Quirks of Human Anatomy
by Lewis I. Held, Jr.

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Table of Contents


1. Background
What role did chance play in our origin?
How much did internal factors influence our evolution?
Are fruit flies really our kissing cousins?

2. Symmetry and Asymmetry
Why are we symmetric outside but asymmetric inside?
Is visceral asymmetry due to molecular chirality?
Why do fingerprints violate mirror symmetry?
Why do blood vessels violate mirror symmetry?
How have animals evolved external asymmetries?

3. Mysteries of the Midline
Why do optic nerves cross the midline?
Why do pre-motor nerves cross the midline?
Why do symmetric organs fuse at the midline?
Where did the neural crest come from?
How did our third eye become our pineal gland?
Are human cyclopes evolutionary throwbacks?
Why do conjoined twins tend to be symmetric?
What causes identical twinning?
Is twinning ever adaptive?

4. Merism and Modularity
What makes the thumb our only opposable digit?
Could we evolve a giant digit like pterosaurs?
Where did our tail go?
Why do our teeth have different shapes?
Why are some features easier to evolve than others?
How do upper and lower teeth achieve quasi-symmetry?
Why do our ears start out on our neck?
Why is our arm built like our leg?
Why does our arm differ from our leg?
Could centaurs (or similar hexapods) ever evolve?
When did we gain the upper hand?
How did evolution sculpt our foot?

5. Sexual Dimorphisms
Why do men and women differ in size and shape?
Why does our species have as many males as females?
Were we all once hermaphrodites?
Why do male fetuses make incipient oviducts?
How did we become the only “naked apes”?
Why are men hairier than women?
Why do only men go bald?
Why do men have nipples?
How did women end up with only two breasts?

6. Silly, Stupid, and Dangerous Quirks
Could choking have been avoided?
Why are our retinas inside out?
Why is childbirth so precarious?
Why are crowded teeth so common?
Is our appendix on the road to oblivion?
Why do we make a yolk sac if we have no yolk?
Why can’t we regrow arms or legs?

7. Mind and Brain
How did humans acquire intelligence?
Why did humans acquire intelligence?
Is intelligence attributable to brain size?
Did we ascend from brute to aesthete suddenly or gradually?


Appendix: Quirks of Human Behavior.



Lewis I. Held, Jr. is Associate Professor in the Department of Biology at Texas Tech University.

© 2009 Thomas B. Brody, Ph.D.