Modern Medical Consequences of the Ancient Evolution of a Long, Flexible Lumbar Spine

A Short Lumbar Spine Makes Ape Bipedality Inefficient

Monkeys, like most quadrupeds, have long, flexible lumbar spines (ie, are “long-backed”¹⁴), which allows for propulsion from the lower back to contribute to propulsion during locomotion, which can be especially useful for arboreal monkeys that leap from support to support. Most monkeys have 6 or 7 lumbar vertebrae when counting by using the numbers lacking a rib, but have 8 or 9 when orientation of the zygapopheses is the deciding criterion (ie, they have up to 9 that permit flexion and extension).^15,16

In apes, stress is placed on the lumbar spine because of increased reliance on forelimb-dominated locomotion coupled with “bridging” between supports in which the trunk is supported between outstretched forelimbs and hindlimbs. In large-bodied apes, leaping between supports is not feasible, as the flexible terminal branches of trees would not support them. Therefore, crossing gaps in canopies necessitates bridging between multiple supports using both the forelimbs and hindlimbs. Compared with monkeys, modern apes evolved to have few lumbar vertebra (ie, they are “short-backed”), which reduced the stress placed on the lumbar spine.^17,18 Modern apes have 5 (gibbons), 4 (orangutans), or 3 to 4 (chimpanzees and gorillas) lumbar vertebrae, and humans have a modal number of 5.^16,19 Suspensory New World monkeys are similar to apes in their locomotor behavior and have also reduced the length of their lumbar spine,^14,20 which suggests a functional convergence.

In addition to a reduced number of lumbar vertebrae, African apes have functionally shorter lower backs that “entrapped” their ultimate lumbar vertebrae (in gorillas) and occasionally their penultimate lumbar vertebrae (in chimpanzees) between their iliac blades, leaving the equivalent of 1 intercostal space between the caudalmost rib and the iliac crest (Figure 1).^21,22 Therefore, the trunk becomes a rigid structure that resists lower back flexion during suspension, climbing, or bridging. Unlike great apes, humans have short, laterally flaring ilia and a broad sacrum, essentially freeing all human lumbar vertebrae from the iliac entrapment seen in African apes and permitting mobility, which is crucial for lumbar lordosis.

Figure 1.

Ventral view of chimpanzee (Pan troglodytes) skeleton. This specimen has 4 lumbar vertebrae, with the ultimate (L4) partially sacralized in morphology and completely entrapped by the iliac blades, with the penultimate lumbar vertebra (L3) partially entrapped. Used with permission by the Museum of Comparative Zoology and Harvard University.

Although chimpanzees are capable of bipedal walking, for example, when they are carrying items with their hands, they use a bent-knee, bent-hip (BHBK) gait. With a BHBK gait, the flexed knee joint remains anterior to the hip, the trunk is angled anteriorly, and the center of mass oscillates mediolaterally with each step.^23,24 Chimpanzees use a BHBK gait because their short, rigid lumbar region prevents them from positioning their trunk's center of mass over their feet in any other way.²⁵ When walking bipedally, chimpanzees require greater thigh muscle activity and must shorten their strides compared with their quadrupedal locomotion.^26,27 Bipedal walking in this posture is both inefficient and fatiguing for the muscles involved, which reduces their ability to perform negative work and increases the risk of injury.^25,28

However, there is evidence among primates that a long, flexible lumbar spine permits upright posture while walking bipedally. Japanese macaque monkeys (Macaca fuscata) have been trained to walk bipedally for entertainment purposes for centuries.²⁹ Performing macaques can walk bipedally for up to 3 to 5 km per day³⁰ Trained macaques walk with longer and less frequent strides and likely expend less energy than untrained macaques. They also exhibit a more upright posture with more extended hips and knees.³¹ Although macaque postcranial morphology is unlike that of humans, trained macaques can walk with an upright posture because they develop a partial lumbar lordosis,³² to a certain degree because of their long, flexible spine, which has an average of 7 lumbar vertebrae.¹⁶

The question remains, if a long lumbar spine is necessary for efficient bipedality, and humans’ closest relatives have short, rigid lower backs, then how do modern humans have 5 lumbar vertebrae? Fossil evidence suggests that humans and African apes share a long-backed ancestor from which chimpanzees and gorillas independently evolved a more rigid lumbar spine.^21,22,25,33 Direct evidence comes from 2 Australopithecus africanus specimens that have relatively complete lumbar spines (Sts 14 and Stw 431), both of which likely had 6 lumbar vertebrae,^34,35 as well as the Homo erectus fossil (KNM-WT-15000) dated to 1.6 million years ago, which had 6 lumbar segments.³⁶ However, this hypothesis remains controversial, as some researchers^16,19,37,38 argue that humans had a short-backed ancestor.

