Imagine yourself on a crash course with certain death. Or maybe not so certain: It depends. You are a microbial hitchhiker on cosmic debris flung from the darkest recesses of outer space. There is a slim chance that the planet you are going to collide with has an environment conducive to your survival and propagation. Considering you have survived both the extremes of pressure and acceleration, as well as exposure to thousands of millions of years of space, it is a good a chance as any. But for the moment, you are helpless, tucked away inside the crevices of rock and ice, lying dormant in your state of stasis. The hour of truth arrives. To your immense delight, your intergalactic venture has borne fruit. The planet is no paradise per se; yet in the throes of its birth. Having survived the ordeal of cosmic dispersal, you are dogged and non-fussy; there are no complaints. You go about usual business, dividing, multiplying, and disseminating your life essence into that primordial world.
For millennia, Man has pondered over the question of the origin of life on Earth. The mystery has been a subject of intellectual rigor through the ages, with a slew of proposed theories ranging from those based on doctrines, to those entirely scientific that strive to address the problem in a coherent and consistent way. Some of them may seem to be borderline science fiction. One of these is the Panspermia theory, which suggests that life on earth has its roots in the stars, and may have been transferred, or ‘seeded’, here from other planets, or even from other star systems, via microbial contamination of meteoroids, comets or planetoids. While this hypothesis may sound far-fetched, evidence accumulated over recent years suggests it may not be that implausible.
Evidently, surface debris is continually being transferred between planets and moons in our solar system, ejected outwards at high velocities by the collisions of asteroids with planetary surfaces. Sometimes, this debris may clump together to form smaller asteroids, which may be propelled to other space bodies. Even a relatively short time after its birth (between 4.1 and 3.8 billion years ago), Earth experienced a phase of Late Heavy Bombardment, enduring a series of widespread and powerful meteor showers. Increasing evidence suggests that a variety of organisms, mainly microorganisms, could have been carried along. Fossils discovered by UNSW scientists in hot spring deposits in the Pilbara region of Western Australia have suggested that the earliest life form, though not of extraterrestrial origin, was present around 3.48 billion years ago, converging with the bombardment phase. But the proponents of the Panspermia theory have a hard time explaining how the micro-organisms got there, since there is not, as of yet, any conclusive evidence of the existence of extraterrestrial life.
Pilbara region of Western Australia, where oldest evidence of microbial life was found.
The relative abundance of organic compounds in outer space could support the idea of extraterrestrial life. A variety of organic compounds have been uncovered on meteors that have landed on earth, for e.g. Carbonyl, a fundamental organic building block, and amino acids, the basic units of proteins. Apart from organic matter, two inorganic elements that allow the precursors of life to form were certainly unavailable on early Earth, but may have been present on early Mars, as noted by chemist Steven Benner. Highly oxidized molybdenum, which may have been crucial to the origin of life, could only have been available on the surface of Mars and not on Earth, because three billion years ago the atmosphere of earth was deficient of oxygen. RNA (ribonucleic acid, the earliest expression of genetic reproduction) falls apart when built in water, a process which can be prevented by the presence of the other element, boron. However, boron was too scarce on early earth to support widespread creation of RNA. “Analysis of a Martian meteorite recently showed that there was boron on Mars; we now believe that the oxidized form of molybdenum was there too”, explains Benner, “It’s yet another piece of evidence which makes it more likely life came to Earth on a Martian meteorite, rather than starting on this planet.” Although these findings are a step forward in helping us understand how early life could form, it does not provide absolute proof of its existence.
Even if there was a possibility of the universe being littered with extraterrestrial life, advocates of Panspermia must still determine how it ended up here. The best candidates for interplanetary travel, they argue, are bacterial spores, which allow bacteria to stay dormant in the absence of nourishment. Extremophiles, microorganisms with extraordinary capability to thrive in the harshest environments on earth, have been known to thrive in temperatures ranging from as high as blood curdling 122°C to as low as a chilling -15° C. Some are also capable of surviving high levels of ionizing radiation-which would otherwise cause radio-chemical damage-most commonly UV radiation. But on the issue of Panspermia, the question arises if bacteria or bacterial spores are resilient enough to withstand the extremities of outer space.
Deinococcus radiodurans, an extremophile (domain: bacteria)
“Bacterial spores: too tough for space?”
To address this question, scientists have carried out experiments aboard the International Space Station, under Project MERCCURI, that have shown that some species of bacteria found on earth have made themselves right at home in space. The vast majority of the 48 samples of bacteria collected by members of the public and microbiologists that were sent grew at a rate very close to that on earth, some even faster. Microbes have also been shown to survive outer space, provided they are shielded from UV radiation (which is very likely when traveling between planets in comets or meteorites).
Meteorite ALH84001 under microscope
However, distances between cosmic bodies are colossal. Spores may have to spend millions of years hidden inside a comet before arriving on a host planet. In 2000, Scientists had already isolated and revived a 230 million-year old bacterial unit-the oldest living thing ever brought back to life-nestled in a tiny, brine-filled bubble inside a salt crystal 1,850 feet underground in the Permian Salado Formation. This discovery suggests that spores could remain viable for up to 250 million years in a dormant or fossilized state, and could possibly survive a voyage through space.
The theory of Panspermia does not address the means of the creation of life, but only how life may have dispersed throughout the cosmos. The mystery of the genesis of life, thus, remains largely unsolved. However; the discovery of novel evidence may lead to the modification of already known facts. It is plausible that space-microbes may not have been the progenitors of all life on earth. On the other hand, extraterrestrial life may not be as alien as we think: perhaps someday humanity will unearth the secret of how a tiny cosmic hitchhiker settled on a rock hurtling through space, breathed life into our planet.
Note: This article was originally published on Raw Science. It can be found at http://www.rawscience.tv/cosmic-hitchhikers-and-the-genesis-of-life-panspermia-theory/
“Evolution from Space” by Hoyle, F. and Wickramasinghe, N.C. (1981).
“Origin of Life: The Panspermia Theory”, 2008, scientific paper by Sonali Joshi submitted to IGP and published on SiS, Northwestern University
Images: Google images for Public use.
Written By: KAMRAN NAVEED SYED & SAIF ULLAH KHAN
About the writers:
Saif is currently an 18 year old A levels student at Lahore Grammar School Johar Town Senior Boys Branch in Lahore, Pakistan and head of the Astronomical Society there. He is passionate about spreading space science education to youngsters. In his spare time,he loves to do swimming and farming. He can be reached at firstname.lastname@example.org
Kamran Naveed Syed: Kamran is currently a 17 year old A levels student at Lahore Grammar School Johar Town Senior Boys Branch in Lahore, Pakistan and an admirable member of the robotics club there. He is zealous about writing anything that is scientifically adventurous. In his spare time, he enjoys playing cricket and listening to electro house music. He can be reached at email@example.com