced differently.

Thorium and uranium, though not directly part of human biology, have played an indirect role in sustaining life. Their radioactive decay heats Earth’s lithosphere, driving tectonic activity. This movement of tectonic plates helps regulate carbon levels by removing it from the crust, preventing a runaway greenhouse effect like the one on Venus.

The “r-process,” a rapid neutron-capture process, is responsible for producing about half of the heavy elements on Earth, those heavier than iron. In this process, a heavy atomic nucleus captures free neutrons rapidly before it can decay. Under the right conditions—extremely high neutron density and temperatures—the r-process creates heavier isotopes.

Ellis and his team estimate that the r-process has generated 96% of the 127I isotope on Earth, essential for human life. It also accounts for much of the bromine and gadolinium in Earth’s crust, along with all the thorium and uranium. Some molybdenum and cadmium also owe their existence to this process.

But where does the r-process occur? One possibility is within the material ejected during a supernova’s rebound. However, the exact physics of this process remains uncertain. Another proven site for the r-process is the merger of two neutron stars, known as a kilonova. These mergers are directly caused by gravitational waves.

As neutron stars spiral toward each other over millions of years, they emit vast amounts of energy through gravitational waves. The event GW170817, detected in 2017 by LIGO and Virgo observatories, resulted from such a merger. Kilonovae are critical sites for the r-process, given that neutron stars consist almost entirely of neutrons. Observations of GW170817 also revealed the ejected material, providing evidence of this cosmic process.

The researchers conclude that iodine, essential for human life, likely originated from r-process events in neutron star collisions triggered by gravitational waves. They propose searching for the isotope 129I in lunar regolith, which remains uncontaminated by human activities.

“Neutron star collisions happen because binary systems lose energy by emitting gravitational waves,” Ellis explains. “These fundamental physical phenomena might have made human life possible.”