Final answer:
The complexity of humans is not only due to a larger number of genes compared to simpler organisms but also the sophisticated use of gene splicing and duplication events, particularly of Hox genes, which allow for more complex body plans. The non-coding portions of the human genome also contribute to this complexity.
Step-by-step explanation:
The increased complexity of humans compared to simpler organisms like flies and worms is not solely due to a vastly larger number of genes. In fact, humans have about 20,000 to 25,000 genes, which is not significantly more than these other species.
What contributes more substantially to our complexity is the way our cells use gene splicing and other processes to create multiple proteins from a single gene. In addition, certain gene families, like the Hox genes, have undergone duplication events that have allowed for more diverse and complex body plans to evolve in vertebrates, including humans. These Hox genes were duplicated at least twice, giving rise to the increased complexity of vertebrate body structures.
Furthermore, the human genome contains a vast amount of non-coding DNA, with only about 25 percent of the base pairs making up genes and their regulatory elements. Only two percent of this actually code for genes. The remainder of the DNA, whose function is still not completely understood, also plays a critical role in the complexity of the genome, affecting gene regulation and genome architecture.