Thucydides said that human nature was unchanging and thus predictable — but he was probably wrong. If you consider natural selection operating in fast-changing human environments, such stasis is most unlikely. We know of a number of cases in which there has been rapid adaptive change in humans; for example, most of the malaria-defense mutations such as sickle cell are recent, just a few thousand years old. The lactase mutation that lets most adult Europeans digest ice cream is not much older.
There is no magic principle that restricts human evolutionary change to disease defenses and dietary adaptations: everything is up for grabs. Genes affecting personality, reproductive strategies, cognition, are all able to change significantly over few-millennia time scales if the environment favors such change — and this includes the new environments we have made for ourselves, things like new ways of making a living and new social structures. I would be astonished if the mix of personality types favored among hunter-gatherers is "exactly" the same as that favored among peasant farmers ruled by a Pharaoh. In fact they might be fairly different.
There is evidence that such change has occurred. Henry Harpending and I have, we think, made a strong case that natural selection changed the Ashkenazi Jews over a thousand years or so, favoring certain kinds of cognitive abilities and generating genetic diseases as a side effect. Bruce Lahn's team has found new variants of brain-development genes: one, ASPM, appears to have risen to high frequency in Europe and the Middle East in about six thousand years. We don't yet know what this new variant does, but it certainly could affect the human psyche — and if it does, Thucydides was wrong. We may not be doomed to repeat the Sicilian expedition: on the other hand, since we don't understand much yet about the changes that have occurred, we might be even more doomed. But at any rate, we have almost certainly changed. There is something new under the sun — us.
This concept opens strange doors. If true, it means that the people of Sumeria and Egypt's Old Kingdom were probably fundamentally different from us: human nature has changed — some, anyhow — over recorded history. Julian Jaynes, in The Origin of Consciousness in the Breakdown of the Bicameral Mind, argued that there was something qualitatively different about the human mind in ancient civilization. On first reading, Breakdown seemed one of the craziest books ever written, but Jaynes may have been on to something.
If people a few thousand years ago thought and acted differently because of biological differences, history is never going to be the same.