An archeological site in Turkey, known as Göbekli Tepe, is currentely the oldest known archeological site with evidence of significant and prolonged construction activity by humans. Evidence suggests that it was some sort of religious/gathering place for people.
The real kicker- The site was built about 12,000 years ago, was active for over 3,000 years and then methodically covered up in the 8th millenia BC. Its excavation contradicted the then popular view that an established agricultural system was necessary for large-scale mobilization and co-operation amongst human beings. Archeological evidence from that site suggests that it was built at the very beginning of the transition from hunter-gatherer to agriculture based societies, as the remains of wild versions of cereals such as wheat have been found at that site.
Here is a YouTube Clip about that site.
and a few pictures that show the scale and detail of the buildings, which were constructed before what we consider ‘civilization’ existed. Note that each T-shaped pillar weighs about 40-60 tons.