This issue’s question comes from our editor, Kirsty Crocket:
“I have heard a theory that SARS (severe acute respiratory syndrome) originated in space, and was carried to Earth by micrometeorite debris entering the atmosphere mostly over China, hence the origin of SARS in that part of the world. What is the truth behind this and how likely is this to have happened in the past and to happen in the future?”
Coatlique was the mother of everything in the universe. She gave birth to Coyolxanuhqui, the Goddess of the Moon, and to a succession of sons who became the stars. Much later, Coatlique became pregnant again, to the shame of her children who plotted to kill her. But before they could act Coatlique gave birth to the raging God of War, Huitzilopochtli, who destroyed his brothers and sisters with the help of a fire serpent, then beheaded his mother and cast her body down. Coatlique became the Earth Mother and her fertile body gave rise to all the life on Earth.
The Aztecs are by no means alone in suggesting that life on Earth originated in the heavens. Many creation myths from ancient peoples all around the world suggest much the same thing. So do they contain a grain of truth?
Certainly, the idea is not new in scientific circles. The philosopher Anaxagoras, in 5th century BC Athens, is the first recorded advocate of the theory of “panspermia”, the idea that life on Earth was seeded from space. A century later though, Aristotle largely rubbished most of
Anaxagoras’s ideas, including panspermia, and put forward his own theory of the spontaneous generation of the lower forms of life. This theory, along with the divine creation of higher forms of life, was to hold sway for more than two thousand years, reinforced by theologians like Thomas Aquinas and upheld by scientists such as William Harvey and Isaac Newton. And you must admit, observational evidence seems to give this idea credence. It’s only common sense that maggots appear spontaneously in decaying meat, frogs pop up from mud and mice creep ready formed out of rotting wheat.
The dogma of spontaneous generation was not put to the test until 1668, when the Italian physician Francesco Redi performed an experiment that showed that only meat to which adult flies had access gave rise to maggots. Redi proposed that the maggots came from microscopically small eggs laid on the meat by the flies, but this wasn’t quite enough to kill off spontaneous generation. When Anton van Leeuwenhoek discovered micro-organisms in stagnant water in 1675, many scientists took up the baton again and decided that these forms of life at least must come spontaneously from dead and decaying matter.
It wasn’t until 1862 that Louis Pasteur finally did for spontaneous generation as a theory for the emergence of the lower forms of life, with an experiment that allowed air to percolate into a flask containing a sterilised nutrient broth, but which excluded dust particles and micro-organisms. Nothing happened. Nothing grew. But the final collapse of the theory left a gaping hole in natural philosophy. If neither divine creation nor spontaneous generation accounted for the emergence of life, how exactly had it begun? A new theory became even more urgent in the latter half of the 19th century after Charles Darwin and Thomas Huxley (Darwin’s Alastair Campbell) diminished the role of God in the development of life in Earth.
In 1907 the Swedish chemist Svante Arrenhuis published a book called “Worlds in the Making” in which he postulated that life had always existed and travelled continuously through space in the form of spores, colonising likely planets as it went. Spores escaped from the upper atmosphere of seeded planets through random motion, the theory held, and were then accelerated through space by radiation pressure from the local sun. Unfortunately for Arrenhuis, just three years after his book was published researchers showed that bacterial spores cannot tolerate the intensive ultraviolet light above the planetary atmosphere and so are unlikely to survive interplanetary, let alone interstellar, voyages without protection.
As the 20th century progressed, scientists began to revisit the idea of spontaneous generation. What Redi, Pasteur and others had actually shown was that the process is not taking place now over timescales observable in the lab. And if you think about it, it is entirely reasonable that life cannot spontaneously generate today because, if it did, it would be quickly incorporated into the food chain of life that already exists. Not only that, but any group of chemicals starting to form what we might call “proto-life” would quickly be broken down again by oxidation reactions in Earth’s present oxygen-rich atmosphere. But four million years ago, when there was no indigenous life and when the Earth’s atmosphere was a reducing one with no free oxygen, could life have arisen then by spontaneous generation?
In 1953, Stanley Miller, a graduated student at the University of Chicago, working with the chemist Harold Urey, passed an electric charge through a flask containing water, ammonia, methane and hydrogen and after a few weeks found he had formed the simple amino acids glycine and alanine. This isn't life, but it is the building blocks of life and subsequent experiments showed that this trend is persistent. So long as you start with molecules that contain carbon, hydrogen, oxygen and nitrogen, amino acids common in proteins are the usual result.
