Take a look at the back of your TV/entertainment centre. A mass of power and signal cables connect the various pieces of equipment in very specific ways. Imagine you wanted to switch the locations of your satellite receiver and your DVD player. The sensible way would be to first unplug the power cables, HDMI cables and scart leads. Then move the boxes and finally reroute the cables and plug them back into the correct sockets. Trying to move them with the cables in situ would result in a tangle.
The various parts of our bodies are interconnected with various nerves, blood vessels, tubes and other types of connective tissue. If evolution is correct then many of our organs have moved relative to each other during our species' history and we should be able to find evidence of that in the way some of those things are routed.
Evolution can't unplug a blood vessel or nerve in the way you might temporarily disconnect a signal cable. Every step it makes must not only leave the body viable, it has to be no less fit than the previous version or it won't survive natural selection.
In part 13 of this series we saw an excellent example of this in the recurrent laryngeal nerve. Routed behind our aorta it has had no option but to keep growing as we evolved from fish to mammal.
Another great example is the vas deferens - tubes that carry sperm from testicles to penis.
In cold-blooded creatures such as fish, the gonads are located deep inside the body near the heart. In warm-blooded animals like humans sperm production takes around 9 weeks and is sensitive to temperature. As bodies got warmer it would have become more difficult for our ancestors to produce viable sperm and natural selection would favour individuals with tetes closer to the skin. Like many other mammals human males keep their testicles in a scrotum at a temperature two degrees below that of their body.
If creationists are correct then we would expect the vas deferens to follow a short and simple route from testicles to penis. If on the other hand we evolved from fish, then the testicles have gradually moved through the abdomen of our ancestors to their current location. Just like moving a DVD player without unplugging the cables that journey might have left clues in the routing of the vas deferens.
The evidence is irrefutable. The tetes took a wrong turn and passed the wrong side of the ureter - the tube that carries urine from the kidneys to the bladder. In future generations there was no option other than for the vas deferens to just keep getting longer so that it now travels from the scrotum up through the pelvis over the ureter, over the pubic bone and back down through the pelvis to the penis.
Our evolutionary history is written all over us.
#31 Ten Questions For Creationists ...
The basic facts about reality covered so far pose an impossible challenge to creationism.
#32 Sexual Selection
How female mating preferences led to some of the most remarkable features of living things.
#33 A Tale About Tails
Human embryology reveals our primate history.
#34 Hiccups and Tadpoles
How hiccups are a relic of our amphibian ancestors.
#35 Nature Red in Tooth and Claw
Nature's ability to inflict pain and suffering in the battle for survival.
#36 Mass Extinctions
96% of life was wiped out in The Great Dying 250 million years ago.