Gradlization of the Modern
Human Skeleton
The latent strength in our slender bones teaches lessons
about human lives, current and past
Christopher B. Ruff
P eople often think of the human skel-
eton as a symbol of death. In one
sense this is true: Bone resists decompo-
periments that help connect the specific
geometr)' of a bone to a certain pattern
of behavior (and vice versa).
hip fracture is 17 percent among white
women and 6 percent among white
men. The great majority of these frac-
sition better than flesh, so it has a greater One of the ctmclusions we've reached tures occur in adults over 50 and result
chance of being preserved after death. is that the skeletons of human beings from minimal to moderate trauma—
However, bone is also a living tissue. have changed over the past 2 million usually a fall from standing height or
The skeleton is remarkably dynamic years, becoming less robust, or more less. Broken vertebrae and wrists are
during life—even in adults—and it re- gracile. Our explanation for this phe- also common in this age group.
sponds to metabolic needs and mechan- nomenon provides new insight into the The risk of fracture among the elderly
ical requirements. When muscles grow modem problem of osteoporosis and isn't uniformly high in every popula-
stronger, the underlying bone adapts confirms that our bones retain their an- tion, howe\'er. Northern Europeans and
by changing its physical shape to bear cient capacity to grow strong. people of European ancestry in other
the increased stress. Likewise, atrophied parts of the world (North America, Aus-
muscles lead to weakened bones. In this Sticks and Stones and Sidewalks tralia, New Zealand and South Africa)
way, our bones tell the story of our lives have Hgher rates than African, African-
Vertebrate skeletons must be both rigid
long after we're gone. American and some Asian and Pacific
and strong, but animals have to balajice
Because most of the archaeological these needs against the cost of pmducing, groups. During the second half of the
and paleontological record consists of maintaining and maneuvering a heavi- 20th century, the fractvire rates among
bones, skeletal remains form the basis er skeleton. Consequently, skeletal size high-risk European populations grew
for most of what we know about hu- tends to match mechanical requirements even higher, but this increase was mod-
man ancestors and our evolution. My closely. There are disadvantages to hav- est compared with the spike in fractures
colleagues and I read the stories of ing grossly under- or ox'erbuilt bones. among residents of Hong Kong, Singa-
these ancient peoples through the bones pore and other rapidly urbanizing popu-
Engineers use a similar concept of
they've left behind. This work builds lations in Southeast Asia. In these areas,
building a structure to meet a specific
on a record of controlled laboratory ex- the low incidence of hip fracture in the
need. This so-called factor of safety is
1960s quickly gave way to a rate similar
the ratio of actual strength to required
to that of Europeans by the 1980s.
strength under maximum load. Biome-
Christopher B. Ruff is a professor and director of chanical engineers such as R. McNeill Older people suffer more broken
the Center for Functional Anntotni/ and Evohdion
Alexander at the University of Leeds bones because the mass and strength of
nt tile folms Hopkins Unirersiti/ Sclinol of Medi-
estimate that factors of safety for verte- bone decrease with age. There is no sin-
cine in Baltimore, Maryland. He receiivd his Ph.D.
in biological antliropologij at the University of
brate limb bones generally range from gle reason why this occurs, or why some
Pennsylvania in 1981 and carried out postdoctoral two to four. Indeed, limb-bone fractures individuals and populations are more
work in the Orthopaedic Biomechanics Labora- are relatively rare. Scientists estimate vulnerable than others. Like other com-
tory at Beth Israel Hospital and Harvard Medical that an individual bone has a one to plex traits, age-related changes in bone
School. Ruff joined the faculty of Johns Hopkins three percent lifetime risk of fracture, result from interactions between envi-
ill 1983. His major research interest is the relation based on data from a variety of species. rcjnmental and genetic factors. Scientists
between mechanical forces and bone structure in have linked changes in bone strength to
There is one condition, however, that
living and fossil animals. Most recently he has been variations in physical activity, the lev-
studying the effects of changes in subsistence strat-
leads to far higher rates of bone failure:
osteoporosis, in which bone becomes els of dietary calcium and vitamin D,
egy on skeletal form in Hobceiw Europe and West- and alcohol and tobacco use. However,
em Asia. Address: Center for functional Anatomy more porous and brittle. This condi-
tion is particularly prevalent among among these, physical activity is the vari-
and Evolution. Johns Hopkins Unii'ersit]/ School
older women. In the United States, the able most likely to account for the get>
of Medicine, 1830 E. Monument Street, Baltinwre,
MD 21205. Internet: cbruff@jhmi.edii estimated lifetime risk of osteoporotic graphic heterogeneity in the incidence of
508 American Scientist, Volume 94
;. Inc.
