Galaxies grow in a hierarchical way, first converting primordial gas into stars, and then accreting new material. Two main processes have been proposed to efficiently supply fresh gas to galaxies, that is, galaxy major mergers and star formation. Although star formation ignited by gas accretion has been considered as the most efficient mechanism for many years, recent works have suggested a possible increase in the fraction of major mergers at early epochs, reviving the debate on which of the two processes dominates over the other. We investigated the importance of major mergers in this scenario by exploiting recent data from the ALPINE survey. This project collected ALMA observations of the rest-frame far-infrared [CII] 158 µm line and the surrounding continuum emission in a hundred of star-forming galaxies one billion years after the Big Bang, when the major phase of galaxy mass-assembly was in place. Along with ancillary multi-wavelength data and spectroscopic observations, ALPINE allowed us to conduct the first panchromatic study of such a large statistical sample of primordial galaxies. We used, for the first time, the morpho-kinematic information provided by [CII] to identify major mergers at z~5. We found that ~40% of normal galaxies at that redshift was undergoing a merging, providing the first constraint on the major merger fraction from [CII] at this epoch. By combining our results with studies at lower redshift, we computed the cosmic evolution of the merger fraction, and compared the processes of merging and star formation as responsible for the growth of galaxies. Our results reveal a significant merging activity in the early Universe, suggesting that major mergers could have an important role in the overall process of galaxy mass-assembly across cosmic time.