April 25, 2011

Misfeatures of human anatomy

Body Vector fieldI have never been interested in developmental biology. I always wanted to know how the finished system worked, not the stuff that presumably only embryologists and obstetricians needed. Evolution on the other hand was interesting: after all, how to understand systems without understanding how they got to their present state?

Of course, these two views don't fit together at all, and I am very happy I realized just how stupid I have been thanks to Lewis I. Held's book Quirks of human anatomy: an evo-devo look at the human body, Cambridge University Press 2009.

The book is a fairly light-hearted introduction to how evolution acts on the genes that then cause the formation of our body, with the overarching theme of trying to explain just why we have ended up with these weird bodies. It looks at how much chance and determinism affects our background, the odd interplay of symmetry versus asymmetry (how do you get an asymmetric heart out of a symmetric embryo? and is it a good idea?), modularity, sexual dimorphism and what makes us human and how we got there (maybe).

The chapter discussing silly, stupid and dangerous quirks is the capstone of the book. This is a goldmine of observations of the problems with our legacy anatomy, a massive argument against any intelligent design or creationism views the reader may hold. While most of us know a few drawbacks or sillinesses of the body - the appendix, male nipples, wisdom teeth, nerves crossing the mid-line, a visual cortex farthest away from the eyes, inside-out retinas - Held adds plenty more: ligament asymmetries in knees and ankles predisposing to sprains, redundant aortic arches that form and then disappear, bad routing of the iliac vein, inability to reshape the eye, pacemaker asymmetry in the heart increasing the risk of death, that we have a link between the windpipe and oesophagus (due to the lungs evolving from the foregut) making us vulnerable to choking, the long detours of the left recurrent laryngeal nerve, sciatic nerve and vasa deferentia, oviducts making tubal pregnancies a real risk, prostrate encircling the urethra (risk for blockage; meanwhile the pancreas forms out of two halves that sometimes accidentally strangle the duodenum), poor drainage of sinuses, useless yolk sacs and of course, the headache of both having a wide birth canal (for big brained children) and a narrow pelvis (for effective walking). The list is long and the causes are in most cases trivial - yet we pay a high price of these misfeatures.

Redesigning humanity to get around these would be a good idea. As I argued with Nick (in a paper often blamed for giving nature too much credit), in these cases of evolution not caring about our goals we should expect relatively safe enhancements. Unfortunately Held also shows just how tricky it is to handle developmental programs: actually fixing the massive esophagus-windpipe bug would involve not just tweaking developmental programs to separate them, but to figure out a way of getting our speech to work too (we use the mouth for articulation, moving air movements solely to the nose would preclude that). Building on top of a mess doesn't allow you much freedom, but redoing it from scratch will leave you with something completely different.

All in all, I found Held's book enjoyable quick reading. It has a massive reference apparatus allowing the reader to get the real dirt on the various genes, quirks and comparative anatomies if needed. It appears to be a good jumping off-point into evo-devo, something I now plan to do.

Posted by Anders3 at April 25, 2011 11:31 AM