Who said life began in the primordial soup? Life may have come from outer-space

Microbes on earth.... and in the cosmos

By Professor Chandra Wickramasinghe

Until recently it was thought that the total number of distinct bacterial species on earth was a mere few thousand. Today, using modern techniques to detect bacterial RNA rather than bacterial cells, it is estimated that the total number of bacterial species might be well in excess of a billion.

The estimated number of bacterial cells in all terrestrial habitats is about 5X10A30 (5 followed by 30 zeros!) exceeding by a huge factor the total number of stars in the entire observable universe. So here is the first truly astronomical attribute of terrestrial bacteria.

The vast majority of these bacteria are thought to be what are called extremophiles, bacteria seeking extreme and hitherto uncharted environments. They have not yet been cultured, perhaps never will. They lie in surface soil and surface water, evidently doing nothing; perhaps they are waiting for the right host to emerge. Perhaps they are falling from the skies.

This latter idea seemed wildly outrageous when we first proposed it over two decades ago. But now, with the passage of time, and the accumulation of new evidence, it is quietly slipping into the realms of orthodox science.

Our story begins in interstellar space, in the gigantic dust clouds that lie between the stars.

The cosmic dust, as seen in the dark patches on the photograph of our Milky Way, was thought 25 years ago to be comprised of inorganic ice particles, similar to the particles that populate the cumulous clouds of the Earth's upper atmosphere. From 1974 onwards, I had argued in favour of an alternative organic model for this dust, and this model has come to be fully vindicated by new data that has come to light.

What the data shows beyond a shadow of doubt is that cosmic dust possesses sizes and compositions that cannot be distinguished, through remote sensing spectroscopic techniques, from freeze-dried or desiccated bacteria. The total mass of bacteria or bacteria-like organic particles in the galaxy is truly phenomenal, some 10A 33 (1 followed by 33 zeros) tonnes, involving about one in every three carbon atoms in the whole of interstellar space.

In interstellar clouds cosmic microbes would be at a temperature of minus 230C, and could therefore not replicate. They lie there in a state of freeze-dried dormancy until such time as they become incorporated in new star and planet systems.

The outer regions of a newly formed planetary system become populated with hundreds of billions of individual comets, as indeed we find today around the Sun in our own planetary system, beyond the orbits of Uranus and Neptune. It is within the lukewarm interiors of such comets, where liquid water and all the right nutrients are present, that anaerobic microbes can replicate. And replication occurs explosively. Given the right nutrients a single microbe in a culture medium doubles in 2-3 hours, these double again in a further two hours, and so on....

Sequential doubling with a continual supply of nutrients thus leads to a culture of the size of a sugar cube in 4 days, and after a total of 120 doublings and in a mere 12 days an entire comet of size 10 kilometres could be completely converted into bacterial matter.

Bacteria have the most remarkable survival properties. Some types of bacteria, the thermophiles, can replicate in superheated water at temperatures well above 100C. A viable strain of the bacillus Streptococcus mitis was recovered within a TV camera, after two years of exposure to conditions on the surface of the moon.

Bacteria can be taken down to near zero temperature and near zero pressure, provided suitable care is exercised in the experimental conditions. Bacteria can survive after flash heating under dry conditions at temperatures of 600C.

Viable bacteria have been recovered from the interior of an operating nuclear reactor. Bacteria recovered from Antarctic ice drills have shown viability after dormancy for close on half a million years. These are not properties one would expect to have evolved at the surface of the Earth, but they are all properties necessary for survival in space.

When different genes are used for constructing evolutionary trees, several equally probable connections emerge. The genes of archeae, bacteria and eukarya all display considerable intertwining and intermixing near the trunk, so much so that it is often not possible to say which came before which. There is even more disturbing evidence to suggest that genes of eukarya (our direct ancestral cell-line) might be more primitive than those of the simpler archaea and bacteria.

