by Alex Boese
By the standards of mainstream science, these were perfectly legitimate points to raise, but, as Hoyle and Wickramasinghe thought through the implications of comets giving birth to life, they came up with other arguments that proved far more controversial. It occurred to them that the rain of organisms from comets wouldn’t have stopped four billion years ago. It would have continued and could have had ongoing effects here on Earth. It could, for example, have interfered with the course of terrestrial evolution. They noted the presence of gaps in the fossil record, where evolution had seemingly taken sudden leaps forward. Perhaps these gaps indicated times when infusions of genetic material from space had caused abrupt changes in species. This suggestion outraged biologists, because it essentially called into question Darwin’s theory of evolution by natural selection.
Then Hoyle and Wickramasinghe moved their argument forward to the present. After all, there was no obvious reason why the rain of comet organisms wouldn’t still be happening today. This led them to suspect that outbreaks of infectious disease might be caused by extraterrestrial germs falling to Earth from comets, echoing ancient fears that these heavenly objects were harbingers of doom.
One somewhat whimsical piece of evidence they put forward in support of this claim involved the features of the nose. Perhaps nostrils had evolved to face down, they speculated, in order to prevent space germs from dropping into them!
On a more serious note, they pointed to a curious aspect of epidemic disease – how difficult it can sometimes be to pinpoint precisely where outbreaks begin, because they often seem to begin in multiple, geographically diverse locations simultaneously. For example, the first cases of the great flu pandemic of 1918 were reported simultaneously in India and Massachusetts. If germs travel from person to person, this is a puzzle. One would think the pattern of infection should show a steady radiation outward from a single point. Hoyle and Wickramasinghe argued that if, on the other hand, pathogens were floating down from outer space, these diverse points of origin were no mystery at all.
To bolster the case for this model of disease transmission, Hoyle and Wickramasinghe transformed themselves into amateur epidemiologists and conducted a study of an outbreak of flu that occurred in Welsh boarding schools in 1978. Based on their analysis, they concluded that the spread of the disease in the schools couldn’t adequately be explained by person-to-person transmission, because the distribution of victims in the dormitories seemed entirely random. They argued that vertical transmission (germs falling from space) better explained the pattern of the outbreak.
They even suggested that it might be possible to connect outbreaks of disease to the passage of specific comets. Here, they drew particular attention to a rough periodicity in global outbreaks of whooping cough, which seem to occur, on average, every three and a half years. This correlated uncannily well with the regular return in the sky every 3.3 years of Encke’s Comet.
By the late 1970s, Hoyle already had a reputation as a scientific troublemaker on account of his advocacy of the steady-state theory and consequent insistence that the Big Bang never happened. But, when he declared that the Earth was under constant attack from extraterrestrial pathogens, many wondered if he had completely taken leave of his senses. If he hadn’t been such a prestigious astrophysicist, the scientific community would have simply ignored him and Wickramasinghe.
As it was, critics blasted just about every detail of the germs-from-space theory. Astronomers pointed out that comets are extremely hostile environments, so it defied belief to imagine life arising inside of them. The one thing that biologists believe to be absolutely essential for life is liquid water. All life on Earth depends on it, but it’s doubtful that it could exist in liquid form within a comet. It would be frozen solid. And even if one assumes that heat from radioactive elements could maintain a liquid core inside a comet, the radiation itself would cook any organism. Either way, life wouldn’t survive.
The astronomical critiques, however, were mild compared to the scorn and opprobrium that biologists poured down upon the theory. Sir Peter Medawar, winner of the 1960 Nobel Prize in physiology or medicine, denounced it as ‘the silliest and most unconvincing quasi-scientific speculation yet put before the public.’
The aspect of the theory that particularly incensed biologists was the fact it didn’t differentiate between bacterial and viral infection. The idea of bacteria forming inside comets seemed far-fetched, but it was at least vaguely within the realm of possibility. But viruses in comets? The notion seemed completely absurd, because viruses require a host to replicate and this wouldn’t exist in a comet. It would be impossible for viruses ever to have evolved in such an environment.
There’s actually an extraordinary specificity between a virus and its host. Each virus is highly evolved to attack only certain types of cells within certain species. How could such specificity ever have been acquired inside comets travelling through space, millions of miles away from the potential host cells on Earth?
Experts also dismissed Hoyle and Wickramasinghe’s efforts at epidemiology as laughable, criticizing them for not taking into account factors such as varying degrees of pre-existing immunity or differences in the amount of pathogens shed by individuals, which can explain much of the unpredictability of patterns of disease transmission.
Hoyle and Wickramasinghe didn’t help themselves when they started adding increasingly fantastical elements to their theory. They speculated that not only was interstellar dust made of organic compounds, but that it might actually consist of vast clouds of freeze-dried bacteria. They suggested that life was so improbable that its evolution must be directed by a cosmic intelligence that was somehow coordinating what types of genetic material to shower upon the Earth. And perhaps, they mused, it wasn’t just bacteria and viruses falling from comets. In their 1981 book Evolution from Space, they argued that comets might also be dropping insect larvae into our atmosphere.
