The
word "invention" is a bit loaded so I'll sidestep it a little bit. Certainly the concept of map and reduce functions have been around for a long time, but MapReduce is a new framework for processing data sets in a scalable way. It's not just putting map and reduce function together.
Similarly you can argue that Google did not invent link analysis, but that misses the point. Their PageRank algorithm had applied link analysis in a scalable way to a new problem domain (Web) that it certainly stands on its own as a useful invention.