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.
"The strongest evidence to support a cosmic origin of life and panspermia is the mind-boggling complexity of life," said Wickramasinghe. "The information content of the simplest living cell is specific in kind and superastronomical in quantity. This points to a system of scale of which exceeds the scale of our planet, our solar system, our galaxy and perhaps much beyond."
That all life on Earth may have an extraterrestrial origin is an enticing idea, but before it can be taken seriously, there are a lot of questions to answer. For example, can organic compounds found on Earth survive the harsh space environment? How would these organics have formed in space in the first place? And even if these organics could survive in space, would they be able to withstand entry through Earth's atmosphere?
Astrobiologists have been taking their experiments to space to see how terrestrial life holds up in this harsh environment, where the lack of oxygen, constant radiation exposure, and freezing temperatures would seem to preclude it.
The first attempt to test this hypothesis was in 1966, when two Gemini missions exposed the bacteriophage T1, a type of virus that replicates in bacteria, and Penicillium roqueforti to the vacuum of space for a few hours. These particular samples were chosen due to their commonality on Earth. But as the experiment would show, the specimens didn't fare all that well and became inactive after a certain threshold of UV radiation exposure was reached.
Research into panspermia got a new breath of life in the 1990s, when the European Space Agency launched its Exobiology Radiation Assembly mission to see how radiation affected different spore samples. As the ESA discovered, solar radiation caused strand breaks in the spore DNA, which induced mutations and significantly reduced their survival. Moreover, all the spores that were cased in artificial meteorites were killed, although the survival of those same spores increased significantly if they were encased in glucose, a simple sugar circulated in the blood stream.
Around the same time, the Russians launched their BIOPAN program, which would expose various organics to space for up to 17 days at a time. Incredibly, the Russians found that some bacteria, spores, lichens and even one animal (the tardigrade) were able to survive in the outer space environment. These organisms are rightly known as extremophiles, the most famous of which is the tardigrade, otherwise known as the 'space-bear.'
Astrobiologists have been finding other examples of lifeforms capable of existing where no one thought life should be possible, albeit life forms that are far less complex.
In 2013, for example, scientists found hundreds of microbes that were able to live half-a-mile under the Antarctic ice. Understanding how these creatures survive in such hostile conditions may help open up the search for life on other ice-locked worlds, like Jupiter's moon Europa.
The last decade has seen a flurry of activity investigating panspermia, including the space-based EXPOSE mission that ran from 2008 to 2015. During this mission, which was meant to approximate radiation on Mars to see how life would fare under these conditions, astronauts on the ISS exposed a variety of biomolecules and microorganisms to space for around a year and a half at a time. As the researchers found, some of these organics, like green algae, were able to survive for up to a year and a half, far longer than anyone expected.
The same year EXPOSE was launched, an analysis of organic compounds found in the Murchison meteorite, a large space rock that fell to Earth in 1969, suggested that these compounds were extraterrestrial in origin. This means that many of the organic compounds that are the necessary building blocks for life were already present in the early solar system and could also survive entry into the Earth's atmosphere. This was further boosted by a 2011 meteorite study by NASA, which suggested that adenine and guanine, basic building blocks of DNA, may have formed in space.
As radio telescope technology continues to improve, astronomers have been able to discover the presence of organics in space at incredible distances. In 2012, some Danish researchers reported finding glycolaldehyde—the simplest sugar and a requirement for making RNA—in a star system that is 400 light years away. The following year, researchers using the Atacama Large Millimeter Array, a major radio telescope array in Chile, found a prebiotic molecule called cyanomethanimne in the ice particles of a giant cloud of interstellar gas some 25,000 light years from Earth. This molecule produces adenine, one of the four nucleobases that form the rungs of the DNA ladder.
All of this evidence seems to support the notion that the organic compounds needed to create life could be created in space. But since we have yet to find any full-fledged microorganisms drifting through space, the presence of these organics begs the question: how would they have ended up on Earth?
In late 2014, Czech scientists bolstered the idea that these organic compounds may have hitched a ride to Earth on a meteorite when they were able to form complex DNA and RNA organic compounds such as uracil and thymine using starting chemicals found in meteorites, and operating under conditions found in outer space.
The following year, the Japanese Space Agency launched the Tanpopo mission to the ISS in 2015. Tanpopo will be exposing amino acids to space for periods of one, two and three years. At the same time, the mission will also be collecting samples of cosmic dust in aerogel to see if microbes can be detected in the upper regions of low earth orbit (around 400 miles above the Earth's surface).
"We are investigating the possibility of interplanetary migration of microorganisms," Kensei Kobayashi, the lead research on the Tanpopo mission, told me during the recent NASA astrobiology conference. "Cosmic dust is a very promising carrier of organic compounds, but it's exposed directly to solar radiation. Our hypothesis is that there are some kinds of extraterrestrial compounds that can survive and be delivered to Earth by cosmic dust."
Researchers at the European Space Agency are preparing to launch the OREOcube, which will be attached to the ISS and will focus on exposing "organic thin films" deposited on an inorganic substrate (read: rocks) to space to see how the Sun affects organic-inorganic interactions. The main goal is to see how photo-chemical evolution may affect the survival and transportation of organics through space. These organics will consist of amino acids, nucleobases and polyaromatic hydrocarbons (PAHs), the possible starting materials for life, which NASA estimates to be associated with up to 20 percent of carbon in the universe.
All this still doesn't preclude the far more mundane hypothesis that life began right here on Earth. A recent experiment at the Czech Academy of Sciences managed to recreate the four bases of DNA in a lab by recreating conditions found on Earth about 4 billion years ago, when our planet was getting its ass kicked by asteroids and meteors.
Even if life did begin on Earth, the theory of panspermia may very well lay the groundwork for the discovery of extraterrestrial life in our own solar system.
"I think that scientists are at last coming round to the view that habitats of life are widespread in the universe," said Wickramasinghe. "If there is life anywhere it is clear that life must be present everywhere. This follows from the inevitable mass exchanges that take place over astronomical timescales between neighboring bodies. Acknowledging the fact of our cosmic origins would hopefully imbue us with a sense of unity of all life."