At which point, the scientific community ceased to take them seriously.
This may make it sound like the germs-from-space theory doesn’t have a scientific leg to stand on. In terms of its reputation, this is probably true. Mention it to scientists and they typically react by rolling their eyes. But this isn’t true about all the individual elements of the theory. Some parts have proven more scientifically robust.
For instance, Wickramasinghe’s observation that interstellar dust contains vast amounts of organic compounds has been confirmed, and he gets credit for first making this discovery. At the time of writing, over 140 of these organic compounds have been identified. Most astronomers wouldn’t agree that these compounds are specifically cellulose, and they definitely reject the claim that space dust is made up of freeze-dried bacteria. Nevertheless, the organic compounds are definitely out there, and scientists other than Hoyle and Wickramasinghe have concluded that this could potentially be very relevant to how life originated.
Many origin-of-life researchers now believe that these compounds might have kick-started the evolution of life by being deposited on the surface of the young Earth by comets. This scenario has been described as ‘soft panspermia’ because, while it doesn’t involve life itself coming from space, it does imagine that the building blocks of life arrived from space rather than being fashioned on Earth, as Urey and Miller believed.
Even the idea that life could originally have formed inside a comet has received some experimental support. In the late 1990s, the astrochemist Louis Allamandola simulated the ultra-cold environment of space dust inside a special chamber at NASA’s Ames Research Center. By doing so, he showed that when ultraviolet radiation struck molecules on the surface of the dust, it caused them to form into complex organic compounds. Even more intriguingly, Allamandola reported the formation of vesicles with cell-like membranes. This was significant because while organic compounds are essential to life, some kind of membrane to keep those molecules separated from the external environment is also necessary. Space dust could potentially have produced both.
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This led Allamandola to speculate that billions of years ago, these cell-like vesicles and organic compounds might both have been embedded within the ice of comets. As the comets passed around the sun, they could have been warmed and jostled around just enough to cause some of the compounds to work their way into the vesicles, thereby producing the first incarnation of a living cell.
Allamandola acknowledged this was possibly a ‘crazy idea’. He might have had the memory of the germs-from-space theory in the back of his mind. Regardless, his experimental data demonstrated that there is still a viable case to be made for the idea that life originated in comets. It hasn’t been completely ruled out of contention. And, if life did originate in a comet and arrived on Earth by that means, could life forms still, on occasion, make their way to Earth from a comet? The idea isn’t impossible. Perhaps there really are comet creatures in the family tree of life.
Weird became plausible: the vent hypothesis
In February 1977, marine geologist Jack Corliss of Oregon State University and two crewmates were cruising a mile and a half beneath the surface of the ocean, near the Galapagos Islands. They were in the Alvin submersible, a craft specially designed to withstand extreme depths, searching for a hydrothermal vent that had been remotely detected the previous year, which they wanted a closer look at. These vents are giant fissures in the ocean floor. Submarine geysers of scalding hot water, heated by contact with underground magma, blast out of them and mix with the freezing-cold water of the ocean.
They found the vent soon enough, but they also found something far stranger. Clustered around it was a thriving ecosystem that included massive clams, giant tube worms and even albino crabs, all living in complete darkness. Life was the last thing they had expected to find because, after all, they were 8,000 feet below the surface of the ocean. The pressure at a mere 2,500 feet is enough to crush a nuclear-class submarine. Biologists had assumed that the ocean floor at such depths would be sterile. In fact, the conventional wisdom at the time was that life could only survive within a relatively narrow temperature and pressure range, which the vent system was way outside of.
This discovery of the vent ecosystem is now regarded as one of the great moments in origin-of-life studies, because it led many to suspect that hydrothermal vents may have been where life began. But, when Corliss first proposed this idea following his return home, it was considered so dubious that most journals refused to publish it, and it took over a decade to gain acceptance.
Corliss didn’t develop the idea on his own. It emerged out of discussions with several of his Oregon State colleagues (John Baross and graduate student Sarah Hoffman). What led them to the concept was that, in addition to being rich in energy, vents boast a number of potentially life-friendly features. For example, the temperature difference between the burning-hot geysers and the chilly surrounding water can facilitate chemical reactions of the kind that could have promoted the complex chemistry required for the emergence of life. Plus, such systems contain significant concentrations of methane, ammonia and hydrogen, from which organic prebiotic molecules, i.e. ‘the building blocks of life’, could have formed.
In 1979, the three researchers co-authored an article detailing this vent hypothesis. They thought it was an important contribution to the debate about the origin of life, but they soon discovered that their scientific peers didn’t seem to agree. The leading scientific journals, Nature and Science, both promptly rejected it. Other editors similarly treated the hypothesis as too far-fetched to merit serious consideration.
The problem was that the vent hypothesis flatly contradicted the then-dominant ‘prebiotic soup’ model, which imagined a far gentler beginning for life. In this theory, organic molecules had first formed in the hydrogen-rich atmosphere of the early Earth and had then rained down into the oceans, where they mixed together in the warm waters like ingredients in a slowly simmering soup, before eventually combining to make living cells.