The canonical tree of life is fast collapsing at its trunk, suggesting that its presumed roots may not be on Earth at all, but perhaps much further out in the cosmos. In our own solar system it is the comets, a hundred billion of them, that serve as both incubators and distributors of microbial life. Recent studies of comets seem to be fully consistent with this point of view.

Infrared studies of comets, such as comet Halley, have shown a spectral signature that matches that of bacterial cells. Comets, when they approach the inner regions of the solar system, throw off bacterial type material at a prolifc rate, typically some 10 million tonnes per day. This happens not just for comet Halley, but for every single comet - including the recent spectacular comet Hale-Bopp, in which case the dust emission rate was even greater by a factor of 100 or more.

Most of the material expelled from comets eventually finds its way into the depths of interstellar space, thus serving to amplify the store of galactic microbial matter. But some of it stays within the solar system to rain down on planets like the Earth.

This in our view is how life on Earth must have originated some 4 billion or so years ago. Upto this time the Earth was bombarded so fiercely by comets and meteorites that no life could have either emerged, or survived here. It has been discovered very recently that the oldest evidence of microbial matter on Earth dates back to before 4 billion years, and this takes us well into the epoch of fierce impacts.

In a soup

Next, let's note that the old arguments for primordial soup on the Earth begins to wear thinner by the day. In the 1950's and 60's laboratory experiments of Harold Urey, Stanley Miller and Cyril Ponnamperuma were hailed as proof of a primordial soup.

Starting from a mixture of inorganic gases such as water, methane and ammonia it was shown that complex organics, including amino-acids could be formed through interaction with various forms of high energy radiation, such as might occur in lightning or through exposure to ultraviolet light from the sun. For this type of process to work it was necessary to have an early Earth atmosphere that was reducing, not oxidizing. Sadly for the primordial soup protagonists, geochemists discovered in the 1980's that the Earth's early atmosphere was oxidizing not reducing. Under such conditions the Urey-Miller scheme simply does not work.

But even if by some miracle it did work, and the Earth's oceans came to be filled with all the chemical building blocks of life - the amino-acids, nucleotides and so forth - this is still a far cry from life itself. Perhaps the most difficult problem to resolve in a purely terrestrial context, one that has not even been touched upon unto the present day, concerns the origin of the information content of life.

The information needed to put life together, even in its simplest and most primitive form, is specific in kind and superastronomical in quantity. How was this highly specific information acquired in the first place from a situation that was initially thoroughly chaotic? From a very simple calculation, one could infer that the minimal number of trials required to discover the crucial arrangements needed for life, as for instance in the enzymes, through random shufflings of the components, far exceeds anything that could happen in all the oceans of the Earth, let alone in Darwin's "warm little pond."

To set the problem in its correct perspective let us suppose that a quarter of a million citizens of this city are each supplied with an unbiased cubic die, and suppose further that we ask everyone at the stroke of midnight to throw their dice. The chance of finding life from random shufflings is similar to the chance of each one in the quarter of a million population simultaneously throwing a 'six'! If honesty prevails one has to admit that the ultimate origin of life is an event so improbable as to verge on the miraculous. So to constrain this event to our tiny planet is not only unnecessarily restrictive; it is pre-Copernican in philosophy. And there is no logic whatsoever to demand that.

Evidence

To sum up the argument thus far, the evidence for life being truly cosmic is growing steadily by the day, and this comes from a variety of sciences - astronomy, biology, geology amongst others. As the curtain begins to fall on the 20th century, a long overdue paradigm shift seems destined to occur.

My final remarks concern some medical implications of the life-from-space theory, which have wide-ranging implications for mankind, but have so far remained controversial. Here remains, in my view, the last bastion of pre-Copernican thinking that has to be conquered. If microbial life is a truly cosmic phenomenon and if terrestrial life arrived here from comets, that process could not have stopped at some distant time in the past. The daily input of cometary material to the Earth is known to be about 100 tonnes. Even if only a percent of this were biogenic, the probable annual input of bacterial cells is some 10A23 (1 followed by 23 zeros) per year, and that of viruses, viroids, fragments of DNA and even proteins, even perhaps prions, that can interact with terrestrial life, could be well in excess of 1OA25 (1 followed by 25 zeros). These numbers are vastly in excess of the totality of viruses exuded by all humans in a whole year.

