It is often said that Ancient Egypt lasted for three millennia. It implies that their institutions were stable and had enormous staying power, but history is rarely so static.

When I think of countries that exist today, such as those in Europe, they were radically different culturally only 1000 years ago and mostly did not exist 2000 years ago.

Considering the shifts that took place over the last 2000 years, surely Egypt must have undergone more and greater changes over a period of 3000 years. So to what extent is it accurate to say that the Egypt of the Old Kingdom is "the same" Egypt as the Egypt of the Ptolemaic Dynasty?

For example, were the pharaohs always viewed as gods? Did the pharaohs always marry their siblings? Did they always mummify their dead? These things are often stated as properties of Ancient Egypt, but surely their culture, religion, and social structures would have undergone changes over time?