They were sweet, unhopped, thin gruels that could only be consumed through straws to avoid ingesting the non-liquid ingredients. An example of this can be seen represented on a Mesopotamian cylinder seal from the 3rd millennium BC city-state of Ur. The flowers of the hop plant that provide the head and flavour in modern beer and act as a preservative were not added until about 200 AD in Babylon and not until post-medieval times in Europe. It is extremely rare to find the remains of alcoholic drinks in prehistoric burials. Most early excavators took great care to scrub all vessels within an inch of their lives, effectively removing all traces of contents.
Preservation is contingent on conditions as well.
An early example of mead comes from a Neolithic beaker from Ashgrove Farm, Methilhill, Fife dating to about 1000 BC, which was found to contain two different kinds of honey, possibly representing the remains of some form of fermented mead-like beverage. The same mixture was found at the Danish Iron Age site of Juellinge. If the preservation conditions turn out to be favourable, the contents might be identified of a bronze cauldron and a ceramic drinking cup found during my own excavations last year of two Iron Age graves near Heuneburg in Germany. The Sumerians left us written instructions for the production of beer without hops, and similar gruel-like ales that must be sipped through a straw are still produced in some parts of Africa, and elsewhere. The earliest ales were more of a mildly intoxicating food than a beverage.