There are many reasons.
1. Climate Change
2. Predators
3. Lack of food
This caused them to cross the Bering Strait
L-dopa is effective against Parkinson's, but its enantiomer D-dopa is not. An enantiomers drug may not be equally effective. The enantiomer, in chemistry, is one of two stereoisomers that are mirror images of each other that are non-superposable, much as one's left and right hands are the same except for being reversed along one axis. It is also known as an optical isomer.
