Science isn't done in a vacuum. In addition to the famous "standing on the shoulder of giants" analogy, science is also collaborative. This is becoming even more so as scientists specialize in narrower fields and the Internet makes communication much more efficient. My latest paper in the Publications of the Astronomical Society of the Pacific had 22 coauthors! There are many different phases of this project and each takes a different skill set. Some people can do it all alone, but most people will want to share the load with others.