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Blazars are Active Galactic Nuclei characterized by relativistic jetslaunched in the vicinity of the central engine (i.e. a supermassive blackhole), that are oriented close to our line of sight. Their peculiar orientationmakes them very efficient tracers of the overall jetted population, and due totheir brightness they can be visible up to very high redshifts. A deepknowledge of these objects can provide fundamental clues to the models offormation and growth of the first supermassive black holes, but their search inthe early Universe must be careful and follow a systematic approach. Thediscovery in the last $\sim15$ years of extremely massive blazars at very highredshifts ($M_{\rm BH}>10^9M_\odot, \, z>4$) revolutionized our perception oftheir earliest evolution: there seem to be different formation epochs forextremely massive black holes hosted in jetted ($z\sim4$) and non-jettedsystems ($z\sim2.5$). This is not easy to explain, since one would expect thatjetted sources accrete less efficiently. Small differences in the populationare also derived from the search of such high-$z$ sources: we will go throughthe open questions in order to understand where the common knowledge stands andwhich steps must be undertaken to better understand the formation and commonevolution of supermassive black holes and jets in the early Universe.

Big and young supermassive black holes in the early Universe