Man-made mountains to climb

"How low can you go" intones the singer of the famous limbo-dance tune as he exhorts contestants to pass under ever-lower bars. Frances Ashcroft might add to that "and how high, how deep, how fast, how cold, how hot, and for how long". Her book Life at the Extremes is a recitation of the physical extremes of human capability and endeavour, and their physiological basis.

Humans are one of two mammals that are found virtually everywhere on earth (rats are the other and they merely follow people). Humans also have a penchant for trying things that they should not, like ascending mountains, riding up in hot-air balloons, venturing to the poles or into deserts, diving deep into the sea, jumping off bridges and out of aeroplanes, or running great distances. This gives people like Ashcroft, who study the physiology of such actions, a ready supply of answers to their questions.

How high can you go? Most people get altitude sickness somewhere around 12,000ft. Around 30,000ft most of us will die fairly quickly if not supplied with oxygen. How deep can you go? Divers have managed about 250ft unaided. How hot can you get? The late 18th-century treasurer of the Royal Society once baked himself (and some steak and eggs) for 15 minutes at over 1000C, the boiling point of water.

Ashcroft's accounts of how humans adjust to extremes is authoritative and sprinkled with the human-interest anecdotes that one has come to associate with good science journalism. I never knew, for example, that when Celsius established his temperature scale, he set 00 as the boiling point of water and 1000 as its freezing point; or that during Napoleon's retreat from Moscow in the winter of 1812, it was so cold that soldiers partially butchered live horses, using them as "living larders". These kinds of stories, and their supporting facts and physiological explanations, appear on nearly every page.

Beyond a compendium of impressive and hard-won facts, what is the message or big idea of Life at the Extremes ? What apart from "extremes" ties it all together? One possibility is that for Ashcroft, a professor of physiology, the medium of physiology is the message. The act of being able to explain how muscles work, how circulatory systems adjust blood pressure and so on, testifies to the success of modern physiology in understanding the body. Equally, these explanations in some sense validate the extreme behaviours by showing that they - unlike UFOs and crop circles - have rational explanations.

A potential danger is hidden in this approach to scientific writing. Physiology, unlike disciplines such as physics, cosmology or biology, lacks a unifying and synthetic theory. Each of Ashcroft's physiological explanations of an extreme behaviour is added, almost independently, to the others, and not as part of a common theory. Scientific explanations meant to enhance the behavioural exploits are instead at risk of becoming their doppelgängers . The question of how fast can Olympic sprinters run gets replaced by the more primary question of how much fast-muscle fibre these sprinters have. One must share Ashcroft's fascination with the explanation, marvel at the beauty of the workings of the body, or at science itself, for the text to carry the reader along. Once it is realised that some other animal is better at most things humans can do (a cheetah could easily win the Olympic 100-metres gold medal), even the original extreme behaviours can begin to seem less impressive.

An alternative to Ashcroft's Guinness-Book-of-Records -meets-human-physiology approach might have been to ask if there is something physiologically special about humans. Are humans especially versatile or adaptable, or less so, or about the same as other animals? How does our endurance or strength compare to other animals?

An interesting question, given humans' vast geographic range, is whether humans show greater phenotypic variability than other animals. For example, the Inuit are short and stocky, the better to conserve heat. Tall, thin bodies characterise peoples who live in hot climates. These differences are built-in: raise an Inuit in a hot climate and they do not grow up tall and thin. There is good reason to believe that there are many other "local" genetic adaptations among humans.

Questions of evolutionary potential could also be posed. To what extent are we blinkered in our assessment of humans by our narrow time in history, reflecting as it does only those adaptations of modern humans? Given enough time, might natural selection be able to produce a race of people who could live year-round above the current upper limit of about 16,000 feet? There are geese that routinely fly over the top of Mount Everest.

The question of potential is not merely of intellectual interest. Humans should perhaps be bracing themselves for a new set of extremes. Increasing population densities will mean extremes of exposure to infectious disease. Loss of the ozone layer means extremes of exposure to ultraviolet radiation. Modern health and sanitation are bringing extremes of age. Extremes of pollution bring high levels of exposure to toxins. And what about the extremes of psychology and stress that accompany fast-paced lives in the information age? What are the cognitive consequences when people are asked to juggle large amounts of information? Can we adjust to these extremes?

These are the topics that may most acutely define the coming years. They are gritty and lack the grandeur of conquering Mount Everest, and physiologists such as Ashcroft may be called upon to explain them.

Mark Pagel is professor of evolutionary biology, University of Reading.