It is an unknown.
[link|http://news.com.com/Why+cats+will+never+live+the+sweet+life/2100-1008_3-5803492.html|http://news.com.com/...08_3-5803492.html]
"What we still don't know is--which came first: carnivorous behavior or the loss of the T1R2 protein? With regard to the gene, is this a case of use it or lose it?"
This is the guy I heard.
During the long interview, he said something along the lines that an omnivirous animal that lost the sense of taste for sweet would be pushed into pure carnivirous. But he could not know for sure.