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mal groups. They have far more species than any other phylum, yet the living species are merely the surviving branches of a much greater diversity of extinct forms. One group of crustacean arthropods, the barnacles, was studied extensively by Charles Darwin. But the origins and the evolution of arthropods in general, embedded in what is now known as the Cambrian explosion, were a source of considerable concern to him, and he devoted a substantial and anxious section of On the Origin of Species to discussing this subject: “For instance, I cannot doubt that all the Silurian trilobites have descended from some one crustacean, which must have lived long before the Silurian age, and which probably differed greatly from any known animal.” His interest, if not his uncertainty, was echoed repeatedly over the following 150 years, with debate over what were the closest relatives of the arthropods and over the relationships between the main constituent groups, and even doubts about whether the phylum is monophyletic (that is, whether it evolved from a single common ancestor that is not shared with any other phylum). Since the publication of On the Origin of Species, most data on the pattern of arthropod evolution have been obtained by studying embryos, adult morphology, and fossils, but the introduction of molecular biological data to phylogenetics and comparative developmental biology in the past 20 years has led to great insights. Gene sequences provide vast numbers of markers of phylogenetic relationships and, over the past 20 years, have redrawn many aspects of the metazoan tree of life. The comparative molecular genetic analysis of development has similarly changed the view of the evolution of developmental mechanisms and the origins of novel morphology, revealing surprising conservation and providing a complement to phylogenetic proximity for determining homology. Even the study of morphology has been changed by molecular techniques, and the palaeontological evidence has been transformed by the steady description of exceptionally well preserved fossils from the Cambrian and, increasingly, from other periods too. In this Review, we discuss recent advances in understanding arthropod origins and relationships from the fields of molecular systematics, palaeontology, morphology and ‘evo-devo’. We show that the source of Darwin’s discomfort about arthropod origins, although not entirely removed, has been substantially alleviated. A new consensus is emerging about the timing of arthropod origins, as well as the relationships among arthropods (including between fossils and living taxa) and between arthropods and non-arthropods. Arthropods are monophyletic Arthropods encompass a great diversity of animal taxa known from the Cambrian to the present day. The four living groups — myriapods, chelicerates, insects and crustaceans — are known collectively as the Euarthropoda. They are united by a set of distinctive features, most notably the clear segmentation of their bodies, a sclerotized cuticle and jointed appendages. Even so, their great diversity has led to considerable debate over whether they had single (monophyletic) or multiple (polyphyletic) origins from a soft-bodied, legless ancestor. The application of molecular systematics to arthropods in 1992, however, decisively resolved the issue in favour of monophyly. In other words, many of the morphological features shared by arthropods are likely to have a single origin and to have diversified across the group. It has long been recognized that two other living groups, the softbodied onychophorans (velvet worms) and the microscopic tardigrades (water bears), are close relatives of the euarthropods. All of these groups are segmented and have appendages, and they are often collectively referred to as the Panarthropoda. All of the available molecular and morphological evidence supports the idea of onychophorans and eu arthropods falling into a monophyletic group or clade, but the position of the tardigrades is less clear. Although they are traditionally regarded as the closest living relatives of the euarthropods, some molecular phylogenies place them basal within the panarthropods, or even as a sister group to the nematodes, but this may be an artefact resulting from their derived and rapidly evolving genome.