The Atacama Desert in Chile could potentially be home to some pretty 'weird' life.
Credit: iStockPhoto
If life started more than once on Earth, we could be virtually certain that the universe is teeming with it. Unless there is something very peculiar about our planet, it is inconceivable that life would have begun twice on one Earth-like planet but hardly ever on all the rest.
Until recently, biologists assumed that all life on Earth is of the same origin, with every organism that ever lived descended from a common genesis. But how do we know that is so? Could there be two or more different sorts of life on this planet? Has anybody actually looked?
When I began mulling these ideas a few years ago, I was amazed nobody had really thought much about evidence for multiple genesis events. Astrobiologists have been busy figuring out how to detect a different form of life on Mars, but it hadn't occurred to many people to hunt for alternative forms of life on our own doorstep.
I did, however, find enough open-minded scientists to attend a workshop at Arizona State University in December 2006 and brainstorm a few ideas. The result was a groundbreaking research paper setting out a strategy to "seek out new life", as the mission statement of Star Trek's starship Enterprise proclaims, not light-years out in the galaxy, but on Earth itself.
In order to do this, we first have to understand why biologists think all known life shares a common origin. The main evidence for a single genesis on Earth comes from biochemistry and molecular biology: oak trees, whales, mushrooms and bacteria may look very different, but their internal workings are all organised around the same system.
They all use DNA and RNA to store information, proteins to serve as enzymes and structural building blocks, and they all store and release energy using molecules known as ATP. Many identical, or at least very similar, genes are found in distinctly different species; for example, humans share 63% of their genes with mice and 38% with yeast.
The real clincher comes from the genetic code, which translates data stored in DNA into instructions for making proteins. DNA stores its information as sequences of molecular units called nucleotides - there are four different nucleotides, normally labelled by the letters G, C, A and T. What makes you you and your dog a dog hinges entirely on the sequence of those letters - and there are millions of them.
The letters spell out, among other things, the instructions telling molecular contraptions called ribosomes how to assemble proteins by stringing together amino acids in the correct order. These directions are given by nucleotides clustered into groups of three (for example, AGT). There are 64 different possible triplet combinations available, which code for one or more of the 21 different types of amino acids.
Despite the enormous number of possible coding combinations, all known species on Earth use the same code.
The fact that such complicated and specific features as ribosomes, ATP and the triplet code are found to be universal would be very hard to explain unless all the species had descended from a universal ancestor: ancient cells that had already incorporated those distinctive features.
By sequencing genes, it is possible to actually construct a common genetic tree and display the shared descent. Over time, species tend to drift apart genetically, so the number of common genes declines.
The slow and cumulative divergence provides a measure for how long ago two given species differentiated. The genetic tree is mirrored in the fossil record, which also charts the steady accumulation of changes and speciation.
Nobody doubts that familiar multicelled organisms lie on the same tree. The animals in the zoo, the plants in your garden, the birds in the sky and the fish in the sea all represent a single type of life. But this is only part of the story: the vast majority of species on Earth are microbes.
As the late American palaeontologist Stephen Jay Gould so graphically expressed it, "Our planet has always been in the 'Age of Bacteria', ever since the first fossils - bacteria, of course - were entombed in rocks more than three billion years ago. For any possible, reasonable or fair criterion, bacteria are - and always have been - the dominant forms of life on Earth."
Under a microscope, many microbes look similar - little blobs and rods, sometimes with bits sticking out. You can't tell by looking at their exterior what goes on inside. But if you examine the innards of a microbe, chances are you will find the same stuff - DNA, proteins, ribosomes - as in you or me.
At least, that has been the experience so far. But microbiologists have only just scratched the surface of the microbial realm.
Our world is literally seething with these tiny organisms. Just one cubic centimetre of soil might contain millions of different species adding up to billions of microbes in all, and the vast majority haven't even been classified, let alone analysed. Nobody knows for sure what they are; for all we know, some of them could be life as we do not know it.
To investigate a species of microbe fully, you first need to culture it in the laboratory and then study its biochemistry by sequencing its genome to position it on the tree of life. This technique, while undoubtedly important, has its problems.
Many microbes don't like being plucked out of their natural habitat and cannot be cultured easily. Some resist gene sequencing.
And, because the chemical techniques used to analyse microbes are customised and targeted to life as we know it, they wouldn't work on an alternative form of biology. Should there be a different type of microbial life out there, it is very likely to be overlooked, simply because it would be unresponsive to the biochemists' probes used so far. In a laboratory sample it might well get thrown out with the garbage.