Figure 1. The bones of anatomically modem humans are, on average, more slender and less strong than those of our ancestors. Although this trend
of gracilization has heen progressing for more than a million years, the pace of skeletal weakening has accelerated over the past few millennia.
However, individuals who perform rigorous exercise develop much more rigid bones, nearly equal in strength to the skeletons of our ancestors.
This computer-generated artwork is based on individual x-ray images.
www.americanscientist.org 2006 November-December 509
1,600 tions be reconciled? The answer comes
from basic principles of engineering.
Predicting Bone Strength
1,200 - Wlien an engineer analyzes a structure
to see how strong it is, he or she takes
into account not only the design, but
also the properties of the construction
800- materials and the size of the stnicture.
This analysis is a little simpler for bone
biomechanics because the material prop-
erties of bone—at least the kind found
400- in most parts of the skeleton—are fairly
constant within and between species.
On this basis it appears that bone tissue
evolved only once. From giant whales
to tiny shrews, the many skeletons that
50-59 60-69 70-79 80-89 have existed during vertebrate history
age (years) are largely made of the same substance.
As a result, those of us who study old
Figure 2. The rate of hip fractures among Hong Kong women grew dramatically between bones can compare samples that have
1966 and 1985, a period of increasing industrialization. The incidence now equals or exceeds been buried for milleiinia. The material
the fracture rate found in some high-risk European populations. Bone fractures among Hong properties of these bones may change
Kong men show a similar trend. A decrease in physical activity, coupled with traditionally low
with time, becoming friable or fossil-
calcium intakes, probably explains the elevated risk. (Adapted from Lau ct«/. 1990.)
ized, but their size and shape are gener-
fractures. In Hong Kong, for example, in- life is a recent development for Hoino ally well preserved.
creasing urbanization and mechanization sapiens. Elderly members of our ances- Because the bones themselves aren't
have led to a reduction in weight-bearing tral populations had many fewer hip suitable for testing, we use their dimen-
activities. Together with a low-calcium fractures than senior citizens do today, sions in a computer simulation, or mod-
diet, this shift to a more sedentary life- even after controlling for their shorter el, to predict their original strength. We
style is the most likely explanation for life spans. In fact, out of many thousands can represent the long bones of the limbs
the recent increase in fracture rates. John of excavated archaeological specimens, fairly precisely using the same type of
Chalmers and K. C. Ho at the Universi- only a handful of hip fractures have ever model that an engineer would use to
ties of Edinburgh and Hong Kong, who been described. Yet the same specimens judge the strength of a staictural beam.
authored the 1970 paper on this subject, do show age-related loss of bone mass The most important properties in this
actually predicted this trend. or density—and at a frequency and se- simulation are quantities that describe
From an evolutionary perspective, the verity similar to modem levels. How can its cross-sectional size and shape. The
high incidence of broken bones late in these seemingly contradictory obser\'a- cross-sectional area of bone determines its
axial rigidity {in other words, how resis-
+115% tant the bone is to deformation under
compression or ter\sion). Other proper-
ties are called the second moments of area
and section moduli, which measure the
bone's resistance to bending in different
planes as well as to torsion (twisting).
These last two variables depend on the
amount of material in the cross section,
but they depend even more on how far
from the center of the cross section that
material is distributed. Section moduli
vary as a product of the third power of
the distance from the central axis, and
second moments of area vary as a prod-
uct of the fourth power.
