A few months ago, I had the chance to view the BBC documentary series "Human Planet". As stunning as the cinematography for the series was, what made the most lasting impression on me was the sheer amount of physical exertion required to eke out a living by people residing in pre-agricultural societies: hunter-gatherers routinely engage in sustained bouts of walking and running that would put the most fervent joggers in the Western world to shame, and it isn't as if they consider themselves to be doing anything out of the ordinary in the process.
Life for the earliest species of Homo, such as Homo ergaster and Homo erectus - hominids without the knowledge of fire, the ability to build permanent shelters, or any weapons more effective than wooden spears usable only at close range, and very probably lacking anything more than a primitive protolanguage consisting of nouns and verbs strung together with minimal grammar - must have been hard in the extreme, which is probably why there are so few intact fossils of our distant ancestors available today; the genetic evidence corroborates that until our species began its exponential expansion in numbers around 60,000 years ago, there were fewer bipedal apes of all species on the entire planet than there are chimpanzees living in the African rainforest even today, or no more than a few hundred thousand individuals at best.
Accepting that our distant ancestors were marginal in numbers, and with little to protect them from the forces of nature other than their (comparetively limited wits) and their primitive stone tools, how did they manage to make a go of it for the 1.7 million years it took to go from Nariokotome Boy to our indolent, corpulent, reality-tv addicted human masses? The answer is that they did it by sheer physical strength and stamina. Our bodies are superbly adapted to running in the tropical heat, and when the sun is blazing overhead on the savannah, a human being in good physical condition can keep on going long after even the fastest antelope will have collapsed from heat exhaustion: this ability to keep running in the heat is what we have to thank for our hairless bodies and numerous sweat glands, and - unique as it is amongst mammals - it would have given our distant ancestors a crucial advantage over the other apex predators on Africa's plains.
Modern humans largely retain the physiological advantages which were enjoyed by our predecessors, even though we are relatively "gracile" (i.e. puny) by comparison with all hominids before us: if a male Homo erectus in peak condition were to be somehow transported into the present, he would probably sweep the 2012 Olympics without much effort. In some part this difference is due to a physiological compromise which has been necessitated by the continued expansion in brain-size over the last 2 million years: human females must give birth to infants whose brains are much bigger than those of Homo ergaster, and this has required a widening of the birth canal in our women (and therefore, to some degree also in men); one side effect of this has been that the mechanical efficiency of running in our species has been somewhat compromised.
Another reason for the relative physical weakness of Homo sapiens by comparison to those who came before it is clearly attributable to a lessening of selective restraints: as technology has made it ever easier to live and reproduce without extreme physical exertion, ever larger numbers of individuals who would have perished without offspring in the Paleolithic are able to pass on their genes. If one wishes to understand why so many people suffer from congenital myopia, or have jaws too small to properly hold all of their crowded teeth, here is the reason why; the latter trend is so noticeable that the prevalence of impacted wisdom teeth in different populations can be clearly linked to how long their ancestors have been farming ...
Still, as great a role as genetic drift might play in the weakening of our species, it is clear that a major - and even possibly the major - reason why we are so much weaker than our predecessors has nothing to do with genes, and everything to do with the slothful lives the great majority of us lead. Even by comparison with Westerners living just 100 years ago, the average adult in Europe or North America today is far idler, much fatter and carrying far less muscle, a difference of a scope which cannot be attributed to genetic causes, given the short timespan involved. Even the most physically active of us struggles to walk or run 30km in a week, when a Hadza hunter will routinely cover distances greater than 100km in the same period: our armies consider a 4km run a "tough" hurdle to surmount to enlist, while hunter gatherers habitually cover 5 times that distance at a running pace. These comparisons are, if anything, flattering, as they take the fittest of us as the baseline for comparison, while some have it that just climbing a flight of stairs is enough to leave most of us out of breath!
As pathetic as the picture painted above may be, it has more than merely academic consequences. One of the things I noticed when watching "Human Planet" was that the Hadza man who had been chosen to demonstrate to the BBC team how to steal kills from lions claimed to be 60 years old, and yet, when examined from the neck down he appeared to be in better physical condition than the average 30-year old American or British male. As I discovered on looking further into the issue, this observation was not as strange as it seemed to me at the time: a lifetime of intense physical activity is in fact the closest thing we have to an elixir of youth, and what we take for granted as "normal" symptoms of the aging process is in large part (though not entirely) due to wastage attributable to inactivity. In fact, even that portion of muscle loss which can be blamed on aging can be guarded against by building up as much muscular strength as possible when young, to provide a buffer for one's latter years.
With a little forethought (and a bit of physical exertion), there is no reason why the average octogenarian should be any less vigorous than the likes of Sir David Attenborough, still visiting the South Pole at the age of 85; by contrast, under the present circumstances we see large numbers of people stooped over or entirely unable to walk while in their 60s. What makes this so regrettable is that all it would have taken for most of these individuals to have avoided such a fate is for them to have passed their lives in a manner more in keeping with what 2 million years of hard living has supremely prepared them for. None of us are meant to be sitting in idleness for 10-14 hours a day while gorging on carbohydrates.