Lumbar Vertebrae Morphology Associated With Human Bipedality

Most mammals are quadrupedal and distribute their mass over 4 limbs, whereas bipedality necessitates that only the hindlimbs are used for support. To maintain erect posture, the trunk must be placed atop the pelvis, which in turn must be positioned directly over the feet in an almost perfect distribution of mass. Humans have, therefore, greatly modified the skeleton to maintain this balancing act.

Compared with apes, humans have a shortened ilium that brings the sacrum closer to the acetabulum and a tilted pelvis that positions the sacroiliac joint directly over the acetabulum.³⁶ However, because of obstetric constraints,³⁹ our sacrum is angled nearly horizontally. When measured as pelvic incidence, this angle is similar in modern humans (mean [SD], 54° [10°]) and our australopithecine ancestors (43.5° [2°]) compared with extant apes (22° [3°]).²⁸

As a consequence of an inclined sacrum, a large lumbosacral angle is required for lumbar lordosis. This angle ranges from 60° to 80° in adult humans, is about 30° in apes, and is nearly 0° for nonprimate mammals.^15,40 To produce a lordosis, human lumbar vertebrae and, to a lesser extent, intervertebral disks, are dorsally wedged so the anterior border of the vertebral body is taller than the posterior border, which is a pattern that increases caudad down the column.^34,36 Dorsal wedging is not seen in modern apes but is found in several fossils of human ancestors (Sts 14, AL 333-73, SK 3981b, KNM-WT 15000, and MH2), particularly in the lower lumbar elements.^34,41,42

Many quadrupedal mammals have morphologic features that act as mechanisms to prevent extension of the lumbar column.⁴³ These mechanisms include transverse processes arising from the vertebral body, zygapophyseal joints that block extension, and a styloid process.⁴³ These passive mechanisms will stretch the intervertebral discs when the spine is extended, such as when offspring either ride on their mother's back or hang from her belly.⁴³ Humans lack these mechanisms, and additional loads from carrying infants or other objects induces compressive loads on the lumbar spine, which may increase lordosis.⁴³ An increase in lordosis places pressure on the zygapophyseal joints, particularly the caudal lumbar vertebrae, which support more weight when standing than the more cranial lumbar vertebrae.⁴⁴

Modern humans’ upright posture places great pressure at the lumbosacral joint because of the increasing caudal load in the lumbar spine, the wedged morphology of L5, and the angulation of the lumbosacral joint. To ameliorate this pressure, humans have 2 unusual adaptations. The first adaptation is a change in zygapophyseal joint orientation from sagittal in the cranial lumbar vertebrae to coronal at the caudal lumbar vertebrae.⁴⁵ This sagittal orientation differs from chimpanzees, whose orientation remains coronal throughout the lumbar column. Interestingly, orangutans have a human-like pattern.³⁴ This coronal orientation helps prevent the anterior displacement of the ultimate lumbar vertebra at the lumbosacral joint.^34,36

The second adaptation is a progressively increasing distance craniocaudally between the zygapophyseal joints in the lumbar spine. The caudal more distant zypapophyses contrasts humans from apes where the interfacet distance at L1 is the same or broader than it is at S1 (Figure 2).^21,34,36 The human-like condition in our ancestors dates back to the Pliocene Epoch: the interfacet distance is greater at S1 compared with the putative L3 in AL 288-1, commonly known as Lucy,²⁵ which may help prevent spondylolosis.

Figure 2.

Dorsal view of human (Homo sapiens, left) and gorilla (Gorilla gorilla, right) lumbar spines. Lines are drawn from the zygapophyseal joints from T12 to L1 to the ultimate lumbar vertebra for both taxa (L5/S1 for human, L4/S1 for gorilla). Note that the lines converge caudad for the gorilla but diverge for the human. Photographs not to scale. Used with permission by the Cleveland Museum of Natural History.