o\o
The vast majority of bone-aging
studies, including those of archaeo-
logical samples, have concentrated on
Figure 3. The thickness of the dense outer cortex is not the hest index of bone strength. Some phy-
the bone's mass, volume, density or a
sicians diagnose osteoporosis when the bone cortex gets thinner. This conclusion may not be cor-
rect if the bone's diameter increases, a change that can accompany aging. This cartoon compares combination of these, However, these
two hypothetical cross sections of bone: baseline (a) and increased-diameter (IJ). Compared with parameters yield an incomplete picture
the blue cortex of a, the green cortex of (' makes up a smaller fraction of its cross section (expressed of skeletal biomechanics. Based on engi-
as percent cortical thickness or percent cortical area). However, the bone with the expanded cross neering principles, animal models and
section (b) would be more rigid (second moments of area) and stronger (section moduli). human observations, the architectural
510 American Scientist, Volume 94
properties described above are probably In one set of experiments, we ob-
more important in determining overall tained optical cross sections from the
strength and the likelihood of fracture. long bones of more than 100 human
In fact, without an engineering per- specimens that were 5,000 to 1.9 mil-
spective, some commonly evaluated lion years old. All were members of the
measurements can easily be nusinter- genus Homo, either direct ancestors or
preted. For example, the cortex—the close relatives of modem humans. The
dense outer shell that makes up the remains came from all over Africa and
shaft and ends of long bones—gets thin- Eurasia, although the earliest speci-
ner with age in nearly everyone. This mens (older than 600,000 years) were
change is most often expressed as a African, and most of the later ones were
decrease in percent cortical thickness, European. We took a few of the mea-
and most clinicians view it as a sign of surements from photographs of broken
increasing fragility. However, this in- fossils, but most of the data came from
terpretation is only valid if the outer di- x-ray scans combined with detailed
ameter of the bone stays the same. If the molds of the originals. (Computed to-
bone itself gets wider with age—^a docu- mography would have been preferable,
mented phenomenon—then its strength but this technology was unavailable at
and rigidity can increase even as the most of the museums that housed these
cortex gets to be a smaller percentage remains.) Because of differences in the
of the diameter Some scientists hypoth- condition of the samples, the best data
esize that the age-related widening of came from the diaphysis, or middle re-
long bones compensates for the loss of gion of the femur (about mid-thigh). To
overall bone mass, although the effect control for differences in body size, we
seems to vary between human popula- normalized the section modulus at this
tions. Anthropologists rarely consider site by dividing by the product of esti-
this factor in their studies of human mated body mass and femoral length. Figure 4. Outwardly similar to its modern
skeletal remains. In a plot of these relative strengths counteqiart (right), the femur of a 1.9-inillion-
versus sample age, the best fit is an year-old Homo species (left) has a much thick-
More Brain, Less Brawn exponential decrease, that is, a decline er cortex. (Adapted from Ruff et al. 1993.)
Biomechanical analysis can tell us
much about past human popula-
200-
tions and skeletal changes over time.
With my colleagues Erik Trinkaus at
Washington University in St. Louis
and Brigitte Holt at the University of
Massachusetts, Amherst, I have inves- t >
tigated the changes in relative bone £ 150-
strength among modern humans and
their ancestors from the past several
million years. I refer to "relative" bone
strength because we find that it is crit-
ical to control for variation in body CO
size: Larger bodies have longer, stron- £ 100
ger bones—a rule that is particularly
true for weight-bearing bones. This
relation applies to adults within and
between hominid species, and also to
individuals as they grow.
50
Although we can easily measure the
length of an archaeological or fossil bone 105 104 10 102
specimen, body mass is more diificult to years before the present (log scale)
calculate. Our methods for doing so are Figure 5. The average bone strength of human beings and human ancestors has fallen during
based on the size of the femoral head the past 2 million to 3 million years. This graph shows temporal changes in the strength (section
(the ball that fits into the hip socket), and moduli) of the femur relative to body size. Time is expressed in logarithmic units because the
estimates of the individual's original to- relation is exponential. The solid line shows a regression drawn through 104 individual fossils
tal body height and breadth (from long (purple circles) attributed to the genus Homo; the dotted line is the theoretical extension of this
bone lengths and pelvic width, usually). line to the near-present The blue triangle is "Lucy," an earlier human relation laustralopith-
We avoid the possibility of circular rea- ecine) not included in the regression. The blue circles indicate mean values for three popula-
soning because the femora! head shows tions of anatomically modem humans: an archaeological sample of Native Americans from the
American Southwest who lived about 900 years ago, and East Africans and U.S. whites from
patterns of growth that are largely inde-
the early to mid-20th century. The error bars indicate plus or minus two standard deviations for
pendent of die shaft, where biomechani- the Native Americans and East Africans; individual body-size data were unavailable for U.S.
cal properties are measured. whites. (Adapted from Ruff 2005.)
www.americanscientist.org 2006 November-December 511
that gets progressively steeper. There is We also compared these values to
a lot of scatter in the data, but the trend more recent human remains from less
is statistically significant. From roughly than 1,000 years ago, using three di-
2 million years ago to about 5,000 years verse populations from North America
ago, human bones became almost 15 and East Africa. The bones from all of
percent weaker. these specimens were, on average, 15
percent weaker than those from 5,000
years ago—in fact, they lay below the
extended regression line from the main
data set. Thus, relative bone strength
decreased even faster during the past
5,000 years than it did over the previous
2 million years.
