Quantum Evolution: Life in the Multiverse. Johnjoe McFadden
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source (plants use water) and an energy source (sunlight). Photosynthetic organisms extract carbon dioxide from the atmosphere and add hydrogen and sunlight energy to make simple sugars. The sugars are then strung together, pulled apart and reassembled to make the cell’s complex biomolecules – proteins, fats, carbohydrates and DNA.
Plants did not invent photosynthesis but stole the idea from bacteria – quite literally. Chloroplasts, the organelles performing photosynthesis inside leaves are descendants of a bacteria called cyanobacteria. Cyanobacteria are far more ancient than plants, and performed photosynthesis on Earth at least a billion years before the arrival of plants. The ancestors of modern plants were probably symbiotic partnerships between primitive fungus-like organisms and the photosynthetic cyanobacteria, perhaps resembling today’s lichens. This partnership slowly became permanent, and all today’s trees, ferns, flowers and grasses are the descendants of this marriage of convenience.
Cyanobacteria are not the only bacteria to perform photosynthesis and probably not even the first. Like plants, cyanobacteria perform oxygenic photosynthesis – they release oxygen as a product of their photosynthesis. The oxygen comes from their hydrogen source: water. Other bacteria can utilize alternative sources of hydrogen – such as hydrogen sulfide (H2S), ammonia or organic compounds – to fix carbon. These bacteria perform an anoxygenic photosynthesis which does not generate oxygen. This form of photosynthesis almost certainly preceded its oxygenic cousin.
Many bacteria and all animals are unable to fix atmospheric carbon dioxide, extracting it instead from alternative inorganic and organic chemical sources. Bacteria are the most versatile chemical feeders, able to extract carbon from a wide range of chemicals, which include organic compounds, carbon monoxide, calcium carbonate, methane, methanol, ether and formic acid. One group of bacteria using methane as both carbon and energy source is common in animals’ intestines, marshes and oxygen-deficient mud. But their most bizarre habitats were discovered on the sea-bed. In the summer of 1997, Chuck Fisher of Pennsylvania State University and Phil Santos from the Harbour Branch Oceanographic Institute were in a mini-submarine, Johnson Sea Link, exploring the sea-bed seven hundred metres below the Gulf of Mexico. They were examining the huge bubbles of methane hydrate forming when natural gas (methane) seeps up from the ocean floor, mixing with water and other hydrocarbons to form a dirty yellow methane ice. Scientists had suggested that methane-eating microbes might also feed on the hydrates, but what Fisher and Santos did not expect to find was a multitude of pink worms using oar-like paddles to crawl over, or burrow into, the ice. They were a new species of polychaete worms. It is unlikely that they eat methane directly, instead the worms probably graze on methane-eating bacteria colonizing the ice. It has even been suggested that the worms might build burrows to cultivate farms of these bacteria.
The next ingredient for life, nitrogen, should not be a problem since eighty per cent of the air we breathe is nitrogen gas. However, we cannot assimilate nitrogen gas from air – too unreactive – we breathe it in and right back out again. Instead we obtain our nitrogen from organic chemicals in food such as, for example, the protein in meat. Plants are able to assimilate inorganic forms of nitrogen such as nitrate (a compound of nitrogen and oxygen), but this does not solve the problem since the only non-biological source of nitrate is lightning strikes which generate temperatures high enough to burn atmospheric nitrogen and yield nitrate.
With only very limited supplies of fixed nitrogen available from lightning, life might have become severely nitrogen-limited billions of years ago. Fortunately, bacteria (including cyanobacteria) discovered how to fix nitrogen in the air to make the soluble compounds ammonia and nitrate. Nearly all biological nitrogen is derived from these nitrogen-fixing bacteria in soil and water. Leguminous plants (such as peas) form symbiotic partnerships with nitrogen-fixing bacteria, allowing them to grow in nitrogen-depleted soil.
The last ingredients of life – the minerals like calcium, sodium, magnesium, phosphorus and iron – are fairly readily available, usually as salts dissolved in water. Most organisms can readily assimilate inorganic sources of these elements, such as the sodium chloride (NaCl): the salt we sprinkle on our food.
