The dinoflagellates are one of the three major phytoplankton groups in modern oceans. They are known since the beginning of the Mezozoic (± 250 million years ago), but what is their origin? Several Proterozoic and in particular Palaeozoic acritarchs (± 540 to 250 million years ago) that are among the first signs of life are morphologically very similar to some modern dinoflagellate cysts. In addition, the stratigraphic, paleoecological and palaeogeographical distribution of these acritarchs is very similar to that of the modern dinoflagellates. However, it remains unclear if dinoflagellates are the organisms that produced these acritarchs. Microfossils of unequivocal dinoflagellate cyst morphology are not known until the Triassic, whereas organic biomarkers and physiology suggest a much older origin.? We therefore propose to compare the ancient acritarches with fossil and modern dinoflagellate cysts. Recently, it appeared that each modern dinoflagellate cyst species has a unique macro-molecular fingerprint. Here, we will use this information on an innovative way. We will combine organic geochemistry, micro-infra-red spectroscopy, culturing and genetic techniques, and microscopic analyses (optical, TEM, SEM) to determine the biological affinities and the macromolecular characteristics of individual microfossils, including acritarchs and dinoflagellates. Our study should resolve - at least partly - the question about the origin of the dinoflagellates, and the biological affinity of the acritarchs.