The Russian biochemist Alexander Oparin and the British biologist J. B. S. Haldane had both independently proposed versions of this theory in the 1920s. But it was in the early 1950s that the theory had really taken hold, when the Nobel Prize-winning chemist Harold Urey and his graduate student Stanley Miller conducted an experiment in which they shot a spark (meant to simulate lightning) through a mixture of what they assumed to have been the primary components of the Earth’s early atmosphere: methane, hydrogen and ammonia gases. They found that, sure enough, organic molecules such as amino acids quickly formed.
The results of the Miller–Urey experiment seemed to offer dramatic confirmation of the prebiotic-soup model, and, based on the success of the experiment, Miller rose to become the undisputed dean of origin-of-life studies in the decades that followed. He trained students who then got jobs at leading universities, carrying with them the gospel of the model and further extending Miller’s influence.
So, when the vent-hypothesis article was sent around by journals for peer review, it ran headlong into this academic wall of the prebiotic-soup faithful, to whom the idea that life might have arisen in an environment as violent and turbulent as a submarine hot spring seemed patently absurd. Miller and his disciples shot holes in the article, arguing that the extreme temperature of the vents would have caused organic molecules to quickly decompose. They also pointed out that the geochemistry of modern-day vent systems depends upon oxygen, which wouldn’t have been present when life first formed, because oxygen was only produced later by photosynthesizing surface organisms. It also didn’t help that Miller was known to be extremely protective of his status as the reigning master of origin-of-life studies.
The article did get published eventually, in 1981, in the obscure journal Oceanologica Acta. And it might have languished there forever, but, against all odds, it caught the attention of a few researchers and began to circulate around, gaining fans. This was in the days before the Internet. So, as the geologist Robert Hazen later recounted, photocopies of the article were passed from one researcher to another, defying the efforts of Miller and his disciples to block it.
The reason for the wider interest was that a few geologists had begun to have doubts about the prebiotic-soup model. Their research indicated that the Earth’s primordial atmosphere hadn’t consisted of methane, hydrogen and ammonia, but instead had mostly been gases belched out by volcanoes: nitrogen, carbon dioxide and water vapour. Organic molecules wouldn’t have easily formed in such a climate, and this significantly weakened the case for a prebiotic soup. To the geologists, the vent hypothesis seemed like a plausible alternative.
When Miller and his disciples realized the vent hypothesis was attracting attention, they aggressively pushed back against it, mocking it as a fad and continuing to stress that the vents were simply too hot to have allowed life to originate. Miller even told a reporter from Discover magazine, ‘The vent hypothesis is a real loser. I don’t understand why we even have to discuss it.’
However, the Millerites couldn’t stop the trickle of discoveries that kept adding weight to the opposing hypothesis, such as the realization by marine geologists that the vent found by Jack Corliss wasn’t a one-off. Similar hydrothermal vent systems lined undersea ridges throughout the Atlantic and Pacific. They represented a vast, previously unknown environment that would have existed on the early Earth.
Supporters of the vent hypothesis also pointed out that the extreme depth of the vents could actually have been a benefit for early life, not a drawback, because any incipient life form would have been protected from surface hazards such as asteroid strikes and ultraviolet radiation.
But the crucial development that turned the tide in their favour occurred when biologists, inspired by the discovery of the vent ecosystems, started looking elsewhere for ‘extremophiles’, or organisms that thrive in extreme environments. They ended up finding them everywhere, surviving in the most unlikely conditions imaginable: in the frozen ices of Antarctica, in the bone-dry Atacama Desert of South America and even in ro
cks up to seven kilometres beneath the ground.
Among the most remarkable extremophiles are so-called tardigrades, aka ‘water bears’. These are microscopic eight-legged creatures that live in almost every environment on Earth, from the deep sea to the tops of mountains. Research has revealed that they can survive near-complete dehydration, temperatures up to 150 degrees Celsius (300 degrees Fahrenheit) and even the vacuum of space. They could easily live through a nuclear apocalypse that would wipe humans out.
The discovery of such hardy organisms upended the old assumption, which the prebiotic-soup model had taken for granted, that life is fragile. It’s not. It finds ways to thrive in a broad range of temperatures and pressures, and this suggests it might have originated in an environment we would consider to be lethal. By the 1990s, the growing realization of this had led to the vent hypothesis being widely accepted as a viable origin-of-life scenario – much to the consternation of the Millerites.
Of course, being accepted as plausible isn’t the same as being considered the answer to the puzzle of life’s origin. Far from it. If anything, an answer to that question seems further away than ever. Since the 1980s, the number of possible locations that researchers are willing to consider has steadily expanded. Could arid deserts have birthed life? Or perhaps frozen polar fissures filled with briny water? What about hot volcanic geysers? Or maybe the first cells formed within the pores of rocks, miles underground. All these speculations now have their supporters. A significant minority even champions an extraterrestrial origin. But it was the vent hypothesis, inspired by giant tube worms and albino crabs living a mile and a half deep on the ocean floor, that first challenged the long-held assumption that life must have originated somewhere gentle.