Answer to Question 3

In the evolution of bipedalism, about 5 million years ago, it was obviously necessary to align the vertical spine above the hip joints.  This is not easily achieved while maintaining an adequate birth canal.  The solution that evolved was to tilt and curve the sacrum.

 

This, of course, makes the sacral endplate, which is the base of the spine, tilted instead of horizontal.  In fact it averages 50 from the horizontal and is sometimes above 70.

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The details of this evolution are covered in the paper "The Obstetric Pelvis of Lucy" by the anthropologist Owen Lovejoy (1) These details are also discussed in my book (2) and technical paper (3) both of which are available free via this website.

 

It is this angle of the sacral endplate from the horizontal that results in the wedged shape the lowest two discs

L4-5 and L5-S and of the L5 vertebra; This is the lordosis in the human spine.

 

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(1) Tague RC, Lovejoy CO The obstetric pelvis of AL 288-1 (Lucy). J Human Evolution 15: 237-255, 1986

(2) The Evolution of Low Back Pain. Book 60 pages. Free download.

(3) The Obstetric Reason for Lordosis and the Implications for Lifting and Low Back Pain. Technical paper. Available at www.lower-back-pain.info/obstetric.html

 

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