Living organisms are extremely versatile in their ability to utilize a wide range of both organic and inorganic chemicals for the elements that make up their biomolecules. Animals need much of their biomass supplied as ready-made organic molecules. Bacteria have minimal requirements: some subsist on little more than a diet of air and rock.
ICE-COLD LIFE
The average temperature in London is about 13° Centigrade, rarely going above thirty degrees or dropping much below zero. Most higher plants and animals are happiest within a similar range of temperatures, so it is hardly surprising that life is particularly abundant in these latitudes. Humans do live in far more extreme environments. In Timbuktu, the Saharan temperature can rise to 50°C, whilst the inhabitants of Dawson in the Yukon valley endure nights where temperatures drop to –30°C. However, even mad dogs and Englishmen would succumb to heatstroke under a Saharan midday sun and frostbite would soon freeze anyone foolish enough to brave the winter nights of Alaska. Man survives these extremes of temperature by building shelters to provide warmth or shade, thus creating a more equable microenvironment protecting him from the heat and cold outside. The range of temperature that humans can endure (without resort to ingenuity) is actually quite narrow, lying somewhere between 5°C and 30°C.
Many animals survive more extreme environments. Often considered a barren wasteland, during its summer months the Antactic is teeming with life. Millions of seabirds and sea mammals nest on its coasts and fringe of drifting pack ice. Even the snow harbours life. Warmed by the summer sun, the interior of the pack ice becomes laced with channels of slushy brine filled with photosynthetic bacteria and algae. Antarctic mites burrow through the snow to graze upon on the microscopic bloom. The summer melt releases billions of these microbes into the ocean, to be harvested by the filter-feeding krill and channelled into the food chain supporting the seals, penguins and whales of Antarctica.
Within the interior, conditions are far harsher. The coldest temperature ever recorded was a chilly one hundred and twenty-nine degrees below zero at the Russia Vostok station in July 1983. Yet Antarctica is far from sterile. It harbours more than a thousand plant species, mostly mosses, fern and lichen. The topmost peaks of mountain ranges that rise above the ice are often colonized by lichen. Indeed, brown yellow and grey spots of lichen are ubiquitous on exposed rocks throughout the world. In Donegal, Ireland, where I was born, lichen has been scraped off rocks for centuries and used to colour wool for the cloth known as Donegal tweed. The same coloured lichen spots cling to the paving stones of disused paths and cover the crumbling ruins of ancient buildings. Lichen is actually two organisms: a fungus and an algae (or sometimes a bacterium) living in symbiosis. The photosynthetic algae provide nutrients that feed the fungus. What the fungus contributes is less clear, but it probably provides support and the ability to extract essential minerals from the rock. The success of this pairing allows lichen to colonize extreme environments barred to fungi or algae alone. However, even lichen cannot perform photosynthesis below zero. Although they survive the freezing temperatures of the Antarctic winter, they must await the warming sun to heat their rock substrate to a balmy 0–10°C before they can grow and reproduce. A similar freeze and burst strategy is followed by most of Antarctica’s flora which await rare warm spells to initiate a frantic flurry of growth and reproduction – generating millions of frost-tolerant spores or seeds – and closing down again when wintry conditions return.
The dry valleys of Antarctica are probably the most inhospitable regions on Earth. Bone-dry hurricane force winds race unimpeded across the Antarctic plateau, bringing temperatures dropping to –52°C. The winds quickly evaporate any traces of moisture from stray snow drifts. Bodies of long-dead seals and penguins lie perfectly preserved, desiccated, in conditions where even microbial decomposition is halted. A group of American scientists led by Diana Freckman of Colorado State University occupy a research station near the permanently frozen Lake Hoare in the McMurdo Dry Valley. Studying the ecology of the area, they have discovered a curiously simple ecosystem within the soil. Though the land is frozen half a mile deep, it is covered by a thin dry soil eroded from the rocks by the scouring wind. This soil harbours