The Penalty for Tool Use
Because the remains are seldom com-
plete, it becomes more difficult to re-
construct body size for fossils of human
ancestors older than 2 million years. The
exception is "Lucy," the famous 3.1 mil-
lion-year-old skeleton from Ethiopia,
which is complete enough that we can
estimate her original body size with a
fair degree of confidence. Judging by
our calculation of Lucy's relative femoral
strength, her bones were even stronger
than those of the early Homo specimer\s
and almost twice as strong as an average
human from several hundred years ago. Figure 7. Adult chimpanzees in the wild weigh
between 30 and 60 kilograms (between 66 and
Lucy was an australopithecine—a 132 pounds), yet primatologists estimate that
member of a very early group on or chimps are more than twice as strong as hu-
near the lineage leading to modem hu- man beings, on average. The musculature
mans. Although Lucy and her relatives
P walked bipedally, they most likely kept
that generates such force is especially visible
on Cinder, a female chimpanzee at the Saint
to the trees more than later Homo and Louis Zoo who lacks body hair as a result of
probably weren't long-distance travel- the disease aloyecia arcata. Remarkably, Cin-
ers. The arm bones from other mem- der is somewhat small compared with other
bers of this group appear to be very members of her species. The bones of chim-
strong, which may reflect this behav- panzees, like their muscles, are much stronger
ior. (Cross sections from Lucy's arm than those of modem humans. (Photograph
courtesy of Carol Weerts, Saint Louis Zoo.)
bones were unfortunately not avail-
able for study.) If true, this hypothesis The bone-strength results also imply
makes Lucy's relative femoral strength that earlier human ancestors had stron-
even more remarkable, since she prob- ger muscles, a hypothesis consistent
ably walked less than do many mod- with the large muscle-insertion scars
em humans.
that anthrtipologists see on many of the
I We see similar results among modem
nonhuman primates, such as chimpan-
zees, gorillas and baboons. Relative to
specimens. (The scar observations, how-
ever, are far from definitive, as the bones
of some modem nonhuman primates
body size, their arm and leg bones are don't carry such marks.) If early humans
much sfronger than those of humans. were indeed more muscular than you or
(The difference is greater for the arm I, they probably got that way (at least in
Figure 6. Australopitheais aftirensis was one than the leg, of course, since these spe- part) because they were more active and
of the earliest bipedal hominids and prob- cies kx:omote using both sets of limbs.) vigorous. This, in tum, is probably re-
ably an ancestor of the genus Homo. This Not surprisingly, nonhuman primates lated to tool use: The rise of technology
specimen, "Lucy," is unusually complete for a also appear to have much stronger that has accompanied human evolution
3.1 million-year-old skeleton, which enables muscles than humans relative to their has, in effect, progressively shielded the
an accurate estimate of overall body size (she size. Thus, the bone strength results
human body from its environment.
stood about 1.1 meter or 3 feet, 8 inches tall). make sense: Stronger muscles generate
Taking body size into account, Lucy's femur greater force, greater force increases the Scientists have never found tools
is considerably stronger than those of more mechanical stress on the bones, and this from Lucy's time period, and in terms of
recent human ancestors, despite the fact that stress induces the bone to adapt over technology, her kin probably interacted
her species spent less time walking than did with the natural world much as mod-
time by becoming more rigid.
their descendants. em chimpanzees and gorillas do. This
512 American Scientist, Volume ''4
absence of technology predicts a very people with differing activity levels. Un- sional athletes the average left-right
high relative bone strength. Early Homo like most animals, human beings don't asymmetry in bone strength is about 5
had simple stone tools, and their bones use their arms for locomotion, leaving to 10 percent. The tennis players in the
were not quite as strong as Lucy's, but them freer to reflect asymmetrical usage study were between 14 and 39 years
still n^uch more rigid than the average and structure. In fact, human forelimbs old and had played for at least 5 years.
modern human's. The comparatively show more left-right asymmetry than All started playing between ages 5 and
weak bones of recent humans are an those of any other mammal. In the nor- 19. Interestingly, these changes were
inevitable consequence of a sedentary mal population, right-handers and left- more pronounced among subjects who
lifestyle and ubiquitous, sophisticated handers have similar magnitudes—but began playing earlier in life. Since we
tools that make physical strength much opposite directions—of bilateral asym- published our data, other studies have
less relevant to survival. For perhaps the metry in the strength of the second also noted that bone adaptation is partly
same reason, human bodies as a whole metacarpal (a hand bone). Righties are age-dependent. Adult skeletons remain
have gotten smaller on average during stronger on the right, lefties on the left. responsive to increased exercise, but
the past 50,000 years, despite very re- One of the advantages of this compari- they respond more slowly and less com-
cent increases that are probably tied to son is that it inherently controls for non- pletely thiin those of children.
