Life at the Extremes by Frances Ashcroft, HarperCollins, 拢17.99, ISBN 0002559463
AMONG the inhabitants of Terry Pratchett鈥檚 fantasy Discworld are lithified trolls. These silica-based characters are in the habit of referring to their human companions as 鈥渂ags of mostly water鈥�. Pratchett鈥檚 description, as well as nicely summing up our physical composition, reminds us that no matter how tough and resilient we like to think we are, we would undoubtedly appear hopelessly delicate to a life form made of sterner stuff.
This was also was my impression after reading the introductory pages of Frances Ashcroft鈥檚 charming Life at the Extremes. She seems to have experienced most of the extreme environments she investigates, and has the power of making the armchair adventurer feel quite frail. Add to that her gift for carving deep into your mind how vulnerable our species is to extreme conditions, and you are in for a thrilling read.
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Most of the world鈥檚 human beings, for example, would not be able to survive a night out of their houses without suitable clothes. An Arctic fox can rest comfortably in the snow without shivering at a mind-numbing -45掳C, but an indigenous Arctic human has a clearly inappropriate and extremely dangerous sensitivity to cold. At the other end of the terrestrial temperature scale, camels manage to survive for week after week under conditions that would kill a human within 24 hours.
And extremes of temperature are not the only environmental threats to our delicate bodies. Diving just a few metres under water can bring on the dreaded 鈥渂ends鈥�. Dare to venture more than 3000 metres above sea level, and we risk being struck down by altitude sickness, a condition that, like the bends, is frequently fatal. With the aid of clothes, buildings, heaters, air-conditioners, bottled oxygen and other gadgets, our species has explored, even colonised, some of the most hostile environments on the planet, but we have received little assistance in these quests from our precious physiology.
The precise mechanisms underlying the myriad ways in which human beings can wind up troubled, sick or dead just by being in the wrong place at the wrong time is the opening theme of Ashcroft鈥檚 book. She guides us through the characteristic human physiological responses to extremes of altitude, pressure and temperature. Along the way, she explains how and why we succumb to mountain sickness, decompression sickness, prickly heat and heat-stroke, hypothermia and frostbite. She also contrasts our physiological performance with that of creatures better suited by their evolutionary history to the vicissitudes of these hostile conditions.
Her comparison of the diving abilities of humans and whales is particularly interesting. Both are twigs on the mammalian family tree, both transport gases around their bodies dissolved in blood, and both are capable of descending to considerable depths in the ocean. Yet we are acutely susceptible to the problem of nitrogen bubbling out of solution and blocking our blood-vessels when we surface at speed-the bends, while whales seem unaffected as they swoop up and down in the ocean. How do whales manage it?
Flat lemonade
The answer is that whales and other deep-diving marine mammals maintain very low levels of dissolved gases in their blood and tissues. Unlike humans, whales breathe out before diving, limiting the amount of air that can pass across the lung surface, while at depth their lungs collapse completely so that no further gases can possibly enter the bloodstream. Because the nitrogen concentration in a whale鈥檚 blood and tissues is kept low, there is no danger of bubbles forming when the animal surfaces. In other words, humans and whales are, respectively, like bottles of fizzy and flat lemonade. Note what happens to the liquid when the top is unscrewed from a bottle of each, and you鈥檒l have an inkling of why humans get the bends and whales don鈥檛.
After covering some grim material, Ashcroft allows the body count to drop off a little as she offers succinct and vivid explanations of our athletic abilities and limits. But the eye-popping stuff soon returns when she graphically describes the dangers of stepping into the vacuum of space: 鈥淭he air would rush out of your lungs; the dissolved gases in your blood and body fluids would vaporise, forcing apart your cells and forming bubbles in your capillaries, so that no oxygen would reach your brain; air trapped in internal organs would expand, rupturing your gut and eardrums.鈥� Thank goodness the vacuum of space is a long, long step away for most of us. Ashcroft guides us on a fascinating tour of the physiological problems associated with space flight, and the ingenious solutions thought up by engineers.
She ends with a paean of praise for the extremophiles. No multicellular plants or animals can survive for long at a temperature of 50掳C, for example, and no single-celled eukaryotes can survive above 60掳C. But some archaea and bacteria are made of much tougher stuff.
Pyrolobus fumarii, for example, has been found within the walls of black smokers-rocky chimneys that grow above volcanic vents in the sea floor-living at a staggering temperature of 113掳C. Other non-eukaryotes can survive in vinegar and lemon juice. Thiobacillus ferrooxidans has an annoying tendency to set up home in low-grade, iron-reinforced concrete, where it produces so much sulphuric acid that it compromises the structural integrity of bridges and tower blocks. Some microorganisms even manage to eke out a living in the middle of rocks buried kilometres below the Earth鈥檚 surface. Given the existence of these extraordinary creatures on Earth, Ashcroft鈥檚 conclusion that similar life forms may one day be found elsewhere in the Solar System and beyond doesn鈥檛 seem at all contentious.
Life at the Extremes is fascinating, accessible to anyone willing to make the effort, and well worth its price. It is a shame, however, that the book is spoilt in places by poor production. Some of the photographs are completely indecipherable, marring an otherwise extremely professional and engaging piece of work.