If you set out to study life as we know it, then what you find will inevitably be life as we know it. It's therefore an open question whether some microbes might actually be the descendants of a different genesis.
How might we go about identifying life as we don't know it? Given the large measure of chance in evolution, it's highly unlikely that organisms from separate origins would have the same biochemistry.
Astrobiologists refer to known organisms as 'standard' life and to the hypothetical alternative forms as 'weird' life. Part of the problem in searching for weird life is that we don't know exactly what to look for. One strategy is to look in weird places, keeping an eye open for anything that is living.
But how weird is weird? Over the past three decades, biologists have been repeatedly amazed to find life surviving or even thriving in environments previously thought to be utterly lethal.
In the late 1960s, microbes were discovered inhabiting hot springs, such as in the Yellowstone National Park in the USA. Some of these hardy organisms can withstand temperatures of 90˚C, and for obvious reasons are called thermophiles.
That was amazing enough, but more surprises lay in store. Exploration of volcanic vents on the ocean floor with the submarine Alvin revealed entire ecosystems in total darkness, close to 'black smokers' - mineral chimneys in the seabed spewing forth dusky fluid at temperatures up to 350˚C. Many other species of microbes have been discovered living in different extreme conditions.
For example, some organisms, which rejoice in the name of psychrophiles, can tolerate extreme cold - maybe as low as -20˚C - before they stop growing. Others can withstand acid strong enough to burn human flesh, while yet others endure equally corrosive alkaline conditions. The Dead Sea turns out to be a misnomer, because it is host to several species of halophiles - organisms that live happily in very high salt concentrations.
Perhaps most remarkable of all are radiation-resilient microbes such as Deinococcus radiodurans, which can survive such high doses of radiation that they have been found living in the waste pools of nuclear reactors (see "Silent spring" Cosmos 21, p34). Collectively these microbial oddballs are known as 'extremophiles'.
Notwithstanding their exotic nature, to date all extremophiles that have been analysed are standard life: they belong to the same tree of life as you and me. Their existence proves that the range of conditions under which standard life can survive is much broader than previously suspected. Nevertheless there are limits.
If there is a shadow biosphere, it might be occupied by weird 'hyper-extremophiles' inhabiting environments beyond the reach of even the hardiest form of standard life, and have so far escaped detection because nobody thought to look for any form of life under such extreme conditions. A good example is temperature: standard hyperthermophiles seem to have an upper limit of about 130˚C - and for good reason. The intense heat disrupts vital molecules, and even with a host of repair and protection mechanisms, DNA and proteins start to unravel and disintegrate if they are subjected to temperatures much in excess of 120˚C.
Suppose we find nothing living between 130˚C and 170˚C in a deep-ocean volcanic-vent system, but then discover microbes thriving there between 170˚C and 200˚C? The discontinuity in temperature range would be a strong indicator that we were dealing with weird life as opposed to standard life that had simply pushed the temperature envelope higher.
Another limit is depth. In the 1980s the maverick astrophysicist Thomas Gold of Cornell University supervised an experimental oil-drilling project in Sweden, and created a stir when he claimed to have discovered life at the bottom of a borehole several kilometres deep. Not many believed him.
Within a few years, other researchers began finding micro-organisms living in the pores of rocks deep underground. But that was just the start. Rock cores from boreholes drilled into the seabed were found to contain millions of microbes per cubic centimetre, down as deep as the drills could go: about a kilometre.
It soon became clear that there is ample room inside our planet for microbial habitation. Nobody knows how extensive this deep, hot biosphere might be, or just how far down it stretches; Gold conjectured that there is as much biomass under the surface as on it.
Be that as it may, we can easily imagine many isolated, or nearly isolated, subterranean ecosystems, each self-sustaining and by and large separated from the regular biosphere.
In fact, three ecosystems have been discovered that are almost completely isolated from the rest of the biosphere. Buried deep underground, these extraordinary microbial communities are examples of hydrogen-powered life. The hydrogen is produced by the dissociation of water coming into contact with hot rocks or, in one case, by radioactivity.
The organisms get energy and make biomass by combining the hydrogen with dissolved carbon dioxide, and giving off methane as a waste product. Many of them are thermophiles or hyperthermophiles, because the Earth's crust gets progressively hotter with depth.
In spite of their splendid isolation, however, all the occupants of these three subsurface ecosystems turn out to be standard life. But it is clear that scientists have so far glimpsed only the tip of the iceberg: an intriguing question is whether some of these pockets might be inhabited by weird rather than standard life forms.