better nutrition and health care. behavioral factors such as overall body Unlike the cross sections from the hu-
si2e, nutrition and hormonal influences, meral shaft, the si^es of the right and left
Tennis, Anyone? which affect both sides equally. elbow joints were more similar in the
Many animal studies have demonstrat- In the 1970s, Henry H. Jones (who tennis players. In animal studies, too,
ed that bones become stronger with ex- was one of my undergraduate advi- the size of the articular surface and the
ercise. For example, the femurs of young sors), James D. Priest and their cowork- length of the bone (which depends on
pigs that ran an hour every day for a ers at Stanford University x rayed the articular growth) are less affected by me-
year were 24 percent stronger than those arms of professional tennis players. chanical loadings than are diapbyseal
of sedentary controls. The increase was They focused on the bone in the upper cross sections. The cortex of the shaft of
purely a product of geometric chang- arm called the humerus and compared long bones appears to be particularly
es—primarily a thickening of the bone the dimensions of the bone cortex in the "plastic," or responsive to changes in
cortex—with no effect on bone mate- playing and nonplaying arms. In the mechanical loadiiigs during life, so it's
rial properties. Other studies have noted dominant arm, the outer surface of the helpful for reconstructing the behav-
similar findings, further supporting the cortex had gotten bigger and the inner iors of past populations. The restrained
practice of using a bone's geometry to surface had gotten smaller. growth and remcxieling of the ends of
estimate past mechanical loadings. Almost 20 years later, we were able the bones may help to avoid incongruity
The human upper limb presents a to get Jones's original measurements at the joint surface that predisposes it to
"natural experiment" of this sort be- and calculate geometric section proper- problems such as artliritis.
cause almost e\ eryone favors one hand ties. By our calculations, playing-side We also examined the humeri of Ne-
or the other for most tasks. Thus, scien- humeri were more than 40 percent andertals (50,000 to 100,000 years old)
tists can compare bone adaptations in stronger on average than non-playing- and anatomically modern humans
dominant and nondominant arms of side humeri. By contrast, in nonprofes- (10,000 to 30,000 years old) for which
Figure 8. Vigorous exercise can lead lo large increases in bone strength. This diagram shows the average difference in cross-sectional dimen-
sions of the humerus between nonplaying ibluc) and playing (blue plus green) arms of professional tennis players measured by Henry Jones
and coworkers at Stanford University. The author and his colleagues re-analyzed the data and calculated average increases in bone rigidity and
strength of 62 percent and 45 percent, respectively, in the playing arms. The changes were most pronounced in players who began training at an
early age, such as Amelie Mauresmo, currently ranked among the top female professionals in the world. Based on data from Ruff ct al., 1994.
www.americanscientist.org 2006 November-December 513
both arms had been preserved. (Such to a lesser workload. The gracilization Jones, H. H., J. D. Priest, W. C. Hayes, C. C.
Tichenor and D. A. Nagel. 1977. Humeral
fossils are somewhat rare.) (3ur ances- of the modem human skeleton is prob- hypertrophy in response to exercise, journal
tors showed bilateral asymmetry in ably a direct result of our consistently of Bone and joint Surgery 59A:204-208.
shaft strength almost as great as that of advancing technology. Lang, T, A. LeBlanc, H. E\'ans, Y. Lu, H. Genant
the modem professional tennis players. This conclusion has implications for and A. Yu. 2004. Cortical and trabecular bone
Based on experimental studies using understanding the etiology of osteopo- mineral loss hxmi the spine and hip in long-
human volunteers, Daniel Schmitt and rotic fractures. As noted above, broken duration spaceflight. joiirmil of Bone and M/H-
19:1006-1012.
Steven E. Churchill at Duke University hips are more common in urbanized,
t^u, E. M., C. Cooper, C. Wickham, S. Doiinan
have suggested that this asymmetry re- less physically active populations. The and D. J. Barker. 1990. Hip fracture in Hong
flects the use of weapons or tools that significant increase in skeletal strength Kong and Britain. Internationa! journal ofEpi-
loaded one arm more than the other. that is gained through physical exer- dttmobgi/19:1119-1121.
