In certain scientific circles,
there is a graph known as
the Bomb Curve.
This graph is only tangentially related
to bombs, and unfortunately completely
unrelated to a nineties cool factor.
The Bomb Curve measures atmospheric carbon-14
levels over time. This level was generally
constant (and very low) over the millennia
for which it could be measured. There
is, however, a spike in the levels observed
beginning in the 1950s, with the advent
of above-ground nuclear weapons testing.
And a fall-off with the realization
that perhaps these tests were a bad idea.
At any rate, modern scientists are keen
to make the most of poor decisions, doing so with
the Bomb Curve, as might be expected.
Atmospheric carbon-14 is incorporated,
no differently from its isotopes, into
plants performing photosynthesis, and
subsequently into the animals eating
those plants, on up the food chain
to homo sapiens. Thus, C-14 levels
in humans correspond to the atmospheric
levels of C-14 at the time of
incorporation. But, throughout the
course of a human life, cells are
replaced (at differing rates) by new
materials, including new carbon.
The upshot being that the time of
a cell's development can be measured,
with advanced mass spectrometric techniques,
based on what atmospheric carbon-14
levels it corresponds to. And, hence,
at what rate those cells turnover,
i.e. are replaced. One such study
found some brain cells essentially do not
turnover. So our brains are, in one way
the oldest thing about us.