In searching the origin of life it is possible to apply two different approaches. The first one is a down-to-top approach, in which, starting from inanimate matter, like for instance basic organic compounds or amino acids, one tries to reproduce the processes and the conditions that gave birth to the first living beings about 3.85 Ga ago. Alternatively, one may attempt to extract hints about the first steps of the evolution of life from the observation of present organisms. This top-down approach is based on the fact that all living forms have a common ancestor, so that it is licit to assume that some of its vestiges are still preserved inside the genetic code of current cells and bacteria.

Indeed, it is well known that there are structures inside proteins, like structural motifs and folds, which seem to be intimately connected with evolution. For example, while there are about 100,000 expressed proteins in eukariotic systems, there are much fewer motifs and folds, a fact that is related to the pathways of evolution.

In this lecture some aspects of the top-down approach will be illustrated. After a brief review of the known structures in proteins and other macromolecules which are relevant for life.