If a mere lurker may suggest. We don't know there wasn't an ozone hole
in the past!
Volcanoes can cause some depletion in ozone and volcanoes have been
very  active in some parts of the period of human development.
There has been global warming and cooling. Some very cold periods which
were coterminous with drought conditions which suggest changes in
vegetation and therefore in the distribution of herd animals.
There are sites which deal with vegetational change, my favourite is
Jonathan Adams'
http://www.esd.ornl.gov/projects/qen/nerc.html
This could result in humans either competing for scarcer resources or
scattering further a field.
M E Wood
(Prehistoric Archaeology, U Edinburgh)
New Zealand