Hox genes (anterior-posterior axis patterning) shows a special evolutionary conservation in the cervical spine, such that almost every mammal is constrained to seven cervical vertebrae, regardless of the length of the neck. There is speculation that this may be a result of simultaneous Hox effects in neural patterning in the upper cervical spine, and that cervical spine variation leads to nonviable neural variations. One study has found an associated increased risk of cancer in children with a cervical rib.
The lumbar ribs are a similar result of differential Hox expression. The presence of lumbar ribs in mice can even be used as an index of the teratogenicity of a substance... but the increased incidence of natural lumbar ribs seems to imply that the distal Hox variation has less serious associations with neural patterning than proximal variations.
As an incidental note, the gorilla (Gorilla gorilla gorilla), has a similar typical pattern of 17 total thoracic and lumbar vertebrae, but tends to have the distribution of 13 thoracic vertebrae and 4 lumbar vertebrae.
References:
1. Narita Y, Kuratan S."Evolution of the Vertebral Formulae in Mammals: A Perspective on Developmental Constraints" JOURNAL OF EXPERIMENTAL ZOOLOGY (MOL DEV EVOL) 304B:91–106 (2005)
2. Galis F. "Why Do Almost All Mammals Have Seven Cervical Vertebrae? Developmental Constraints, Hox Genes, and Cancer" JOURNAL OF EXPERIMENTAL ZOOLOGY (MOL DEV EVOL) 285:19–26 (1999)
3. Merks J, Smets A, et al."Prevalence of RIB anomalies in normal Caucasian children and childhood cancer patients" "European Journal of Medical Genetics Vol 48, Issue 2, April-June 2005, Pages 113-129.
