While numerous scientists are engaged in the quest for alien life by deploying rovers on Mars, launching space telescopes, and utilizing large radio antennas to scan the cosmos, geobiologist Joseph Kirschvink speculates that the first clues of extraterrestrial existence might actually be within a Martian rock at NASA's Johnson Space Center that fortuitously landed on Earth.

In Kirschvink’s Caltech office, a monochrome image of a meteorite adorns the wall. Radiometric dating indicates the rock originated about 4 billion years ago, during a time when Mars was significantly warmer and wetter. Approximately 16 million years ago, it was ejected from Mars due to a meteorite collision that sent fragments into space.

The rock eventually settled in Antarctica's Allan Hills, where it was discovered by meteorite hunters in 1984. Named ALH84001, it was identified as Martian based on the analysis of gases trapped within its pores, which matched the atmospheric composition observed by the Viking missions that landed on Mars in the 1970s.

ALH84001 also appeared to exhibit evidence of past life, implying that life may have once thrived on Mars and potentially traveled to Earth through space. Kirschvink posits that life in our solar system may have originated solely on Mars, suggesting, “I believe there were bacteria on Mars 4 billion years ago.”

This would mean that all life on Earth, including humans, could trace their lineage back to Martian microorganisms.

For Chris McKay, a planetary scientist at NASA’s Ames Research Center, this notion is somewhat disappointing. “My mission is to look for life on other worlds,” he states, emphasizing the search for what he calls a second genesis.

McKay explains that if life is discovered elsewhere in our solar system and shares a similar biochemistry to Earth life—such as possessing DNA and familiar proteins—then it would be impossible to determine whether life is a rarity or a common occurrence in the universe. It remains unclear whether Mars transferred life to Earth or if Earth influenced Mars. In either case, both planets could be anomalies in an otherwise lifeless universe.

Conversely, finding a genuine second genesis would indicate a universe brimming with life. “The difference between one genesis and two is astronomical,” McKay asserts.

Four independent pieces of evidence suggested that ALH84001 might hold signs of life. The most visually striking was the presence of tiny tubular shapes within the rock, resembling bacterial cell fossils found on Earth. Researchers also identified mineral deposits typically produced by biological activity, along with organic compounds commonly associated with terrestrial microbes. What excited Kirschvink the most was the discovery of magnetosomes—tiny magnetic crystals used by certain bacteria to navigate Earth’s magnetic field.

In 1996, NASA boldly claimed to have found evidence of life on Mars during a large press conference and published their findings in the journal Science. President Clinton hailed the discovery, stating, “If this finding is validated, it will undoubtedly be one of the most remarkable insights into our universe that science has ever revealed.” Biologists, geologists, planetary scientists, and physicists sought samples of the meteorite to validate or challenge these claims.

Over the years, many scientists concluded that the evidence did not definitively support the idea of life on another planet. Numerous experiments indicated that the supposed “fossil bacteria,” mineral deposits, and organic compounds could have arisen through non-biological processes.

However, Kirschvink continues to argue that the magnetite crystals in ALH84001 are difficult to explain as anything other than remnants of Martian life. In 1996, he and colleagues analyzed a sample and found billions of magnetite crystals. Using a scanning electron microscope, they determined that 27 percent of these crystals were indistinguishable from those formed by Earthly bacteria.

Their results were published in 2000, and Kirschvink maintains that no plausible inorganic process can explain the magnetosomes. “The simplest explanation? Biology.”

If Mars was the cradle of life, Earth would serve as the nurturing environment.

In his office, Kirschvink displays images of magnetosomes on his computer, which resemble tiny beads. He points out that both the magnetosomes found in Earthly bacteria and in the Martian meteorite exhibit two distinct traits: their shape and purity. While natural magnetite crystals are octahedral, evolution has shaped those in bacteria into elongated beads, enhancing their magnetic properties and making them superior compasses.

