Heidi,
Here is my take on Peter's blood type development theory:

All the blood types developed about the same time in our history, however, the initial conditions of life favored the O blood type to such an extreme that it is primarily the blood type that would be found in these early humans.

Some time later (I don't remember these dates), agriculture became prevalent in specific areas, due to increased populations. This favored the A blood type causing it to become more prevalent in those areas where agriculture was now a dominant way of life.

Later, dairy herds and nomadic existence became predominant in a different part of the world (than where type A was favored). This allowed type B to increase in prevalence, since this type is favored by these other factors.

Finally at another even later date, history supports that large movements of peoples between these two areas occurred. This allowed the A and B blood types to meet, and intermarriages resulted in type AB.

So there are a couple of important points or assumptions in my understanding:

1) All the blood types have co-existed since the 'beginning'.
2) Different factors favor one blood type over another in different geographical areas.
3) Large movements of people were not common between these areas until fairly recently.

With these assumptions, it is feasible but not probable, that the AB blood type could have existed much earlier than Peter reports. (Due to an individual with B blood type moving into the area where A's are populous, and of course, surviving there with progeny.) HOWEVER, the occurence of the AB blood type would be in miniscule amounts before this time, since the odds would be stacked against their survival.

As you see, I think Peter is using statistics and probabiliies when he puts dates with the beginnings of the blood types. I think he means large noticeable increases in the proportions of the blood types when he talks about their beginnings. Does this makes sense? Please shoot some holes in my understanding of Peter's theory, so I can think about it some more.