Multiregional evolution, as it might be used to model the
relationships between human fossil remains, framed here in time and
space.
The arrows depict avenues of potential genic exchange,
although they tell us nothing about the actual magnitudes of gene flow,
which were all different and varied over time.
The populations
these fossils represent did not contribute equally to the gene pool of
modern humans; like populations today, they surely had different
demographic attributes. Some expanded, some declined, some became
extinct, but they all could exchange genes as part of a single species.
Like populations today, they divided and merged, so that any group may have had multiple ancestors and multiple descendants.
This
figure contrasts with traditional diagrams of human evolution which
depict fossil populations as branches on a diverging tree or bush and
reflect analyses of populational difference that start with assumptions
of divergence and isolation that we believe are incorrect.