I am currently doing a project(in college) on the history and future of the role of a co-pilot and to start it off I wanted to just summarize what the roles of the co-pilot are.
Firstly, "co-pilot" is not a term that is really used anymore. When people say that they usually mean the First Officer. It's a bugbear of mine because "co-pilot" implies that there is one real pilot and a half pilot, when the reality is an airliner needs at least two fully qualified pilots to fly it safely!
When it comes to actually flying the plane, the two pilots have designated roles - Pilot Flying (PF) and Pilot Monitoring (PM - sometimes this is called Pilot Not Flying). Both the Captain and the First Officer share these roles - usually one of them is PF on the outbound leg and the other one is PM on the inbound leg. The PF manipulates the controls to actually fly the plane, whereas the PM handles communications and other required tasks that don't involve actually flying the plane.
The biggest difference between a Captain and a First officer is responsibility. The Captain is the Pilot In Command (PIC), who has ultimate responsibility for everything that happens throughout the flight. There are some practical things that only Captains can do, like flying into some tricky airports, landing in strong crosswinds, and on many aircraft only the Captain can use the steering wheel (or Tiller) for taxiing.
Historically there was a strong heirarchy - the Captain was the boss and the co-pilot was, you could say, a servant. This posed big problems, because the Captain is not always right. The advent of Crew Resource Management (CRM) has brought this down to a much more desirable level - where there is still a heirarchy to ensure decisions are made, but not so much that they are unquestionable. The future of the First Officer role will be one of continuing to recognise that they play an important role in a functioning cockpit, rather than just there to serve the Captain. And hopefully people will stop calling them co-pilots!
Your question's title says "commercial aviation", and in the question you indicate you're interested in the history. With that in mind, I'll add to Ben's excellent answer.
Commercial aviation includes not just airline but also non-airline flying, including corporate flying. In corporate flying, the duties of the co-pilot varied greatly depending on what the equipment was, who the pilot was, and what company was operating the aircraft. I've deliberately used the pilot and copilot terms because prior to 1985 when I did my last corporate flying, I heard those terms more often than captain and first officer when it came to corporate work. The latter terms were "airline talk" to us, and using them for what we were doing seemed pretentious, although that was starting to change.
The typical corporate usage by small companies of small aircraft, including small business jets usually did not include carrying a flight attendant. That meant the co-pilot often did things a flight attendant would otherwise do, including mixing drinks before engine start and cleaning up vomit after the flight. I did both numerous times.
On the airline side and before proper usage of CRM, the first officer might be used as a scapegoat, especially in third world countries and in Asia. An apparent example of this was the China Airlines 605 accident at Hong Kong's Kai Tak in November 1993. Look at the transcript of that accident. The captain is P1 and is flying the aircraft, the first officer is P2 and is performing PNF duties up until 46 seconds before they hit the water. After that, P2 seems to be manipulating the controls. At the time of the accident, the assumption was the captain had handed control of the aircraft to the first officer when it was obvious they were going to crash. To the best of my knowledge that assumption was never challenged.