  An explicit representation formula for all positive ancient solutions of the
heat equation in the Euclidean case is found. In the Riemannian case with
nonnegative Ricci curvature, a similar but less explicit formula is also found.
Here it is proven that any positive ancient solution is the standard Laplace
transform of positive solutions of the family of elliptic operator $\Delta - s$
with $s>0$. Further relaxation of the curvature assumption is also possible. It
is also shown that the linear space of ancient solutions of polynomial growth
has finite dimension and these solutions are polynomials in time.
