alt Hacker NewsAs stated, this is too vague to be much of a hypothesis. Animals, including humans, communicate multimodally, so gesture and vocalisation are not mutually exclusive evolutionary stages. To claim that one evolved before the other, you'd need to define some relevant markers, such as grammar, cultural transmission or some anatomical adaptation.
Well I wasn't about to spend hours looking up sources and details specifying this hypothesis. All I remember now is that it's one that's taken seriously.