  Galaxy mergers are thought to be one of the main mechanisms of the mass
assembly of galaxies. Recently, many works have suggested a possible increase
in the fraction of major mergers in the early Universe, reviving the debate on
which processes (e.g., cold accretion, star formation, mergers) most contribute
to the mass build-up of galaxies through cosmic time. To estimate the
importance of major mergers in this context, we make use of the new data
collected by the ALMA Large Program to INvestigate [CII] at Early times
(ALPINE), which observed the [CII] 158 $\mu$m emission line from a sample of 75
main-sequence star-forming galaxies at 4.4 < z < 5.9. We used, for the first
time, the morpho-kinematic information provided by the [CII] emission to obtain
the fraction of major mergers ($f_{MM}$) at z~5. By adopting different
prescriptions for the merger timescales ($T_{MM}$), we converted this fraction
into the merger rate per galaxy ($R_{MM}$) and per volume ($\Gamma_{MM}$). We
then combined our results with those at lower redshifts from the literature,
computing the cosmic evolution of the merger fraction. This is described by a
rapid increase from z~0 to higher redshifts, a peak at z~3, and a slow decrease
towards earlier epochs. Depending on the timescale prescription used, this
fraction translates into a merger rate ranging between ~0.1 and ~4.0 Gyr$^{-1}$
at z~5. Finally, we compare the specific star formation and star-formation rate
density with the analogous quantities from major mergers. Our new ALPINE data
reveal the presence of a significant merging activity in the early Universe.
However, whether this population of mergers can provide a relevant contribution
to the galaxy mass assembly at these redshifts and through the cosmic epochs is
strongly dependent on the assumption of the merger timescale.