It is entirely possible that a future drilling project, on land or at sea, will hit a pocket of weird life. Even if we don't get lucky and actually penetrate such a pocket, we might still obtain indirect evidence for concealed weird life, such as weird viruses that prey on it.
There are plenty of other places that could be home for isolated weird extremophiles. The inner core of Chile's Atacama Desert is one place - it is so dry and oxidising that bacteria can't metabolise. The U.S. space agency NASA has a field station there, but so far there is no evidence for any carbon chemistry that could be attributed to weird life.
Other possible locations include the upper atmosphere, cold dry plateaus and mountain tops (where high-ultraviolet flux is a problem for standard life), ice deposits at temperatures below -40˚C, and lakes heavily contaminated with metals toxic to known life. We don't need to confine our search to a single parameter such as temperature; it's possible that some combination such as temperature and acidity together is more relevant.
The challenge is to spot the weird microbes if they are present at very low relative abundance. One idea we are working on at my lab is to make a variant of Gil Levin's Labelled Release (LR) life detection experiment that went to Mars on the Viking landers in 1976 and 1977.
After all, this experiment was designed to find organisms of an unspecified variety, using a very general definition of life that relied only on the ability to cycle carbon through its system - something that we expect shadow life to do. The secret of the LR experiment lies with its astonishing sensitivity.
It works by providing a nutrient broth tagged with radioactive carbon (C-14). Any carbon cycling due to metabolism is detected by looking for C-14 in emitted carbon dioxide. Because even the tiniest levels of radiation are easy to measure, the LR experiment can register trace amounts of activity.
If there are weird bugs out there on high mountaintops or in the core of the Atacama Desert - and assuming they don't choke on the broth - Levin's experiment could find them.
The first step will be to determine whether or not they are just an even more extreme extremophile belonging to the standard tree of life, or descendants of another genesis.
It would be much harder if weird life and regular life are intermingled. A persistent science fiction theme is that alien beings are living clandestinely among us, indistinguishable from humans.
Now it seems that a Lilliputian variant of the alien infiltration theme could actually be true. If weird microbes look like standard bacteria and inhabit the same environment as us, they may have already been spotted, but lacking a visible uniform that proclaims membership of an alternative club they wouldn't have excited comment - they would remain hidden in the microbial crowd.
There could literally be alien organisms right under our noses (or even in our noses!), as yet unrecognised for what they are. The thorny problem is how to identify them.
One way is biochemically. Two microbes may look similar yet have very different chemistry going on inside. If we could know in advance what an alternative biochemistry might be, we could then test microbial samples for signs of it.
The trick is to guess right. As we don't know precisely what we are looking for, this is quite a challenge.
But we can make some educated guesses. An obvious example is the 'chirality' of the molecules of life - much like our hands are mirror images of each other, sugars of standard life have a 'right-handed' structure and amino acids are 'left-handed'. If life were to start over again, there is a chance it would choose the opposite handedness next time.
Even if this 'mirror' life resembled standard life in all other respects (for example, by using the same nucleic acids and proteins), it would stand out - not visually, but biochemically. For example, it might only gobble up 'mirror' sugars and amino acids instead of the standard ones that life, as we know it, relies upon.
Richard Hoover and Elena Pikuta at NASA's Marshall Space Flight Centre in Huntsville, Alabama, went out looking for 'mirror' organisms that would eat these opposite-handed sugars and amino acids. They discovered a novel extremophile from a highly alkaline lake in California that ate the mirror soup with gusto. Yet unfortunately it was only a standard microbe that had cleverly adapted to cope with mirror food.
So the chirality story is a bit perplexing and clearly more complicated than we originally envisaged. Nevertheless, using chirality as a signature for weird life remains an obvious and easy technique.
Another clue could come from the building blocks that weird life might use. Standard life uses 21 types of amino acids to make proteins, but many other varieties exist.
In 1969, an unusual meteorite fell near the Australian town of Murchison, north of Melbourne, belonging to a rare class known as carbonaceous chondrites. The Murchison meteorite contains abundant organic material - so abundant it smells of petrol - including many amino acids that standard life doesn't use. A few people have jumped to the conclusion that the meteorite was once inhabited by alien microbes that decomposed, leaving their exotic amino acid contents for us to find among the corpses.
