19 May 2006
Into The Shadows - Searching For Alien Life On Earth
By Rusty Rockets
People with obsessive-compulsive hand-washing syndrome should stop reading right now. As well as all the microbes we do know about, some scientists suggest there could also be alien microbes living on Earth as well. This as yet undetected alien population of microbes could exist quite happily next to our own, and as such has been referred to as the "shadow biosphere". How does this alternate biosphere affect us? Well, nobody is really sure. In fact, nobody is entirely certain that alien life does exist alongside known life, but some scientists think that there is a scientific basis for believing that it does.
Philosophy professor Carol Cleland first coined the term "shadow biosphere" in 2005, while heading an investigation into the definition of "life" at the NASA-funded CU-Boulder Center for Astrobiology. Professor Cleland described Earth's shadow biosphere as: "�a microbial biosphere that is so chemically and molecularly different from life as we know it that it wouldn't be in direct competition with familiar life; familiar life couldn't metabolize it and it would occupy ecological niches that were under populated by familiar organisms. Such organisms might have proteins made of completely different amino acids or amino acids with the opposite chirality, or it might have nucleic acids whose sugars have the opposite chirality, to name a few possibilities," explained Cleland in her project report on "Philosophical Issues in Astrobiology".
Professor Cleland's speculations on the shadow biosphere draw on the work of scientists investigating the possibility of alien life existing among us. First and foremost among them is Paul Davies, Professor of Natural Philosophy at the Australian Centre for Astrobiology, whose hypothesis paper, published in the journal Astrobiology under the title "Finding a Second Sample of Life on Earth," is original, well rounded and convincing, with the additional bonus of suggesting ways and means of detecting this mysterious alien life.
Other scientists, like Professor Chandra Wickramsinghe, refuse to buy into speculations of a shadow biosphere. "I think it is no different from postulating invisible gnomes or ghosts.�The hypothesis is not testable, and therefore I would argue it is not science," complained Wickramsinghe. Wickramsinghe admitted that he is not entirely familiar with the shadow biosphere concept, but his outright dismissal comes as something of a surprise considering his work on another controversial hypothesis - panspermia.
Perhaps Wickramsinghe is right in his summation of an alternate biosphere. Is there really any rational reason for investigating what seems on the face of it quite a radical hypothesis? Cleland seems to think so. "First, most scientists believe that the emergence of life is highly probable under the right chemical and physical conditions; in other words, the emergence of life on Earth was not a miracle of any sort (religious or scientific).� Second, we know that life as we know it on Earth today could have been at least modestly different at the molecular level. For instance, there are over 100 amino acids that familiar life could have used to build its proteins (which form the bulk of its structural and enzymatic material), and yet all known life on Earth uses the same 20. In addition, life could have utilized a different genetic code or a different suite of nucleotide bases to code for those amino acids by means of nucleic acids.� Finally, we know that alternative molecular building blocks were available on the early Earth; meteorites likely provided the Earth with an abundance of different kinds of amino acids, for instance. If the origin of life on the early Earth was a highly probable affair, due to the presence of the right chemical and physical conditions, it seems likely that different regions would have had different suites of molecules available for the construction of the first forms of life. Hence, the Earth could have hosted different forms of proto-organisms and at least some of these proto-organisms might have survived or evolved into more advanced forms of microbial life."
Davies, too, does not put much stock into a single type of life popping into existence only once. In his paper, Professor Davies begins by pointing out that it is quite likely that life on Earth has appeared on multiple occasions prior to becoming established as we currently know it. The reasons why life as we know it did not just pop into existence and remain thereafter is because of cataclysmic events such as Earth impacts, as the heat of these impacts, says Davies, would have had a sterilizing effect on any nascent life. He also says that each new synthesis of life may not have developed in the same way, that there is not one single way in which life will develop under favorable circumstances.
Davies goes on to list a number of possibilities that would have stemmed from these premises, which mostly point to how newly synthesized alien life could have either completely perished, or survived calamitous Earth impacts. "One possibility is survival in subsurface refugia," says Davies, "If alien microbes dwelt more than 1 km below ground, they may have survived even the largest impacts if they were located far enough away from the impact sites. This possibility would depend on the rate of cooling of Earth's crust. The second possibility is that material ejected from Earth by impacts could have preserved a fraction of the microorganisms embedded therein."
