It seems that true mummification didn't occur until around 2600 BCE, Redjita - the efforts to that point were not entirely successful and there was a fair bit of experimentation for a considerable time. Mummification was also only available to the wealthier classes as has been pointed out earlier.

What I have been able to find is this: poor people were interred into shallow graves at the edge of the desert where the natural dryness and heat dessicated their bodies. This was the universal and pre-existing method of burial prior to the arrival of outsiders around 3400 BCE that came possibly from Mesopotamia - this outsiders would become the Dynastic Race. It seems that it was around this time that a two-tiered burial system began to be used. Incidently it was also at this time that the earliest developments in hieroglyphs appeared which suggests this was imported with the newcomers too.

Unfortunately, I don't have much information on the two-tiered burials. I'd have to do a bit more digging.