|All life on Earth has a similar DNA-based chemistry
Conspiracy theorists will readily tell you that the U.S. military is hiding alien corpses in a secret facility in the Nevada desert. But paleontologist and University of Washington geology professor Peter Ward thinks that scientists should be looking for a different type of alien life on earth: alien microbes. Ward is the author of several popular books about astrobiology, including the controversial Rare Earth, co-authored with Donald Brownlee. In his latest book, Life as We Do Not Know It, Ward addresses an issue often avoided by astrobiologists. Although all known life on Earth has a similar DNA-based chemistry, life found on other worlds may not. In chapter 6 of Life as We Do Not Know It, reprinted below, Ward takes this argument one step further. There may, he says, be unfamiliar life forms on Earth, as well. And scientists would be wise to search for them.
Are There Aliens Already on Earth?
Excerpt from Life as We Do Not Know It, by Peter Ward
Back before the first dinosaurs, before the first fishes, before the first worms, before the first plants, before the first fungi, before the first bacteria, there was an RNA world - probably somewhere around four billion years ago, soon after the beginning of planet Earth's existence….
--Matt Ridley, Genome
The concept of extinction did not exist until the nineteenth century. Until then there was no sense that species evolved, lived for a time, and then went extinct. Because religion still held sway over so much of the world, there was a sense that everything God had ever created would still be found - somewhere. Even when large bones and shells of creatures clearly unlike those familiar to the noted naturalists of the day were uncovered from sedimentary strata, the belief was that the creatures in question were still alive somewhere in the vast, little-explored world. But by the early 1800s there was little of the world still unexplored, and it fell to Baron Georges Cuvier (struggling to keep his head on his shoulders through the French Revolution and its aftermath) to demonstrate the reality of extinction. Cuvier, the so-called father of comparative anatomy, obtained the skull and teeth of a proboscidean that was distinctly different from either the African or the Indian elephant. He announced that the fossil elephant parts came from a species now extinct.
|In a eukaryote, the DNA is located in the nucleus of the cell. A DNA molecule is composed of two helically spiral strands, each composed of a linear chain of sugar and phosephate molecules. Credit: MIT
Today, in a world where the extinction rate among endangered species is occurring at an unknown but surely significant rate, it is pleasant to think of a world without extinction and one so little explored that even in the early 1900s Sir Arthur Conan Doyle could put his Lost World with its diverse saurian legions atop a large plateau in the Amazon region and readers could retain a sense of why not? Today we live in a world that is the opposite. There is a sense that it holds no more secrets, that the vast majority of its biota has been discovered and cataloged, that even the immense oceans, now steadily giving up their most famous wrecks to the Bob Ballards of the world, hold little mystery. But this smugness that nature and its secrets are now conquered might be a bit premature. Two recent events have made this abundantly clear. First, in 2002, oceanographer Debbie Kelly of the University of Washington (and another member of our astrobiology faculty) made a startling discovery deep beneath the Atlantic Ocean. Amid the mid-Atlantic Ridge, using submersibles and remote cameras, Kelly and her crew found their own Lost World and named it the Lost City. They found the equivalent of the black smokers [seen at Pacific Ocean hydrothermal vent sites], but this time the substrate was white limestone rather than the darker peridotite rock of the Pacific Ocean smoker systems.
The structures discovered are spectacular. Some are nearly two hundred feet tall, growing like monstrous white stalactites up from the bottom of the sea. These chimneys vent a diverse chemistry of fluids with temperatures ranging from less than 40°C (104°F) to over 90°C (194°F). The fluids coming out of the white chimneys are enriched in methane, hydrogen, and hydrocarbons other than methane and are thus rich sources of energy in form that can be utilized by life. And unlike the black smokers of the Pacific, the microbes living among and in these white chimneys are fueled by the chemistry of rock-altering reactions, not simply the heat coming from deep earth. The Lost City hydrothermal field is thus unlike any known submarine vent system and, according to its discoverers, may be our closest analog to early Earth and early Mars, on the basis of our understanding of the chemistry on those planets at the beginning of their history. The temperatures are moderate, the fluids have a high pH, and there are low metal concentrations and high concentrations of energy sources. The Lost City, which may be our best hope for finding a Lost World of early life on earth, was unknown until 2002 and has been visited only once. There is certainly an abundance of new microbes - and perhaps environments as well - yet to be discovered there.
|Hydrothermal vents provide fuel for life on the seafloor. Credit: lostcity.washington.edu
It is not just the deep sea that is yielding new microbial discoveries. Soon after the discovery of the Lost City field, Craig Venter turned his gaze toward the sea. Venter sampled seawater from the Sargasso Sea of the Atlantic Ocean and, using the same powerful gene mapping techniques that were perfected in the Human Genome Project, surprised the biological community in identifying, from relatively small samples of seawater, 148 previously unknown types of bacteria. This was a shock to the complacent microbiologists who had thought that we pretty much had a handle on what was out there in the ocean. Furthermore, Venter identified 1.2 million new genes from these samples. These add to an already heady number of known Terroan species. [Note: Ward coined the term Terroan life to refer to DNA-based life on Earth.] Of the earth's currently defined creatures, about 750,000 are insects, 250,000 are plants, 123,000 are arthropods exclusive of insects, 50,000 are mollusks, and 41,000 are vertebrates, with the remainder composed of various invertebrate animals, bacteria, protists, fungi, and viruses, leading to a total of about 1.6 million named and identified species on the earth. The majority of these leave no fossil record. The precise figure is not known, since there is no central register for the names of organisms, and because of this, many species have been named several times. Taxonomist Nigel Stork points out that this level of synonymy may approach 20 percent. For example, the common "ten spotted ladybird" found in Europe has forty scientific names, even though it represents but a single species. Such mistakes are made because many species exhibit a wide range of variability. Often the more extreme examples of a given species are mistakenly described as new or separate species.
|The carbonate structures at the Lost City Field include this chimney more than 30 feet in height. The white, sinuous spine is freshly deposited carbonate material. The top shows evidence of collapse and re-growth, as indicated by the small newly developed cone on its top.
