Simplify the following text for a Grade 8 native English-speaking person.
Original: The ability of humans to change the genetics of animals took a large step forwards in 1974 when Rudolf Jaenisch was able to produce the first transgenic mammal , by integrating DNA from the SV40 virus into the genome of mice .
Simplified: In 1974 , Rudolf Jaenisch produced the first genetically changed mammal . He put DNA from a virus into the genome of mice .