I would say you need tangible research work in the field that you'll be applying to. This takes the form of things like working in a research lab, writing an honors thesis, poster or oral presentations at conferences, independent research projects, etc. Basically, you need to have something that will make the admissions committee believe that you are able to contribute to their program and project, and that usually takes the form of past success/experiences with similar projects. With that said, I am sure you can get an interview without having co-authored if you have other things that make up for it. However, I would say a connection with faculty is important assuming that they will be providing you a recommendation letter.