Modern Clinical Consequences of a Long, Flexible Spine

A relatively long lumbar spine coupled with a greater lumbosacral angle and wedged lower lumbar vertebrae is necessary for modern human bipedal locomotion. However, these adaptations create a precarious balance that needs to be maintained. Although lower back pain is not exclusively associated with our adaptations for lordosis, the frequency of lower back pain (about 40% of people have this condition during their lifetime⁴⁶) suggests that maintaining proper lumbar curvature is important for our well-being. Nonetheless, several clinical issues, including spondylolysis, spondylolisthesis, and associated back pain with obesity and pregnancy are the result of unique human adaptations.

Spondylolysis is a fatigue fracture of the pars interarticularis,⁴⁷ most commonly at the L5 vertebra.^48,49 Spondylolysis is not a condition seen in apes,²⁵ and it appears to be more common in humans with greater physical activity. It is found at greater frequency in athletes, particularly those who participate in sports that involve hyperextension of the lumbar spine, than in the general population.^50,51 It was also seen at a greater frequency in a medieval British population, which suggests that their activity patterns may have led to more common fractures.^52,53

However, activity pattern alone may not predict susceptibility to spondylolysis and may correlate with interfacet distance. People with spondylolysis have less pronounced increases in interfacet distances, which is most notable at the L4 and L5 levels compared with unaffected people.⁵⁴ Furthermore, greater strain on human zygapophyses at the L4 and L5 levels may promote spondylosis.⁵⁵

Lumbar zygapophyseal orientation becomes more coronal caudad in humans. People with degenerative spondylolisthesis were found to have more sagittally oriented zygapophyseal joints than people who do not have spondylolisthesis or have spinal stenosis.⁵⁷

Spondylolisthesis may also be treated conservatively. In a retrospective study⁴⁹ of athletes aged 6 to 20 years, about 38% of athletes were shown to have worsening vertebral displacement, and about 42% had no progression over years of training. Researchers found that athletes did not have greater progression than did nonathletes.⁴⁹ Treatment can involve physical therapy, nonsteroidal anti-inflammatory drugs, osteopathic manipulative treatment (OMT), and other modalities. In fact, OMT has been shown in multiple studies to significantly reduce low back pain in short-, intermediate-, and long-term follow-up from clinical presentation.⁵⁸ This result was shown regardless of location and when comparing OMT against active treatment or placebo.⁵⁸ The problems associated with spondylolysis or spondylolisthesis either are sufficiently rare or occur late enough in life to apparently not have a strong negative selective effect.²⁵

Obesity is an additional risk factor for low back pain in both men and women.⁵⁹ Increased body mass index is correlated with a greater incidence of low back pain.^59,60 People who are overweight or obese have greater lordosis and lumbosacral and sacral inclination angles compared with people who have a normal weight.⁶¹ The greater lordosis and inclination angles can place additional loads on the apophyseal joints relative to the vertebral bodies,⁴⁴ which may be associated with low back pain.⁵³ Because there is a greater load transferred at the L4 and L5 vertebral arches compared with other lumbar levels, these joints are a potential site of low back pain.⁵⁵ An increased sacral inclination angle has been shown to correlate with spinal stenosis, spondylolisthesis, and facet joint problems, which can all cause low back pain.⁵³

Spinal alignment is also affected by the forward rotation of the pelvis. Sagittal spinal misalignment from either an affected lordosis in the lumbar or cervical spine or kyphosis in the thoracic spine can cause back pain.⁶² A person who is obese is more likely to compensate for spinal misalignment with his or her lower limbs than pelvis, which is significant because compensation via pelvic retroversion is more likely to relieve back pain caused by misalignment.⁶³ Because people who are obese already have an anterior tilt, retroversion compensation is substantially more difficult.^61,63

These causative relationships between obesity and low back pain make them relatively common comorbidities, yet obesity can also inhibit treatment outcomes for a patient's pain. When compared with people who are underweight, normal weight, and overweight, people who are obese were less likely to see improvements in disability and were also less likely to see a reduction in pain levels after thorough treatment.⁶⁴ The natural inclination would be to manage obesity and then manage low back pain, but this is not always an easy practice.

