ABSTRACT
If one accepts Gordhansky’s famous phrase
‘Nothing in biology makes sense except in
terms of evolution’, then the group of steroid
hormones known as the oestrogens have
indeed conferred survival advantage through
their involvement in many vertebrate
physiological systems. The simplest living
organism that has been shown to possess an
oestrogen receptor and oestradiol 17 as
ligand is the eukaryotic unicellular yeast
Saccharomyces cerevisiae.1 Among multicellular
animals the oestrogens are found first in the
phylum Echinodermata, represented today by
the starfish, and in the phylum Mollusca,
represented by the octopus and squid, whose
ancestors, evolving from simpler forms some
400 million years ago, were apparently the
first to possess the cytochrome P450
aromatase enzyme system necessary to convert
C-19 androgens to C-18 oestrogens.2 Studies
of steroid hormone evolution show that in
comparison the glucocorticoids and
mineralocorticoids are of more recent origin,
appearing first in the elasmobranchs-sharks
and rays-and that aldosterone is first found
in the teleosts or bony fish. When one
considers that it was only 6 million years ago
that the evolutionary line leading to Homo
sapiens branched away from the line leading to
our nearest cousin, Pan troglodytes, the
chimpanzee, it can be seen that our species
inherited a signalling system that had been
part of physiology for the major part of
eukaryotic life on earth. Nature is
parsimonious. A molecule that possesses a set
of physico-chemical characteristics that are of
advantage in a primitive system may be
retained to discharge its function in more
complex evolving systems provided that such
new functions do not confer a net
disadvantage upon survival.