The magnetosomes are also remarkably purer than typical magnetite. “Magnetite is a catch-all mineral,” Kirschvink explains. “It absorbs various contaminants, compromising its magnetic qualities. These crystals, however, are pure,” he remarks, pointing at an image on his screen. “We’ve never observed these outside of biology.” Among the four lines of evidence presented by NASA researchers in 1996, only the magnetosomes remain unchallenged, he claims. “Not even close. Many have tried, but I’ve seen nothing [non-biological] capable of producing those crystals—not even one with those characteristics.”

Nonetheless, the majority of Kirschvink’s colleagues are not persuaded by his arguments. McKay observes that magnetosomes in bacteria typically appear in chains, resembling a string of pearls, rather than as isolated crystals. “If the magnetosomes were found in a necklace formation, it would be convincing to everyone, not just Joe. If he found that in a meteorite, I would say, ‘Indeed, it is so!’” When asked if a strand of magnetosomes would serve as undeniable proof that Mars once harbored life, McKay replies, “The string would; single crystals would not.”

Steven Benner, a biochemist and director of the Foundation for Applied Molecular Evolution in Gainesville, Florida, concurs with McKay’s viewpoint but asserts that Kirschvink’s theories remain unrefuted. “Joe holds a minority opinion,” he states. “It’s an intriguing hypothesis. Ultimately, I don’t believe the case for Martian life in the meteorite has been entirely dismissed. I think Joe presents the strongest argument for a biosignature in that meteorite.”

Kirschvink is aware of the criticisms and knows that he must locate a chain of magnetosomes to sway his scientific peers. The challenge, he explains, lies in isolating magnetosomes from the Martian meteorite's hard rock matrix without damaging the very structures he seeks. The only tool capable of achieving such precision is an ion milling machine, which shoots atomic beams at the meteorite, carving material around the magnetosomes. This could reveal whether the identified single crystals are part of larger chains within the rock. Kirschvink plans to undertake this analysis with help from researchers in Japan.

ALH84001 predates the earliest known life on Earth. Thus, if Kirschvink discovers his magnetosome chains, it would imply that life likely began on Mars before it did on Earth. Although Mars currently presents a cold, arid landscape with an atmosphere only one-hundredth as dense as Earth's, it was far warmer and wetter 4 billion years ago. NASA’s rovers currently traversing Mars have uncovered evidence of ancient lakes and riverbeds, suggesting the presence of shallow seas enveloped by a significantly thicker atmosphere.

While many scientists believe that liquid water is crucial for life, Kirschvink contends that early Earth may have had an excess of water. “The strongest evidence indicates that early Earth was entirely ocean-covered,” he states. Without any land, the fundamental chemical components of life would struggle to form. “The reason is straightforward... to bond two amino acids to form a protein, water must be removed.” This would have been impossible if amino acids were submerged in an ocean. Life required some land—a literal beachhead—to initiate. While ancient Earth may have lacked dry land, Mars undoubtedly had it.

“Much of this is contentious since we’re discussing a world from 4 billion years ago,” Kirschvink acknowledges. “However, it’s evident that Mars possessed southern highlands and what increasingly resembles a northern polar ocean basin. If volcanic landscapes were present alongside rainfall, streams, and rivers—if life had managed to emerge there, it would have flourished.” This scenario, which Kirschvink finds highly plausible, carries extraordinary implications: Life, having originated on Mars, may have migrated to Earth via meteorites. This would mean that all life on Earth, including humans, could be descendants of Martian microorganisms. Kirschvink believes that we won’t uncover our first extraterrestrial beings on distant planets—we merely need to look in the mirror. “I genuinely believe we are Martians,” he asserts. He argues that life on Mars is unlikely to represent the second genesis that McKay seeks.

