Where did life come from? It remains one the biggest questions in biology, and has preoccupied scientists for hundreds of years. The problem, simply put, is that the universe began as a roiling cosmic soup of inanimate matter, but eventually gave rise to a diverse multitude of lifeforms. Since the Earth is the only place in the universe where we know for sure that life exists, the search for its origins has historically been limited to terra firma.

Over the last few decades, biologists have increasingly shifted their focus outwards toward the cosmos and wondered: could life on Earth have originated in outer space?

This hypothesis, which dates back a hundred years or more, is known as panspermia. It was first theorized in a scientifically rigorous way by Chandra Wickramasinghe in 1974. Since at least Aristotle, the idea that life must've started on Earth was more or less taken for granted in the scientific community, until Wickramasinghe proposed that some dust in interstellar space contained carbon, which would make it organic—a theory he would later prove to be correct.

"There were some ridiculous counter-arguments to the effect that the idea of complex molecules in the interstellar medium is theoretically impossible because of the harsh radiation conditions that prevail," Wickramasinghe told me via email. "In recent years it has been impossible to deny the existence of complex organic molecules outside the Earth in interstellar clouds and comets. But there is still an adamant insistence that life on Earth must have started on Earth, even with organic molecules from space being added to a home brewed organic soup."

Importantly, the cosmic organic molecules that Wickramasinghe helped discover are not considered 'life,' but they are the building blocks that make life possible. These molecules are nitrogen-based nucleotides that can be combined to form larger biomolecules such as the DNA and RNA that are the core operating systems of all living things.

"The strongest evidence to support a cosmic origin of life and panspermia is the mind-boggling complexity of life."

In the years since Wickramasinghe's initial proposal, a number of alternative theories of panspermia have been advanced, such as the astrophysicist Thomas Gold's tongue-in-cheek idea that life on Earth was dumped here by an advanced alien race, which would make all of us "cosmic garbage." Perhaps the most compelling version of Wickramasinghe's theory is known as lithopanspermia, which contends that organic compounds hitched a ride through space on a comet or asteroid before crashing into Earth.