  Cell membranes are a key element of life because they keep the genetic
material and metabolic machinery together. All present cell membranes are made
of phospholipids, yet the nature of the first membranes and the origin of
phospholipids are still under debate. We report here the first detection in
space of ethanolamine, NH$_2$CH$_2$CH$_2$OH, which forms the hydrophilic head
of the simplest and second most abundant phospholipid in membranes. The
molecular column density of ethanolamine in interstellar space is
$N$=(1.51$\pm$0.07)$\times$10$^{13}$ cm$^{-2}$, implying a molecular abundance
with respect to H$_2$ of (0.9-1.4)$\times$10$^{-10}$. Previous studies reported
its presence in meteoritic material but they suggested that it is synthesized
in the meteorite itself by decomposition of amino acids. However, we find that
the proportion of the molecule with respect to water in the interstellar medium
is similar to the one found in the meteorite (10$^{-6}$). These results
indicate that ethanolamine forms efficiently in space and, if delivered onto
early Earth, it could have contributed to the assembling and early evolution of
primitive membranes.
