From bacteria near undersea hydrothermal vents to single-celled algae in Antarctic ice, scientists have learned that life persists in some of Earth’s most extreme environments. David Toomey’s new book, Weird Life: The Search for Life That Is Very, Very Different from Our Own, explores whether the universe might harbor life even more extreme than these extremophiles. Here’s David on the concept of panspermia, the possibility that our most distant ancestors could be of extraterrestrial origin.

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In the nineteenth century, many natural philosophers believed that life arose spontaneously from inorganic matter, and that it was arising continuously by a process they called spontaneous generation. In 1860, French chemist and microbiologist Louis Pasteur effectively put an end to such ideas. After conducting a series of careful experiments in which a meat broth was kept separate from contamination by microorganisms, he found that after an extended period, the broth showed no new growth. In short, spontaneous generation had not occurred. It followed that life arose only from other life, and that life’s ultimate origin was sometime in the past, perhaps a very distant past. Many natural philosophers assumed that that origin was on Earth. Many, but not all. A few years after Pasteur’s experiments, British physicist Lord William Thomson Kelvin suggested that the first life on Earth might have arrived from elsewhere via “seed-bearing meteoric stones.”

In the second half of the nineteenth century and well into the twentieth, scientists had no means to replicate the journey of an organism through space. For this reason Kelvin’s hypothesis, which has come to be called “exogenesis,” was largely untestable, and it was filed away among science’s unanswered—and perhaps unanswerable—questions.

The few scientists who did entertain the possibility of exogenesis had difficulty naming an organism that might be up for the trip. Space, after all, is a notoriously hostile environment. Then, in the 1970s, biologists began to discover a great many life forms that survived and even thrived in some very unpleasant conditions, conditions that most of us would call extreme. There were tube worms in the scalding hot water near hydrothermal vents in the ocean floor, algae in the slushy brine in veins of Antarctic ice, microbes in water as acidic as that in an automobile’s battery, and even a fungus in the water core of the Chernobyl nuclear reactor, the last metabolizing in a place whose radiation levels were a thousand times greater than that which would kill a human. Organisms that might endure the rigors space travel seemed within the realm of the possible.

At the same time, the discoveries of NASA’s unmanned Mars missions, from the Viking landers of the 1970s to the Curiosity rover operating even as you read this, showed that some four billion years ago—Mars had a thick atmosphere of carbon dioxide, shallow seas of liquid water, and balmy temperatures, all of which would have made for a congenial abode for life as we know it. As it happens, four billion years ago was also a time when meteors were striking the solar system’s inner planets (Mars included) with great frequency, thus providing any microbes—if there were any, that is—an occasional means of interplanetary transport. In fact, scientists have identified a number of meteorites with a Martian origin. One of them, named ALH84001, was made famous in 1996 when David McKay, chief scientist for astrobiology at NASA’s Johnson Space Center, and his research group suggested that it bore evidence of life, including microscopic filaments that they suspected might be fossils of microorganisms. Although the group’s conclusions remain controversial, they made a great many nonscientists aware of the possibility that life might travel through space. The theory describing the possibility was called “panspermia,” a word derived from the Greek terms for “all” and “seed.”

Such a trip, say from Mars to Earth, would not be easy. A candidate microbe would have to be inside a rock positioned not so near a meteorite impact that it would be vaporized, yet near enough that that it would be launched from the surface, up through Mars’s atmosphere and into interplanetary space. There, the organism inside would need to survive vacuum, temperature extremes and intense radiation for years, centuries or millennia. Finally, still inside the rock, it would need to withstand a fiery entry into Earth’s atmosphere, concluding with an impact violent enough to leave a crater.

By the 1990s, scientists had developed the means to replicate the trip, or at least parts of it. NASA’s orbiting Long Duration Exposure Facility, to take one example, left spores outside the spacecraft, unprotected except for a thin aluminum cover, for six years. The spores were returned to Earth, and were successfully germinated. It was by no means proof of panspermia, let alone exogenesis Rather, this and experiments like it were designed with expectations in the manner of Thor Heyerdahl’s Kon-Tiki. As Heyerdahl’s voyage did not prove that preindustrial peoples voyaged thousands of miles on balsa-wood rafts, it did show that they could have. The upshot of all these experiments was that spores can withstand a violent launch and re-entry, and that so long as they are shielded from ultraviolet radiation with a few centimeters of soil or rock, they are quite capable of surviving in space for decades, long enough for travel among planets within the Solar System.

By the 1990s too, scientists had conceived of other means by which panspermia might be effected: bacteria or similar organisms embedded in dust particles propelled by solar winds or simpler yet, unprotected spores wafted to the upper levels of a planet’s atmosphere and pushed into space by magnetic fields. And there were ideas that some called “fringe.” Sir Fred Hoyle was a British astronomer who had described the process of nucleosynthesis inside stars, and was perhaps best known as the man who named the prevailing theory of the universe’s origin “big bang.” (As it happens, Hoyle did not hold with the theory, and intended the phrase derisively.) Late in his career he and Sri Lankan-born British mathematician Chandra Wickramasinghe proposed that panspermia is ongoing. They suggested not only that microscopic organisms have long entered the Earth’s atmosphere, but that they continue to do so. Hoyle and Wickramasinghe connected the periodicity of influenza epidemics with the periodicity of certain meteor streams, and concluded that the two were related: in short, that flu viruses were brought to Earth by meteors.

If that particular hypothetical instance of panspermia is (at least to some) risible, the larger theory, to many, remains viable. Of course, evidence is nonexistent, and is likely to remain so for some time. The only way to be sure that life originated elsewhere would be for scientists to identify an organism on another planetary body like say, Mars, and then find that it so closely resembled life on Earth that the two must be related. And then, the scientists would need to make a case for it having travelled from there to here, and not from here to there.

David Toomey is an associate professor of English and the director of he Professional Writing and Technical Communication Program at the University of Massachusetts Amherst. He lives in Amherst.