Which of the following best describes the experimental evidence that the FOXP2 gene is vital to the normal development of vocalization in vertebrates and yet has evolved among different vertebrate lineages?

• A. Replacing the FOXP2 gene in mice with the human form of the gene had a neutral effect on development of vocalization in mice.
• B. Knocking out one of the two FOXP2 genes in mice and humans had no effect on the development of vocalization in either species.
• C. Replacing the mouse FOXP2 gene with songbird versions of the gene caused the transgenic mice to vocalize like songbirds.
• D. When scientists knocked out the FOXP2 gene in human subjects, they observed the development of severe speech and language impairment.
• E. Replacing the FOXP2 gene in mice with a humanized version of the gene had no negative effects on the mice but did lead to the development of brain cells in neural circuits that are associated with speech development in humans.