But this conclusion is a stretch; it's more likely that these organic molecules formed somewhere in space. It's not hard to make amino acids in the laboratory, so presumably there are many natural ways for them to form, too. The early Earth may have been coated with carbonaceous material from meteorites and interplanetary grains that fell like manna from heaven, providing raw materials from which the first life may have emerged.
If this is correct, the original cells would have been able to pick and choose from the organic cocktail. To the best of our knowledge, the 21 chosen by known life do not constitute a unique set; other choices could have been made if life started many times.
Then there's the genetic code, which is universal for standard life. We can imagine an alternative type of life made up of DNA and the same 21 amino acids, but using a different genetic code. It would be easy to overlook organisms with this 'near miss' biochemistry, yet they would betray themselves if studied in detail.
More likely, if weird life started from scratch independently of standard life, it would use a different set of amino acids, so it would also have to employ a different genetic code. We can even imagine life in which two of the four nucleotides are absent, replaced by a different nucleotide, or in which there are, say, six instead of four. Because there is little chance that micro-organisms using fundamentally different biochemistry would respond meaningfully to standard biochemical techniques, weird microbes of this sort might be all around us, so far unidentified.
If weird life is discovered, the first priority will be to determine whether it belongs to a genuinely separate tree of life, or is merely a hitherto undiscovered branch on the known tree of life. Suppose we are presented with two radically different forms of life, which we are tempted to attribute to separate trees, each with an independent origin (or independent transitions from non-life to life). On further investigation, we may find that 'below ground' the two trunks join in a common root system: that is, the different forms of life belong on a single tree after all, but they branched apart before the last common ancestor of all standard life.
The known tree of life consists of three distinct 'bushes' that branched apart billions of years ago. One bush contains the bacteria. Another contains eucarya and has all multicellular life, from humans to hedgehogs, and also complex single-celled organisms such as the amoeba. The third bush consists solely of microbes under the collective name 'archaea'.
But how do we know that there isn't a fourth bush, waiting to be discovered, that split away even earlier than the trifurcation into bacteria, eucarya and archaea? If we ever found a new exotic form of life, we would need to eliminate the 'fourth bush' explanation before concluding that it provides evidence for a second tree.
How can a low-lying branch be distinguished from a separate tree? The answer would depend in part on just how weird the weird life is. The devil would be in the detail.
To be sure that any weird life really is descended from a second genesis, it would have to be sufficiently different from standard life for no plausible common ancestor to have existed. That criterion would be hard to establish if the two biospheres overlap and use a lot of common chemistry. Still harder would be if the two forms became partially integrated biochemically, for example, by swapping genes or other structures, thus muddying their separate lineages and confusing the whole evolutionary story.
We can't rule out one form of life 'taking over' another, sci-fi style, by infusing key components of itself into a receptive host, especially if two separate forms of life found themselves on convergent evolutionary tracks. All this would be an unwelcome complication. It would be sad and annoying if life started on Earth many times over, but converged and merged, so that we had no hope of untangling its multiple roots.
Has shadow life already been found? Air-dwelling bacteria have been discovered that nucleate ice crystals by secreting special enzymes, giving them a clever way to reach the ground in snowflakes. Another intriguing phenomenon is the strange rock coating, found in most of the world's arid zones, known as desert varnish.
Its origin has been something of a puzzle since Charles Darwin remarked on it. The coating certainly contains microbial life, and also unusual combinations of minerals - and the chemical composition is very different from that of the host rocks.
It is not clear whether the varnish is a product of life, or a complex mineral layer that has been invaded by life opportunistically. It does, however, provide a readily accessible source of moderately weird material that merits further study.
Probably the most persistent claim that weird life has already been discovered concerns tiny forms known as nanobacteria. These little blobs measure only a few hundred nanometres across (a nanometre is one billionth of a metre). They resemble bacteria but are thought to be too small to contain ribosomes.
Nanobacteria have been reported in rocks, oil wells and blood. They have been implicated in numerous diseases, ranging from renal disorders to Alzheimer's, and have even attracted the attention of pharmaceutical companies.
The claim that these little structures are living organisms, as implied in the use of the term 'bacteria', is highly controversial; if they are, it's hard to see how they could be standard life. They might be a weird form of life that assembles proteins in a novel way, or uses some other type of enzyme.
Or they might not be living at all. One theory, suggested by synthetic biologist Steve Benner of the University of Florida, is that some nanobacteria might be a form of RNA-based life that doesn't need ribosome-made proteins because RNA does the job of both proteins and DNA.
