Even though scientists still argue about what life is, discoveries made in the past few weeks have shown that much more of it teems on our planet than previously thought: organisms are thriving under the ocean floor; in ice caps; even in orbit miles over our heads.
This kind of life is the most successful on Earth. The number of people on the planet - a few billion - pales compared with bacteria, which number 5,000 billion billion billion, a cosmic total.
They are found anywhere that higher life forms breed - including in our bodies. More bacteria live in our mouths than the number of people who have ever lived. But they also seem to thrive in the most forlorn nooks and crannies on Earth, forming a huge twilight zone of life.
Every time that a bacterium is found grazing in a desolate spot, whether on a damp rock deep underground, in a stratospheric air current, or wriggling in superheated water near a volcano, scientists gain confidence that bacteria - or something like them - probably exist elsewhere in the universe.
People have traditionally been fascinated by the bigger organisms that walk, crawl and fly around the planet. But they should spare a thought for the smaller things in life, such as an apparently insignificant bug with the forgettable name of SAR11.
The SAR refers to how the plankton was first identified in the Sargasso Sea area of the North Atlantic when Prof Stephen Giovannoni hunted for genes in water samples more than a decade ago. "SAR11 was the first major group of uncultured bacteria to be discovered by gene cloning and sequencing techniques, and it has since played a leading role in the development of microbe hunting methods," said Prof Giovannoni, of Oregon State University, Corvallis.
He likens the microbial world to "an unexplored frontier populated with species that have never been identified, named or studied". A few weeks ago, a new analysis based on samples taken from across the planet revealed that Prof Giovannoni might have stumbled upon the most common creature on the planet.
His latest project reveals that SAR11 may make up more than one third of all bacterial cells in the ocean surface waters, and almost one fifth of cells in deeper water. "They are possibly the smallest cells known, and all together they weigh about as much as all of the fish in the oceans," said Prof Giovannoni, who reported the find with colleagues from the University of California, Santa Barbara, in the journal Nature.
They are about 1/5000 of a millimetre long and have a volume that is about one hundredth of that of the common gut bug E coli. The new measurements of the number of actual SAR11-like cells show that they make up about half of the microbial community in Atlantic surface waters. Globally there may be a vast number - 24,000,000, 000,000,000,000,000,000,000 - give or take a few, making them candidates for the most successful creatures on Earth.
These tiny ocean microbes are so small that billions will fit into a teaspoon; so abundant that, at 200 million metric tons, they may influence the climate.
"Vast populations of SAR11 increase during the summer and decrease during the winter, in a cycle that correlates with the build-up and decline of dissolved organic carbon in the ocean surface," said Dr Robert Morris, lead author of the study. This suggests that SAR11 has an active role in the oceanic carbon cycle, which affects the concentrations of carbon dioxide in the Earth's atmosphere that contribute to global warming.
Under the oceans, another frontier has been discovered that expands the realm of seemingly hostile environments where organisms can live. In the light of recent work, the deep ocean crust can now be viewed as an immense "biosphere" of life in its own right that covers most of the Earth. According to Prof Giovannoni: "People have wondered what types of organisms might live within Earth's crust. This has given us one of the best looks we've ever had at that environment."
In the 3.5 million-year-old crust almost 1,000 feet beneath the bottom of the ocean off the coast of Oregon, Prof Giovannoni and colleagues found moderately hot water moving through heavily fractured basalt that was depleted in sulphate and enriched with ammonium. The discovery suggests biological activity in a high-pressure, undersea location far from the types of carbon or energy sources upon which most life on Earth is based.
Instead of digesting organic molecules, as most life does, the novel bugs appear to dine on inorganic molecules such as sulphide or hydrogen, and carbon dioxide. The level of biological activity of these bacteria was sufficiently high that ammonia levels in the subsurface samples were 142 times higher than those in nearby sea water.
"This is one of the best views we've ever had of this difficult-to-reach location in the Earth's crust and the life forms that live in it," said co-author Dr Michael Rappe. "We knew practically nothing about the biology of areas such as this, but we found about the same amount of bacteria in that water as you might find in surrounding sea water in the ocean. It was abundant. As more research such as this is done, we'll probably continue to be surprised at just how far down we can find life within the Earth, and the many different environments under which it's able to exist."
While pressure and depth do not seem to deter bacteria, nor does extreme cold. Another recent discovery was of 3,000-year-old microbes in an ice-sealed briny lake in Antarctica. This ancient, unusual and extreme ecosystem in Lake Vida - among the largest of the lakes in Antarctica's McMurdo Dry Valleys - was found about 2,000 miles south of New Zealand by a team led by Prof Peter Doran of the University of Illinois, Chicago.
The research overturns earlier assumptions that Lake Vida was frozen solid. Using ground-penetrating radar, ice core analyses and long-term temperature records, the researchers showed that Vida has a vast amount of ancient organic material and sediment, and a cold, super-salty, layer of liquid under about 60ft of ice - an environment that remains liquid at temperatures below -100C, well below the freezing point of pure water.
The super-concentrated salt lake has been isolated from the atmosphere for at least 2,800 years. Carbon-14 dating showed microbes gathered from ice near the brine to be more than 2,800 years old. Ice core samples taken from above the pool of brine revealed frozen bacteria and algae that came back to life after gradual melting.
The chilly brine itself may also harbour living things. To find out, the National Science Foundation and Nasa are funding a follow-up study to extract samples.
While bugs have been found deep underground, there is evidence that they also soar overhead. On January 20, 2001, balloon-borne sterile "cryosamplers" were sent into the stratosphere - at altitudes of up to 25 miles - by a team led by Prof Jayant Narlikar, director of the Inter University Centre for Astronomy and Astrophysics in Pune, with other Indian scientists.
Last year, a team of astronomers and biologists lead by Prof Chandra Wickramasinghe reported evidence of viable bacteria in air samples at 25 miles up. The quantities suggested that, worldwide, about a ton of bacteria is raining down on us each day. Recently, in the journal FEMS Letters, Dr Milton Wainwright of Sheffield University reported that he has managed to isolate a fungus and two types of bacterium from one of the space-derived samples collected at 25 miles.
Given that contamination is not responsible, some believe the find backs the idea of panspermia, which argues that Earth is being bombarded by micro-organisms from space. However, sceptics will argue that the likeliest source of the bugs remains Earth itself. Even so, the fact that bugs must have made it into orbit long before humans ranks as a fascinating find.
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