Fishy maths in the Buddha's lineage

Taken from my 2001 diary:

I have been reading about the lineage of Gotama Buddha
(e.g. http://www.geocities.com/CapeCanaveral/Runway/5787/buddha-lineage.html).
It appears that he spent time as a human, interspersed with
existence in celestial realms. If you do the maths, the lifespan
of a deva in even the lowest heaven is about 9 million years.
Homo sapiens have existed for only about 100,000 years. The
Homo genus has existed for a few million years only - and most of
that time was probably spent being ugly, hairy and stupid.
So how do you account for this discrepancy between accounts of
Gotama Buddha's previous existences and palaentological evidence?
Also, does that mean that most devas look like reincarnated dinosaurs?