However, the animal that’s arguably helped us the most in understanding both our biology and evolution more broadly is the common mouse —and once again, the mouse has proven its quality.

Scientists have engineered a hybrid mouse with a gene that predates all animal life. The team replaced a single gene in the mouse stem cells with a version from an ancient, single-celled ancestor ...

The genes were injected during the mouse’s embryonic state and created a chimera, which is the term for an animal that sports two distinct sets of DNA.