Live Science on the fluke that eventually led to our current situation:
Over 500 million years ago a spineless creature on the ocean floor experienced two successive doublings in the amount of its DNA, a “mistake” that eventually triggered the evolution of humans and many other animals, says a new study.
The good news is that these ancient DNA doublings boosted cellular communication systems, so that our body cells are now better at integrating information than even the smartest smartphones. The bad part is that communication breakdowns, traced back to the very same genome duplications of the Cambrian Period, can cause diabetes, cancer and neurological disorders.
“Organisms that reproduce sexually usually have two copies of their entire genome, one inherited from each of the two parents,” co-author Carol MacKintosh explained. “What happened over 500 million years ago is that this process ‘went wrong’ in an invertebrate animal, which somehow inherited twice the usual number of genes. In a later generation, the fault recurred, doubling the number of copies of each gene once again.”