Although the notion that life could have been seeded on Earth through comet or meteorite impacts has existed for over a century, it remains a minority viewpoint among scientists, most of whom hesitate to accept a Martian ancestry for Earth life. Mars is located over 30 million miles away—a significant distance for either humans or microorganisms. However, a rock launched from Mars could reach Earth in as little as six months, and given the resilience of bacteria, it’s plausible that life could have arrived intact. Bacterial spores have survived on the outer surface of the International Space Station for 18 months while exposed to the vacuum and harmful radiation of space. In a recent experiment, Swiss researchers applied strands of bacterial DNA to a rocket’s exterior, which remained viable even after the spacecraft’s fiery re-entry into Earth’s atmosphere.

They may need to delve deep, but Kirschvink believes there is over a 50 percent chance that we will encounter our first alien.

If Martian bacteria did arrive here billions of years ago, they would have been introduced into a vast nutrient-rich environment—Earth’s oceans, filled with dissolved carbon dioxide, iron, and phosphorus. While life might struggle to emerge in an aquatic world, once it evolved—perhaps in a drier setting—it could thrive in Earth’s seas. Newly arrived Martians would face no competition. “Everything life needed would be in that ocean, ready for life to emerge,” Kirschvink explains. “Then: Splash! A meteorite from Mars breaks apart, releasing entombed bacterial spores. A single organism capable of reproduction could proliferate exponentially, meaning the first self-replicating microorganism that arrived would dominate.”

What became of Mars? If life did originate there, why did it never evolve into a diverse ecosystem akin to Earth’s? Why did its ancient rivers and oceans vanish? Mars, it seems, was simply too small to sustain life indefinitely. With barely one-tenth the mass of Earth and less than half its gravity, Mars could not retain its atmosphere. Life-essential gases gradually escaped into space, leading to Mars’s transformation into the frigid desert it is today.

“If Mars was the mother planet, Earth would have been the nurturing planet,” states Kirschvink. His narrative carries a tragic undertone regarding the origins of life. One factor that might have made Mars an ideal birthplace for life—its smaller size, which allowed it to cool more rapidly than Earth—also meant that life could not continue to thrive there. “A special solar system may be required for a Mars-like mother planet to infect its neighbors. It may be that these mother planets are destined to perish while their neighboring planets, like Earth, adopt the offspring.”

As Kirschvink studies ALH84001 for magnetosome chains on Earth, other scientists are preparing to explore Mars’s surface. Researchers at NASA’s Jet Propulsion Laboratory have outlined a mission to return Martian rock samples to Earth. While this mission currently lacks a timeline or funding, McKay believes that if bacteria ever existed on Mars, their fossil remains are waiting to be discovered. “Magnetosomes are quite resilient,” he notes. “They are the bacterial equivalent of bones.”

Some scientists speculate that future missions might uncover more than just magnetosomes and fossils on Mars. Gaetan Borgonie, a biochemist from the University of Ghent in Belgium, suggests that life might still exist underground, similar to organisms he discovered in unexplored subterranean ecosystems on Earth.

Beginning in late 2008, Borgonie led a team that discovered the deepest land-dwelling creatures on Earth—a new species of worm residing over 2 miles below the surface in a South African gold mine. They named the worm Halicephalobus mephisto, after the demon in the Faust legends. Though only half a millimeter long, the worm is significantly larger and more complex than the simple bacteria scientists anticipated at such depths. Dubbed the "worm from hell" in some reports, it feeds on bacteria that thrive on minerals in the surrounding rocks.

This discovery is steering scientists in their quest for extraterrestrial life. Much of life on Earth is small, simple, and deep, and the same may hold true for Mars. “If life originated there, those organisms would have had 3 or 4 billion years to evolve, and they might still persist. Life always finds a way. Always,” Borgonie asserts. “If NASA or ESA begins digging on Mars, they will need to dig deep, but I believe there’s a better than 50 percent chance we’ll encounter our first alien.”

He gestures overhead, mimicking the path of an icy eruption. “It’s like free samples, take one!”

