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Almost all animals undergo embryonic development, going from a single-celled zygote to a complex multicellular adult. We know that the patterning and morphogenetic processes involved in development are deeply conserved within the animal kingdom. However, the origins of these developmental processes are just beginning to be unveiled. Here, we focus on how the protist lineages sister to animals are reshaping our view of animal development. Most intriguingly, many of these protistan lineages display transient multicellular structures, which are governed by similar morphogenetic and gene regulatory processes as animal development. We discuss here two potential alternative scenarios to explain the origin of animal embryonic development: either it originated concomitantly at the onset of animals or it evolved from morphogenetic processes already present in their unicellular ancestors. We propose that an integrative study of several unicellular taxa closely related to animals will allow a more refined picture of how the last common ancestor of animals underwent embryonic development., Work in the I.R.-T. lab has been funded by a European Research Council Consolidator Grant (ERC-2012-Co-616960), a Gordon and Betty Moore Foundation grant (MMI Experimental Model Systems grant, 4973.01) and a grant (BFU2017- 90114-P) from Ministerio de Economıa y Competitividad (MINECO), Agencia Estatal ́ de Investigación (AEI) and Fondo Europeo de Desarrollo Regional (FEDER; European Regional Development Fund).