On top of that, research by John Young and his student Jan Martel at the Rockefeller University in New York has led them to conclude that nanobacteria, or nanobes, aren't in fact alive. The duo suggest they are instead chemical complexes made up of organic material combined with common calcium carbonate (limestone), forming amorphous shapes superficially resembling diminutive cells.
The researchers are keen to point out that, even so, nanobacteria are not unconnected with the topic of life's origin, because they provide a natural example of chemical self-assembly: a step on the road to life, perhaps, even if the nanobacteria are not themselves alive. They draw a comparison with prions: protein-like chemicals that can become malformed in a type of chain reaction, giving rise to illnesses such as kuru and mad cow disease.
A radical form of weird life would be organisms that use different chemical elements. Life as we know it is based on the unique properties of carbon chemistry, but it also uses several other key elements, specifically, hydrogen, nitrogen, oxygen, phosphorus and sulphur. There has been some speculation that silicon could substitute for carbon, a conjecture that got as far as an episode of Star Trek, but hasn't been pursued very seriously by biochemists because silicon can't form the extraordinary range of complex molecules that carbon can.
A more plausible candidate came from my collaborator Felisa Wolfe-Simon of Harvard University in Boston, who suggested that phosphorus could be replaced by arsenic. Arsenic can do the same structural and energy-storage jobs as phosphorus, but it can go one better, by providing an energy source too. In fact, there are microbes that exploit arsenic, but they don't inhale it, so to speak: the arsenic compound gets stripped of its energy and the arsenic is then summarily expelled.
Arsenic is a poison precisely because our bodies have a hard time telling it apart from phosphorus. Wolfe-Simon hopes to find weird microbes with arsenic incorporated in their vitals, and for which phosphorus would be the poison.
NASA is currently funding a project for her to look for them in Mono Lake in California. An ecological marvel in the eastern Sierra close to Yosemite National Park, Mono Lake is a picturesque haven for exotic wildlife, and none is more exotic than the microbial inhabitants. The lake has exceptionally high arsenic concentration, and is home to many peculiar organisms, some of which seem to use the abundant arsenic to their advantage.
The great expert on Mono Lake's arsenophiles is Ron Oremland of the U.S. Geological Survey in Menlo Park, who is hosting Wolfe-Simon's project. In order to find weird life, micro-organisms from mud samples in the bottom of the lake are subjected to ever-increasing levels of arsenic. In Mono Lake, standard microbes may have adapted to handle arsenic, but their tolerance does have its limits, and at some level of concentration the cells overdose, dying quietly of arsenic poisoning like tiny victims in an Agatha Christie novel.
Genuinely arsenic life, by contrast, will lap up the cocktail and thrive. By performing successive culturing operations at higher and higher levels of arsenic concentration, the experimenters expect that any arsenic-based microbes, even if initially present in only trace amounts, will soon out-multiply the standard life competition, and so come to dominate the microbial population.
To date, none of the microbes he has studied is an authentically weird form of life. However, the search for arsenic life has only just begun.
Another approach to finding weird life is to sample life as widely as possible from the oceans. In 2004, the American biologist Craig Venter, who helped sequence the human genome, stunned the scientific world once again when he announced he had isolated a staggering 1.2 million new genes and 1,800 previously unidentified microbes in a sample of water taken from the apparently barren Sargasso Sea in the North Atlantic Ocean. In a telling comment, Venter said, "We're looking for life on Mars, and we don't even know what's on Earth."
Several ocean sampling projects are now under way, providing a golden opportunity to discover any weird life that may be lurking in the sea. A three-year international project called Tara Oceans, begun in September 2009, is performing a global sampling exercise, primarily directed at studying the impact of carbon dioxide accumulation on marine biodiversity.
The project will also look at deep-ocean ecosystems and sample microbiology from all the world's oceans. The project's scientists will be on the lookout for a shadow biosphere too, deploying a range of techniques for identifying weird life, and returning selected samples to our BEYOND Centre for laboratory analysis.
The discovery of a form of life that could have arisen only via a second genesis would be the most sensational event in the history of biology, with sweeping consequences for science and technology. It would also have immediate implications for astrobiology, as we could then be sure that the universe really is teeming with life, as so many commentators glibly assert.
Paul Davies is the director of BEYOND: the Centre for Fundamental Concepts in Science at Arizona State University and a member of the Cosmos Editorial Advisory Board. This is an edited extract from his new book, The Eerie Silence: Are We Alone in the Universe? (Allen Lane, $49.95).