It would be hard to prove that alien life once existed and later perished, admits Davies, but if they did survive it means that they either coexisted separately or intermingled with subsequent life forms. Perhaps having some influence of Earth's evolutionary history. One interesting speculation is the idea that we may not all have a common ancestor. "The universal biochemical system and shared genetic code are often cited as examples of a common ancestor, but it is conceivable, though admittedly highly unlikely, that these features resulted from convergent evolution from multiple origins," writes Davies. If the tree of life analogy were used, "we would be dealing with multiple trees rather than multiple branches sprouting from a common trunk."
Cleland is more forthright, believing that a shadow biosphere continues to interact with known life here on Earth. "What we currently know of microbial communities suggests that [the shadow biosphere] would probably interact with our form of life.� Microbial communities contain enormously large numbers of different varieties of microbes, and these microbes interact with each other in various ways. There is little reason to suppose that shadow microbes couldn't participate in such communities," explained Cleland.
One of the major criticisms of multiple and distinct life forms co-existing, says Davies, is the instability of a situation in which life forms co-exist, yet compete for resources. If Earth were contiguously inhabited by Type A life and Type B life, then whichever form enjoyed a differential advantage could come to dominate and eventually drive the other form to extinction. But Davies claims that this is overlooking an important fact. "Bacteria and Archaea are distinct forms of life that occupy similar ecological niches, yet they have co-existed for at least 2 billion years."
Cleland also provides an answer to the resource problem of distinct life forms. "Even supposing that shadow microbes were somehow disadvantaged by our form of life, they could have evolved in such a way as to, in essence, take themselves out of competition with our form of life, occupying regions on Earth that are inhospitable to our form of life, which doesn't mean that these regions would be completely devoid of familiar microbes," she said.
Critics such as Professor Wickramsinghe also make the point that: "If the second biosphere has taken root, as is suggested, some substantial chemical/biochemical evidence has to be around." Cleland says this is the central problem to the shadow biosphere hypothesis, claiming: "Current techniques for exploring the microbial world [microscopy with staining techniques, culturing, and PCR amplification of genes] wouldn't detect them because these tools are very specific to the molecular biology and biochemistry of our form of life."
Davies has similar reservations, stating that scientists are looking for the wrong things, in the wrong places, with the wrong equipment. "Alien life would in all probability be restricted to microbes. Scientists have devised a suite of tools customized for studying known life; alien microbes are likely to be missed or discarded, in even the most general microbiological analyses involving bio-prospecting," says Davies.
Davies also suggests that the shadow biosphere may even be dormant, where only the right environmental conditions will finally spark its population into life. The idea that any alien microbes should have been detected by now is a bit disingenuous, as, says Davies, the first member of a new phylum of Archaea - Nanoarchaeota - was found only relatively recently. "For all these reasons we could be surrounded by living, dormant, or dead alien microbes without being aware of it," says Davies.
Davies does suggest some ways in which scientists could possibly detect the shadow biosphere. These include looking for geological evidence, novel environments, genetic fossils and biological filters. Some of these have been covered before, such as the false-alarm over what were thought to be methane emissions coming from Mars, which would have indicated life. But the most interesting detection method involves looking at chirality. An object or a system is said to be chiral if it differs from its mirror image, meaning that it comes in two forms. It's yet another contentious hypothesis, says Davies, but it is "plausible that chirality represents a frozen accident. Early life broke the symmetry at random, producing the observed chirality with 50 percent probability. It follows that there is a 50 percent chance that a second genesis, or similar life, would select the opposite chirality." If this were true, it would be possible to test for an alien biosphere, as a nutrient broth with opposite chirality contents (an "anti-soup") could be used as a culture medium, in which known life might be unable to grow, but oppositely chiral alien life may still flourish, reasons Davies.
Looking to the future, what would it mean if we did discover alien life living among us? Along with the detection of alien microbes may come new knowledge about the causation for certain illnesses and other inexplicable phenomena amongst known life. Perhaps these alien microbes are the real missing link in the evolutionary history of known life. Who knows? Such speculation is, at present, only limited by the imagination.
Professor Cleland says she is currently under contract with Cambridge University Press to complete a book on the subject of hidden biospheres, entitled The Quest for a Universal Theory of Life; Searching for Life as we don't know it. Professor Davies is continuing his own explorations into the amazing world of astrobiology, and you can hitch a ride by visiting his website.