Credit: University of Washington/Woods Hole Oceanographic Institution
Does this mean that the number of species on Earth today is less than the currently defined 1.6 million? Probably not. Most biologists studying biodiversity suspect that the number is far more than this, but an intense debate ranges about exactly how many more. The most extreme estimates are in the range of 30 to 50 million species, meaning that taxonomists have named a bit over 3 percent of species on Earth and thus, in the 250 years or so since Linnaeus set out the task of describing every species on Earth, have barely begun their work. Other, more cautious souls posit a much lower number of between 5 and perhaps 15 million species. Yet even with this lower number it is clear that the work of describing the earth's fauna has a long way to go. There may be millions of new bacteria to discover and identify alone. We thus see that we may have recognized only a fraction of the total of earth organisms. Is the same true about the diversity of life chemistry on Earth as well as the diversity of species?
These discoveries demonstrate that indeed there is much yet to be learned about the diversity of life on Earth and more about its habitats and biochemistry. This is especially true for the Archaean domain. These microbes show a huge diversity of metabolic pathways just now being discovered. Just as there are many new microbes being discovered, so too was it a surprise how incomplete our knowledge is of something as basic as the fixation pathways of carbon dioxide by microbes on Earth. Where we thought that there were only a few ways that microbes could create food from the carbon in CO2, it has become clear that we are only scratching the surface of what microbes can do. This gap in our understanding is obviously a huge hurdle for those pondering what might be encountered on the many diverse habitats that could be present in the solar system.
Here is a sense of how fast the discoveries are mounting and an indication of how much is yet to be discovered and how much might still be hidden. By 1987 microbiologists had identified twelve major phyla within the domain Bacteria, based on ribosomal RNA analyses. By November 2003 there were 52 phyla identified, and by 2004 the number had risen to more than 80. That is why, when Venter found 148 new types of bacteria, from surface seawater at that, it was a major surprise.
If there are so many new microbes as well as vast new features of the planet being discovered, what else is hidden from us? Might there be something even more spectacular out there waiting to be found, such as non- Terroan life? The proposition no longer seems so ridiculous. In fact, we might ask if a whole alternate biosphere exists on earth in tandem with our familiar DNA biosphere. This is really a dirty little secret. While we continue to affirm that there is but one form of life chemistry of Earth, in reality we do not know. Moreover, if there were a second dominion of life on Earth, what would it be? The most probable of such life as we do not know it would be RNA organisms, the group that I have defined as the cellular forms. As we have seen, many believe that cellular RNA life was the stepping-stone to modern-day DNA life but then went extinct. I certainly do. Yet strange living fossils, such as the coelacanth fish and the monoplacophoran Neopilina (a very early mollusk that was at the base of the tree of molluscan phylogeny), show that once in a while a living fossil comes out of the woodwork. Might the same be true of RNA cellular life? We still have RNA viral life. Why not the larger kind?
|University of Washington oceanographer Deborah Kelley contrasts the white porous (almost wasp-nest-like) texture of a sample from the Lost City's carbonate chimneys with a sample from the sulfide chimneys studied since the 1970s.
Credit: University of Washington
Both coelacanths and monoplacophorans were found by accident. These living fossils were fished out of the deep sea. But both are relatively large. The coelacanth, a bright blue fish with lobed fins, the supposed ancestor of us land-living vertebrates, is up to six feet long; the monoplacophoran is considerably smaller, only about a half inch in shell length but nevertheless something that can be seen without a microscope. For microbes, identifying something really new is a much more difficult proposition. A very large microscope is necessary to see even one, and unlike the fish and the mollusk, which can be easily recognized as radically different from the many other members of these tribes, almost all microbes look alike. It takes an inspection of their genes to recognize their degree of difference from other microbes. Herein lies a huge rub: It is not clear that we would recognize an RNA organism if we found one. We may have already found millions of them that remain unknown and unidentified, not recognized as non-DNA life. Why? Because the method we use to differentiate DNA life is by comparing DNA, or ribosomal RNA. We target the ribosome for study. But RNA life would not have ribosomes! Our RNA life would have a different kind of RNA, presumably, and thus would be invisible to our tests!
The potential presence of still-living RNA life on earth has intrigued biologists for some time. Microbiologist (and Nobel laureate) Joshua Lederberg has advanced ways that we could search for another possible non-DNA life on earth, protein life. His solution is to culture samples of microbes with radioactive phosphate. Terroans would incorporate this poisonous material into their nucleic acids and perish, whereas any possible protein life would survive. Another method would be to try to grow microorganisms in a medium without phosphate. Again, anything that grew would not be our familiar earth life. Lederberg has called on his colleagues to start a search for non-DNA life using substantial research funds, for the search would be neither trivial nor easily accomplished.
Unfortunately the search Lederberg suggests has never been attempted. That's a shame, for on much less evidence and chance of success, the SETI organization continues to look for intelligence among the stars when there is still so much mystery on the ground beneath our feet.
Related Web Pages
Census of Marine Life
Expedition Slide Show
The Ocean Food Web: II
Spying on Biodiversity
Finding Nemo 10
The Ocean Food Web: I
Seeing Forests in the Tree of Life
Lord of Gondwanaland
Rare Earth Debate Series
Interactive Presentation: The Life and Death of Planet Earth
Tree of Life
The Tree of Life Web Project
First Complete Gorgon Fossil Found (ABC-AU)