Interestingly, all five Neandertals and cise, if maintained throughout a per- Mosekilde, L. 1995. Osteoporosis and exercise.
19 of the 24 early modem humans have son's lifespan, may help to prevent Bone 17:193-195.
stronger right humeri, indicating right- such fractures. Roy, T A., C. B. Ruff and C. C. Plato. 1994. Hand
handedness. In the larger group of early dominance and bilateral asymmetry in struc-
The bones of our ancestors show ture of the second metacarpal. American lour-
modem humans, the frequency at which that the human skeleton was once i!alofPln/i:iailAnthropoiogif94:20?r-2n.
we found stronger arm bones on the stronger than it is today. Studies of Ruff, C. B. 2000. Btxly size, body shape, and long
right side—about 80 percent—is similar modern athletes, however, demon- bont? strength in modem humans, journal of
to the rate of right-arm-bone asymmetry strate that we are still able to achieve Human Evolution 38:269-290.
in a wide range of recent human popu- such strength. The skeletons in our Ruff, C. B. 2005. Mechanical determinants of
lations. Not surprisingly, it's also similar evolutionary closet can teach us some bone form: Insights from skeletal remains.
journal ofMusculoskeletal andNeuronal Interac-
to the frequency at which people favor valuable lessons about modern life- tions 5:202-2}2.
their right hand. (Many sources state styles and their consequences. Ruff, C. B., E. Trinkaus, A. Walker and C. S. Lars-
that 90 percent of the population is right- en. 1993. Postcranial robusticity in Homo, I:
handed, but they're usually referring to Bibliography Temporal trends and mechanical interpretii-
writing preference, wliich can be biased Alexander, R. M. 1981. Factors of safety in the tion. American journal of Phydccil Anthropologvi
staicture of animals. Science Progress, Oxford 91:21-53.
by various cultural factors. In cross-
cultural studies, the frequency of right- 67:109-130. Ruff, C. B., A. Walker and E. Trinkaus. 1994.
Auerbach, B. M, and C. B. Riiff. 21X16. Limb bone Postcranial aibusticity in Homo, 111: Ontog-
arm preference is a little lower for activi- eny. American Journal of Physical Anthropology
bihiteral asymmetry; Variability and com-
ties such as throwing and hammering.) monality among mtxiem humans, louriwl of 93:35-54.
Humm Emiution 50:203-218. Schmitt, D., and S. E. Churchill. 2003. Experi-
The Leg Bone Connected to the ... Brandwood, A., A. S. Jayes and R. M. Alexander. mental evidence concerning spear use in Ne-
1986. Incidence of healed fracture in the skel- andertals and early modem humans. JourmI
It is clear that limb bones, at least, re- of Arcliaeological Science 30:103-114.
spond to increased mechanical force by etons of birds, mollusks and primates, lotiriml
ofZoohgi/ 208:55-62.
changing their geometry, adding bone
Chalmers, J., and K. C. Ho. 1970. Cetigraphical
material to strengthen the outer cortex.
variations in senile osteoporosis. The associa- For relevant Web links, consult this
The bilateral asymmetry found in pro tion with physical activi^. journii! of Bone and issue of American Scientist Online:
tennis players and pre-agrarian humans loiiit Surgery 52B:667-675.
suggests that bt^ne strength can increase Holt, B. M. 2tX)3. Mobility in Upper Palet>lithic
by 40 percent or more under the proper and Mesolithic Europe: Evidence from the http://www.americanscientist,.org/
conditions. Conversely, reduced me- lower limb. American Journal of PIn/sical An- lssueTOC/issue/902
chanical loads lead to the loss of bone. thropolagy 122:200-215.
For example, after six months in space
under zero-gravity conditions, the leg
bones of astronauts aboard the Inter-
national Space Station were 20 percent , BOBBY LE5K-0
weaker on average than before, based
on section moduli derived from bone
mineral scans of the femoral neck. Para-
lyzed patients experience even greater
losses over longer time periods.
These examples may represent ex-
tremes, but they demonsfrate the poten-
tial for bone to adapt when circumstanc-
es change. In a few years, the strength of
a person's bone structure can change as
much as the total average change over
the past 2 million years of human evolu-
tion. Although some of this evolution-
ary change may reflect nonmechanical
factors, including genetic changes, the
most parsimonious explanation is that
the human skeleton has simply adapted
514 American Scientist, Volume 94