Patrick McGovern at UPenn studies ancient beer actually. He has collaborated with Dogfish Head Brewery to re-create some of the ancient beers he has discovered the ingredients of. That means you can buy some and taste for yourself what ancient beer may have tasted like! If you are of legal drinking age of course. There's even an Egyptian version.

I personally have tasted Midas Touch, Chateau Jihau, and Theobroma. My opinion is that these beers are all pretty thick and sweet.

I saw Patrick McGovern give a talk at the Getty Villa in California a few years ago. McGovern's idea about alcohol is that it originated by ancient humans discovering that rotting fruit has more sugars and calories than regular fruit. He gave examples of how modern, non-human primates eat rotting (therefore fermenting) fruit. The consumption of fermenting fruit escalated into full scale production of alcohol, and McGovern believes that beer or mead would have been the first type of alcohol produced. He gets the "recipes" for the beers by analyzing the proteins left in ancient ceramic vessels. The proteins cannot identify specific fruits, but they can give a broad idea of what was in a vessel that we know would have held beer or mead.

Ancient beer was, for one, clouded with sediments. The pictures of beer drinking in many glyphs from the fertile crescent show people drinking from a large jug, using long straws. The beer was made from a specific type of bread, and then was occasionally flavored, particularly in Egypt. Different flavorings included berries, honey, and spices, each advertised in their own way. Another major difference between modern beer and ancient beer was the absence of hops. Hops are a relatively recent invention, and did not exist in ancient times.

Source: A history of the world in six glasses by Tom Standage