The ancestral shape hypothesis: an evolutionary explanation for the occurrence of intervertebral disc herniation in humans

This study explicitly tested the ancestral shape hypothesis, which holds that intervertebral
disc herniation preferentially affects individuals with vertebrae that are towards
the ancestral end of the range of shape variation within H. sapiens and therefore are less well adapted for bipedalism. We tested two predictions of
this hypothesis with shape data recorded on the last thoracic and first lumbar vertebrae
of orangutans, chimpanzees, healthy humans, and humans with Schmorl’s nodes, which
are bony indicators of intervertebral disc herniation. The first prediction was that
there should be differences in shape between healthy human vertebrae, chimpanzee vertebrae,
and orangutan vertebrae, due to the different modes of locomotion of the taxa. The
second prediction was that pathological human vertebrae should share more similarities
in shape with chimpanzee or orangutan vertebrae than do healthy human vertebrae. The
results of the analyses were consistent with both predictions. We found that the last
thoracic and first lumbar vertebrae of healthy humans, orangutans, and chimpanzees
differ significantly in shape, which is in line with the first prediction. We also
found that human vertebrae with Schmorl’s nodes share more similarities in shape with
chimpanzee vertebrae than do healthy human vertebrae, which is consistent with the
second prediction. Thus, the study supports the ancestral shape hypothesis.

A potential alternative explanation for our findings needs to be considered. The vertebral
shapes associated with Schmorl’s nodes may be a consequence of intervertebral disc
herniation rather than its cause. It is certainly the case that vertebrae can remodel.
For example, the shape of the vertebral body is known to change with increasing age.
Body height tends to decrease and there is often an increase in surface concavity
as the endplate collapses 59]. However, we do not consider intervertebral disc herniation causing changes in vertebral
shape to be a good explanation for our results. One of the main shape differences
identified between healthy human vertebrae and those with Schmorl’s nodes relates
to the neural foramen 25]. Previous work indicates that the shape of the neural foramen does not change after
the neural arch fuses to the vertebral body 60],61] at around six years of age in humans 62]. Therefore, any factor that influences the shape of the neural foramen must act during
spinal development. Bone remodelling during development could influence the shape
of the vertebrae, including the neural foramen. Although this could explain why there
is a difference in shape between pathological and healthy human vertebrae, it does
not explain the relationship identified between pathological human and chimpanzee
vertebrae. This explanation would require that bone remodelling result in vertebral
shape changes that systematically approach a shape functionally related to quadrupedal
locomotion. This, we submit, is less parsimonious than the ancestral vertebral hypothesis.

A possible functional explanation for the association between vertical disc herniation
and vertebral shape is provided by Harrington et al. 24]. These authors suggest that the diameter of the vertebral disc influences its ability
to withstand tension during compression. Their argument rests on LaPlace’s law 62], which states that the ability of a fluid-filled tube to withstand tension decreases
with increasing radius. According to Harrington et al. 24], the rounder bodies of pathological vertebrae would have a larger diameter than the
more heart-shaped bodies seen in healthy vertebrae, making the intervertebral disc
less able to withstand stress 24],62]. We also found that pathological vertebrae have shorter pedicles compared to healthy
vertebrae. The pedicles act as structural buttresses for the vertebral body and play
an important role in load bearing during axial compression 63]-68]. It has been hypothesized that the shorter pedicles identified in vertebrae with
Schmorl’s nodes may be less able to withstand physical strain placed on the spine
25],45]. Since bipedalism causes a large amount of axial loading on the lower vertebrae 30], we hypothesize that the combination of round vertebral bodies with short pedicles
may provide less support for the spine during bipedal posture and locomotion.

Our results have implications for medical science beyond shedding light on the causes
of intervertebral disc herniation. One is that vertebral shape may be a factor that
could help predict an individual’s susceptibility to vertical intervertebral disc
herniation. The shape analysis techniques used in this study can also be used on medical
images, such as CT scans. It may be possible for clinicians to investigate an individual’s
vertebral shape and identify those who may be at risk of developing the condition.
This ability would have significant diagnostic and preventative value, especially
for high-risk individuals, such as athletes 69]. In addition, a better understanding of the role that locomotion and posture plays
in the health of the spine could aid in the treatment of individuals afflicted with
symptomatic vertical intervertebral disc herniation. Locomotion is recognized as an
important factor in rehabilitation for sufferers of back pain 70], and understanding the role that vertebral variation can play in spinal health could
aid physiotherapists to refine activity and exercise regimes. Thus, the findings of
this study may not only help medical practitioners to understand why some individuals
are more commonly afflicted with back problems than others, but may also lead to advances
in the identification, prevention, and treatment of people suffering from intervertebral
disc herniation.

In addition to offering these potential clinical benefits, our results provide further
support for the claim that an evolutionary perspective can shed important light on
human health problems 71]-74]. Evolutionary medicine has identified the value of considering evolutionary adaptations
to enable better understanding of human developmental issues, chronic diseases, and
nutritional needs 74], but the influence of skeletal morphology on human health has received little attention.
Our study highlights the potential of using osteological analyses of skeletal variation,
including comparative analyses between humans and non-human primate species, in evolutionary
medical studies. Bipedalism has been suggested to impact human spinal and joint health
28]-30],75],76], but few studies have been carried out to evaluate this proposition 30]. The identification of an ancestral vertebral shape that influences the occurrence
of a common spinal pathology supports the idea that the relatively rapid evolution
of bipedalism in the hominins may continue to impact modern human health.

The main goal of our study was to shed light on a major contemporary health problem
with the conceptual and analytical tools of evolutionary biology, but our results
also contribute to the understanding of human evolution. Specifically, they shed additional
light on the evolution of bipedalism, and in particular, the functional anatomy associated
with it. Previous studies have identified morphological characteristics purported
to relate to bipedalism 77]-80]. The present findings add features to this list—a larger neural foramen relative
to body size, taller, narrower pedicles, and a more heart-shaped vertebral body. There
are two persistent debates in palaeoanthropology regarding the evolution of bipedalism
and a better understanding of the functional anatomy of bipedal vertebrae may contribute
to their resolution. The first debate regards the timing of the emergence of bipedalism
in the evolutionary record. The understanding of how human vertebrae are unique among
hominoids enables the identification of fossil vertebrae adapted for bipedal locomotion;
this will help researchers infer which species were bipedal, provide additional insight
into how bipedalism evolved, and suggest whether it followed a gradual or punctuated
pattern of evolution. The second debate surrounding the evolution of bipedalism is
whether early bipeds walked with their knees and hips in a flexed position, like chimpanzees,
or if their mode of bipedalism resembled our own 81]-84]. The ability to identify vertebral shape characteristics unique to humans and compare
these with features unique to modern chimpanzees may provide additional insight into
the functional anatomy required for habitual bipedalism and help understand the evolutionary
trends that led to the modern human gait.

With regard to future research, several possibilities suggest themselves. Firstly,
if the ancestral shape hypothesis is accepted, it prompts the question of how this
shape influences the occurrence of vertical intervertebral disc herniation. This could
be investigated with biomechanical studies of the interaction between locomotion,
vertebral morphology, and the soft tissues of the spine. Secondly, this area of research
would benefit from the use of 3D shape analyses of human and non-human ape vertebrae
to investigate how 3D vertebral morphology relates to locomotion and human spinal
health. Lastly, the clinical value of this research would be substantially increased
with the inclusion of in-vivo medical images of individuals with and without back
problems.