1.1.2 All records are selective
It is important to bear in mind that none of these records
represents an unbiased picture of the past. We do not have a
time machine and therefore must rely upon evidence that has
survived to the present. This process of survival is selective.
In the archaeological record we find many stone tools but
few wooden ones: arrowheads but not shafts. In the
paleontological record we find plenty of skeletal fossils, but
soft tissues leave traces only very rarely. In the historical
record we may not encounter those texts that displeased
contemporaneous or subsequent heads of state, either
because they were destroyed, or not written in the first place.
Similarly, in the genetic record, survival is quite literally
selective. Natural selection and other processes have shaped,
and continue to shape, our genome in different ways. Even
ancient DNA evidence, although not influenced by
subsequent natural selection, is biased: for technical reasons it
can tell us far more about the genetic diversity of our female
ancestors than it can about our male forebears. Since the
survival of ancient DNA is influenced by physical and
chemical conditions, samples will be more plentiful from
some regions of the world than from others.