This line may seem like small potatoes but Wallace discovered it before Darwin’s theory of evolution and long before scientists developed the theory of plate tectonics and continental drift. Scientists were still uncertain /why/ these two regions had such different wildlife. Tigers and monkeys on one side, kangaroos and platypus on the other. What gives?
Even though they are (relatively) close now, Australia and Asia haven’t been connected since the supercontinent Pangaea broke up about 175 million years ago. At that time, Australia was next to the Indian subcontinent and Antarctica. India gradually drifted upwards into Asia, Antarctica southwards to the pole, and Australia has been an island ever since. The consequence of this is that Australian species have been evolving, isolated, for about 80 million years.
The line has been modified a few times - Huxley proposed a small shift (pictured), and two other lines (Weber’s and Lydekker’s) have also been suggested.