Kirschvink shares this sentiment. When asked, as a fellow Martian, whether life might still exist on Mars today, he responds that it’s definitely not on the surface. “The atmospheric pressure is so low that water boils. Most of the planet remains well below freezing most of the time. However, we know that once life evolves, it can adapt to extreme environments. It has done so on Earth. It has gone deep; it has ascended high. It didn’t originate in those environments but migrated into them. If life began on Mars, it likely adapted to those harsher conditions and may still be managing to survive.”

Not everyone agrees. Steve Mojzsis, a planetary geologist at the University of Colorado at Boulder, argues that the absence of clear signs of life on Mars undermines the idea that it ever existed there. “Life on Earth occupies the driest deserts, the coldest glaciers, the highest mountains, and the deepest sediments—it permeates our planet; there’s a layer of biological activity everywhere. Mars? There is no evidence of it whatsoever. We’ve dispatched a fleet of spacecraft to Mars—it’s the inverse of 1950s space horror movies—and we can’t find anything.”

The resolution may arise from further exploration. McKay is advocating for a mission to search for subterranean Martian life by drilling about a meter into frozen ground near the planet’s north pole. Named Icebreaker Life, this project could launch in 2018 if NASA grants approval. McKay initially aimed to drill 10 meters to penetrate surface layers that might have been sterilized by intense solar radiation. However, building a 30-foot drilling rig on a spacecraft would exceed the mission’s budget.

An alternative—transporting a disassembled drill on a spacecraft to reassemble it on Mars—would also be impractical, as no existing robot possesses the dexterity needed for such a task. Consequently, McKay and his team have opted for a one-meter-long drill that will stand upright once the spacecraft lands.

McKay has tested some Icebreaker Life components in Antarctica, demonstrating that a prototype drill can penetrate 3 feet of frozen ground in about an hour. Once operational, Icebreaker Life will use a brush to collect samples every 2 inches of drilling, passing them to instruments that will test for enzymes and other signs of life—potentially even intact, living microorganisms.

If life were to be discovered on Mars, genetic analysis would determine its relationship to Earth life. Either Kirschvink’s theory about Mars as the mother planet would be validated, or McKay would find his second genesis. Even without a second genesis on Mars, the possibility remains that life could have independently arisen elsewhere—on one of the many planets orbiting distant stars, or even further within our solar system.

At the top of McKay’s list of locations to search for life is Enceladus, one of Saturn’s moons. It harbors an ocean beneath layers of ice and features over 100 geysers that propel ice into space. If life exists in Enceladus’ ocean, the geysers may contain chemical signatures of its presence. McKay can barely contain his excitement when describing the moon’s plumes, gesturing as if tracing the path of an icy eruption. “It’s like free samples, take one!” he exclaims.

McKay has already conferred with Japanese researchers to discuss a collaborative mission to Enceladus. He envisions sending a probe through the moon’s geysers, deploying a mechanical catcher’s mitt coated with sticky gel to gather material expelled from the geysers. The spacecraft would then return the samples to Earth. This technology was previously employed to collect matter from a comet’s tail in 2006, presenting no new engineering challenges. Since Saturn is over ten times further away than Mars, a shared ancestry for life on Enceladus and Earth would be less probable, thereby enhancing the likelihood of McKay’s second genesis being found in the depths of that remote ocean.

If Mars, Enceladus, and other potential worlds in our solar system are found to be barren, we know there are many other planets that may be hospitable to life. NASA’s Kepler space telescope has identified over 1,000 planets around other stars, and astronomers estimate that our galaxy could host as many as 40 billion Earth-like worlds. Should life be discovered on any of them, the vast distances would almost guarantee a second genesis. What are the chances that such a vast expanse of cosmic real estate is uninhabited? As Scottish philosopher Thomas Carlyle remarked more than a century ago, “If they are not inhabited, what a waste of space.”

Tim Folger writes about science and environmental issues for National Geographic, Discover, Scientific American, and other publications. He is also the series editor for the annual anthology, The Best American Science and Nature Writing.

This article was originally published on Nautilus on June 18, 2015.