Before giving a few examples of diseases that may be driven from outside, let us look at microbial (or viral) disease from the point of view of the survival of a pathogen. If a pathogen is unable to find and to attack a suitable host, it obviously becomes extinct.

But just as certainly, too much of an ability to attack a host also threatens extinction, following the wholesale death or extinction of the host. Long-term survival of any pathogenic micro-organism demands a perpetual walking of a tightrope, not attacking too weakly, and not attacking too strongly. Regarded in this way, it seems surprising that so many pathogens appear to have walked the tight-rope successfully, not just in our own day but throughout history as well.

According to the present point of view there is no question of walking on a tightrope. No pathogen will survive very long unless it is subject to repeated renewals from outside.

The lower the population density of the host, the more unlikely it is that a pathogen survives through person to person transmission. So how did the measles virus, for instance, and other similar pathogens, maintain themselves among the relatively small human populations that existed in prehistoric times? The conventional answer is that they survived lean times by attacking some other host, some host more common than humans.

But this clearly does not work. Measles, for instance, is specific to humans, at any rate in the present day.

Throughout recorded history a long succession of epidemics have struck our planet suddenly and mysteriously, and disappeared equally suddenly and perhaps equally mysteriously

In a few instances, doctors in ancient Greece, Hippocrates, for example, gave remarkably good descriptions of the diseases of their day. Some of the descriptions could be identified with modern diseases, whilst others could not. The patterns of human infections would seem to have changed.

The first descriptions of recognisable modern diseases tend to be found from about the end of the 16th century, for example the first description of influenza. Why were there no good descriptions before? The answer may be that influenza was probably introduced at precisely that moment in time. (See Box)

As we prepare to enter a new millennium it seems amply clear that we cannot afford to remain oblivious to the widespread cosmic prevalence of micro-organisms. There are experiments in progress to search comets and planets in our solar system for microbial life. A few months ago a team of Indian scientists have embarked on a project to search the Earth's upper atmosphere for the presence of extraterrestrial micro-organisms. If the next great plague were to come from space, as many seem to have done in the past, then space technology could serve to forewarn us, and hopefully help the medical profession to plan strategies that might alleviate its worst consequences.

Influenza mystery

At the end of the last century the distinguished English physician Charles Crieghton maintained that influenza could not be a transmissible disease. He adduced evidence from three epidemics in 1833, 1837 and 1847 to note that populations living in vastly separate areas were affected almost simultaneously.

From this Crieghton concluded that the causative agent of the disease descended over the land like a miasma, in much the same way as would an aerosol introduced at the top of the atmosphere. Crieghton's evidence has not gone away even after the viral nature of the causative agent was discovered. Instead, it has become stronger with every passing year.

The year 1918 is infamous in medical history for a devastating world-wide pandemic of influenza, which is said to have killed more people than all the battlefields of the First World War.

In writing the history of the 1918 pandemic (New England Journal of Medicine, May 1976) L. Wienstein pointed out that there were several distinct waves of influenza in 1918, the first not having very serious effects. It was the second wave, caused presumably by a modified form of the influenza virus that did the big damage. Of this second wave Weinstein wrote:

"The lethal second wave involved almost the entire world over a very short time. It was detected in Boston and Bombay on the same day but took days to weeks before it reached New York City.."

How in 1918, when travel from Boston to Bombay took of the order of four weeks by boat, did a virus manage by person-to-person transmission to cross the thousands of miles between these two cities in a day?

An easy explanation of all these facts would be in terms of a virus, or fragment of a virus, perhaps a trigger, arriving from space and drifting to ground level through a turbulent lower atmosphere.

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