I have solved the mystery.
The reason why only 30% of human DNA performs any useful function is that the rest of it is comments.
Once we decode a typical human genome, we see that the code begin as follows:
* Human Genome
* Version 2.1
* (C) God
/* Revision history:
* 0000-00-01 00:00 1.0 Adam.
* 0000-00-02 10:00 1.1 Eve.
* 0000-00-03 02:11 1.2 Added penis code to male version. A bit messy --
* will require a rewrite later on to make it neater.
* 0017-03-12 03:14 1.3 Added extra sex drive to male.h; took code from
* 0145-10-03 16:33 1.4 Removed tail.
* 1115-00-31 17:20 1.5 Shortened forearms, expanded brain case.
* 2091-08-20 13:56 1.6 Opposable thumbs added to hand() routine.
* 2501-04-09 14:04 1.7 Minor cosmetic improvements -- skin colour made
* darker to match my own image.
* 2909-07-12 02:21 1.8 Dentition inadequate; added extra 'wisdom' teeth.
* Must remember to make mouth bigger to compensate.
* 4501-12-31 14:18 1.9 Increase average height.
* 5533-02-12 17:09 2.0 Added gay option, triggered by high population
* density, to try and slow the overpopulation problem.
* 6004-11-04 16:11 2.1 Made forefinger narrower to fit hole in centre of
/* Standard definitions
#define SEX male
#define HEIGHT 1.84
#define MASS 68
#define RACE caucasian
/* Include inherited traits from parent DNA files.
* Files must be pre-processed with MENDEL program to provide proper
* inheritance features.
"Father unknown -- guessing\n
/* Set up sex-specific functions and variables
/* Kludged code -- I'll re-design this lot and re-write it as a proper
* library sometime soon.
/* G_spot *g; Removed for debugging purposes */
/* Initialization bootstrap routine -- called before DNA duplication.
* Allocates buffers and sets up protein file pointers
DNA *zygote_initialize(Sperm *, Ovum *);
/* MAIN INITIALIZATION CODE
* Returns structures containing pre-processed phenotypes for the organism
* to display at birth.
* Will be improved later to make output less ugly.
Characteristic *lookup_phenotype(Identifier *i);