Well, I'll be honest with you. I was a theoretical physicist but my Ph.D. work was on a problem that was not particularly interesting to me at the time. And I used to subscribe to Scientific American and I found that there were all these wonderful discoveries happening in biology and I also knew that a number of physicists had gone into biology and been successful. So, I decided to switch.
Recorded Adam Smith, Telephone interview recorded minutes after the prize announcement, Nobel Prize Outreach, 7 October 2009 source