Like the length and morphology of the human lumbar spine, our modern rates of obesity have ancient roots. Our nomadic ancestors evolved to store fat whenever food became available.⁶⁵ As technology and societal organization progressed, the need to store fat because of food scarcity was dramatically reduced.⁶⁵ Therefore, easy access to high-energy food along with a decrease in energy expending gives rise to a population with increased obesity rates.⁶⁵ A majority of children who are obese have deficiencies in balance and gait patterns.⁶⁶ They also are more likely to have overuse injuries that make them prone to withdraw from weight loss programs.⁶⁶ Considering these factors can limit exercise, the general strategy of diet and exercise as a management protocol for obesity becomes all the more challenging.

Similar to obesity, pregnancy affects bipedal posture by shifting the center of mass anteriorly.⁶⁷ The increase in mass from pregnancy in quadrupeds does not result in a major shift in center of mass. However, in humans, lumbar lordosis and pelvic inclination increase as pregnancy progresses.^67,68 This increased lordosis can be accommodated by differences in morphology between men and women—men only have wedging at L4-5, whereas women typically have dorsal wedging in L3-5. Women also have greater prezygaphophseal areas at L2-5 compared with men, which allows for greater loading on the dorsal pillars.^55,67 Despite these differences, back pain in pregnant women is correlated with sagittal and transverse abdominal diameter and lumbar lordosis.⁶⁹ Lumbar lordosis increases with increasing body mass as pregnancy progresses and so does the frequency of back pain.^70,71

Osteopathic manipulative treatment has shown to be beneficial in decreasing pain. In a randomized controlled study⁷¹ on back pain during the third trimester of pregnancy, patients who received OMT and usual obstetric care showed the least back-specific functional deterioration compared with control groups that received usual obstetric care and sham ultrasonography.⁷² The OMT techniques include soft tissue techniques, myofascial release, and muscle energy. Furthermore, pain was alleviated in patients who received OMT for sacroiliac dysfunctions.⁷²

Reduction in normal lumbar lordosis can also have clinical consequences and is a defining feature of flatback syndrome. Flatback syndrome is most commonly iatrogenic, often from surgical treatment of scoliosis using a Harrington distraction rod, which leads to loss of lumbar lordosis.⁷³ Patients present with a forward trunk inclination, inability to stand upright without flexed knees, extended upper thoracic and cervical spine, back pain, and thigh pain due to overuse of hip flexors.^73,74 The placement of the Harrington rod greatly affects the development of flatback syndrome. If the rod is placed at or cephalad to L3, lordosis is maintained, whereas a rod placed caudad to L3 may cause flatback deformity.⁷⁵

To diagnose flatback syndrome, a thorough history and physical examination must be done. Osteopathic examination findings may show a decrease in the patient's lumbar lordosis, a decrease in lumbar range of motion, and an extended pelvis. A full-length lateral radiograph with the knees and hips fully extended can be done to view the decreased lordosis and measure the degree of spine curvature.^75,76 Patients with flatback syndrome can be treated both conservatively and surgically. Initial treatment consists of physical therapy and pain management, which includes exercises to increase hip and back extension, as well as exercises that encourage anterior pelvic rotation. From an osteopathic manipulative medicine standpoint, treatment of the patient can include muscle energy techniques to relax the hip flexors and extensors, as well as counterstrain techniques to address any tender points.⁷⁷ If the patient's symptoms persist and the sagittal balance progresses, then surgical management is indicated.⁷⁵

Conclusion

These modern ailments demonstrate the necessity and the delicate nature of lumbar lordosis for human bipedal locomotion. Humans have a long, flexible lumbar spine to produce this lordosis. However, because of the need of a tilted pelvis and sacrum, humans have a greater lumbosacral angle, which places tremendous pressure on the lower lumbar spine. Despite adaptations to limit the pressure, serious conditions, such as spondylolysis and spondylolisthesis, can arise. Conversely, eliminating lordosis can also have clinical consequences, as seen in people with flatback syndrome and Harrington rods for scoliosis. Human bipedality has allowed us to inhabit a wide range of habitats and become a highly successful species; however, the downside of this adaptation continues to affect us to this day.