These experiments have not been without their critics however. It is now thought, for instance, that the atmosphere of the early Earth did not contain predominantly reducing molecules. And although lightening storms were probably very common, they were not continuous as the electric arc in Miller’s experiment was. Still, it is widely accepted that conditions on the primitive earth are likely to have led to the production of some amino acids, if not in the quantities and variety implied by Miller and other researchers. So where did the rest come from?
In 1969, a meteorite fell on the town of Murchison, in Victoria, Australia. It proved to be rich in amino acids and so far over 90 have been identified. This suggests that amino acids can survive unchanged in the harsh environment of space and so may have been present in the Earth as it formed. From these amino acids and other simple compounds more complex structures can be built. For example, one of the products in Miller’s experiment was hydrogen cyanide (HCN). In 1961, the Spanish American biochemist Juan Oro ran a similar experiment but included HCN in his original mixture. His products included adenine, a component of nucleic acids, of RNA and DNA, and of adenosine triphosphate (ATP) the major energy-releasing molecule in cells. In this way, common processes in the oceans of the primordial Earth could have built up proteins and nucleic acids.
Remember, there were no organisms in the early oceans, nor any free oxygen. These proteins and nucleic acids would have persisted unchanged at depths out of the reach of ultraviolet light and other cosmic radiation and there, given enough time, combinations could form until, by chance, a nucleic acid molecule capable of replication gave rise to the beginning of life.
So, is the theory of panspermia dead and buried? Not a bit of it, panspermia is alive and kicking strongly. Interest in the theory was rekindled in the 1970s by the British astronomers Fred Hoyle and Chandra Wickramasinghe. They, and other teams working in the same field, have identified complex organic compounds in dust clouds in interstellar space. Hoyle and Wickramasinghe argued that primitive organisms could have evolved on the surface of such dust grains and then been transported to Earth by comets and meteorites.
The 19th October 2000 issue of Nature reported the claims of a group of researchers to have revived bacteria that were dormant as spores in 250 million year old salt crystals from new Mexico, USA. While some suspect more recent contamination in this case, viable bacterial spores are widely accepted to have been recovered from 30 million year old amber. Recently, we have found life to be hardier and more tenacious than we ever thought possible, with “extremophiles” discovered living around deep sea volcanic vents and under the Antarctic ice.
Spores might conceivably survive long space journey hidden safely away from cosmic radiation, metres deep in a meteor. Even then, over the vast timescales needed for interstellar voyages the chances of a charged cosmic particle striking bacterial DNA increase, making it unlikely that many individuals would survive. But as Hoyle pointed out, it doesn’t take many.
If interstellar seeding seems less than likely then, what about interplanetary?
Mars, being smaller and further from the Sun than the Earth is, would have cooled more quickly. It seems from recent observations that there was once liquid water on the surface of Mars and the planet may have had an early “warm, wet period” when life could have developed before it had a chance to get started on Earth. We have evidence that some meteorites found here originated on Mars and one of them, ALH84001, is controversially thought to harbour traces of ancient Martian bacteria. Life could have started first on Mars and then been seeded to the Earth. Perhaps we’re all Martians.
This wasn’t enough for Hoyle and Wickramasinghe though. They contested that microbial life not only seeded life on the early Earth, but that it continues to rain down on the planet today, brought on meteors and in comet tails. Not only that, but they also proposed that new diseases that seem to spontaneously arise, like the Spanish flu pandemic in 1918 that killed 20 million or more people, or the recent SARS outbreak, are the result of the introduction of new, spaceborne microbes. They cited the fact that many new respiratory diseases seem to come out of the Far East as evidence for their theory. As the micro-organisms are delivered into the Earth’s upper atmosphere they become caught up in the jet stream. Lower winds, pushed up over the Himalayas, pick up the organisms from the jet stream, then descend onto the plain of China, delivering the bugs there first.
Just before Hoyle’s death, he and Wickramasinghe published a paper in the Indian journal “Current Science” showing a correlation between the timing of flu pandemics and periods of high sunspot activity. The mechanism they proposed is that intense solar activity near the sunspot maximum increases the strength of the solar wind, which is channelled to Earth along the planet’s magnetic field lines, carrying a even more micro-organisms down into the jet stream. This theory has been roundly criticised but has raised new interest in the whole idea of panspermia.
So, did life originate here and are new organisms still being delivered daily? The jury is likely to stay out on that one for a long time yet.