As far as we know, when did humans first brew beer, and where? Around when would you have been able to get your hands on something resembling a modern lager?
As Slyboty notes correctly, beer is a very ancient drink predating the oldest written records (and likely any archeological discoveries.) For millennia though beer was made only from fermented malt with various add-ons.
The major change that gave origin to modern beers though is the addition of hops, for the first time creating a beer closely resembling modern lagers. The first documented use of hops in beer is dated 822 A.D. [source] although records of hops cultivation dates back to 736 [source] which suggests use of hops in brewing predates written records by a considerable margin.
The simple answer is "nobody knows." I must apologize for being a bit of a history nut here and this may tell you far more than you ever wanted to know...
Fermentation of grain is universal, as is fermentation of fruit and honey. You find it in the new world with drinks like chicha (an Andean drink where saliva provides the amylase to break down corn starch into sugars for fermentation), among Native Americans in North America as well, in early Europe, in early China, and early Japan as well.
Grain fermenting is a remarkably complex process because yeast can't digest starch so you have to have some way of turning starches into sugars. These break down into more or less three strategies:
- malting (roasting sprouted grains). Interestingly doesn't work for rice.
- helper cultures, like they use to make sake. Add a fungus that breaks down starch and then a yeast to eat the sugar. Indonesian "Tape" (rice or cassava "beer" or paste) is done this way as is Chinese rice beer and wine. As a bonus this also works on other starches, like yams (used in Malaysia).
- Chewing the grains to mix with saliva and then spitting in a bowl as with the Andean Chicha drink.
South America has all three methods represented, as does early Asia.
The fact that different strategies are employed around the world suggests to my mind that brewing did not start somewhere and spread but was independently reinvented in different places. It is probably for this reason extremely old, far older than we have direct evidence for.
As for beer, the question becomes what you mean. If you mean fermented grains, we have no idea. It's very very old and might go back to neolithic times if not even earlier (though I suspect that the organized grain agriculture of neolithic times was an absolute prerequisite to grain fermentation).
If you mean malt, yeast, and hops, we still don't know. We have evidence that hops was cultivated for beer in Augustine's time, and Old Norse distinguishes between beer and ale by the use of hops but Old English "beor" means something different (probably very strong mead with herbs?), suggesting perhaps that the hops distinction post-dated the split of Old Norse (and thus 8th century), but that doesn't get you very far. What is pretty clear is that it certainly predates our earliest records of the practice. By how much and when did it become a separate drink? I don't think we will ever know. At any rate the early prevalence of hops as one herb used in brewing among many in early England and Scandinavia suggests (though does not prove) that hops were adopted (again as one herb among any) for beer brewing by the Germanic tribes, some time before we have written records.
Now as for lager, these are far more recent.
Finally as a note, until maybe a hundred years ago, before modern capping and beer cans, most beer would have been sold uncarbonated, but speciality beers were carbonated much like champagne is today. My own home brewing experiments (I even brew as a history nut) with non-carbonated beers suggest that they are of an altogether different character than carbonated ones (owing to the carbonic acid in carbonated drinks). The flavors are lighter, sweeter, brighter, and more distinct, and entirely different than "flat" beer. This is because dissolved carbon dioxide imparts extra acidity to beer.
EDIT: It also occurs to me that controlled fermentation is one of the most common ways of preserving foods in the pre-industrial ages. This includes lactic fermentation (that gives us shelf-stable cheese, salami, sauerkraut, etc), but also alcoholic fermentation must be included there too. There's no reason to think that the first fermented grains were fermented by accident.
According to LiveScience,
beer dates back to the dawn of cereal agriculture, loosely pinpointed at 10,000 B.C.E. in ancient Mesopotamia, the region of southwest Asia currently occupied by Iraq.
I also recall watching a documentary a few years back (unfortunately can't remember the name to properly cite it) which claimed that humans started settling and agriculture because they accidentally consumed some sort of primitive beer.
I'm not sure about when the first beer was made, but the oldest existing brewery is Weihenstephan, which dates back to 1040 AD, a good 26 years before the battle of Hastings.
They have not only lager, but a Hefeweizen, Dunkel, and a few other brews.