The history of Egypt's contributions to world civilizations can be dated back to circa 3000 B.C., when Upper and Lower Egypt were forcibly united by King Menes, with the following period 2780‑2200 B.C., being called the Old Kingdom when the pyramids were constructed.  Following several centuries of weak central government the Middle Kingdom from 2000‑1750 B.C., came into existence.  This era has been called the "classical age", as an indication of its high intellectual level, and medicine apparently held a prominent place in the pursuit of learning.  This great scholarly period was ended by the invasion of the Hyksos, a Semitic people from Lower Egypt, circa 1674 B.C., and although a New Kingdom was established between 1580 to 1350 B.C., Egypt did not regain the high level of scholarship that had existed during the Middle Kingdom until the Alexandrian period, 300‑100 B.C.