You pretty much need to be a science or engineering major to be competitive for MD/PhD. GPA and MCAT tend to be higher than for MD, but that varies.
The one thing that is absolutely necessary for MD/PhD admissions is significant amounts of research. The typical applicant had a couple of school years plus summers doing research. Many (probably close to 50%) take a year or two off after college to do research full-time.
However, publications are far from required--in my experience (as a senior MD/PhD student and someone who has participated in the admissions process for most of my time in the program) it is a significant minority who have publications. Being able to talk about your research, having evidence of significant involvement in your research (eg poster presentations), and getting good LORs from your research mentors are all more important.