One of the primary foci of research in astrophysics is on developing a rigorous understanding of the first galaxies. This entails studying the physical processes such as accretion, cooling and star formation in first galaxies, and also investigating the consequences of these processes in the present day Universe. We investigate the star formation in the early galaxies and its subsequent evolution using the eagle simulation and find that the star formation has a smooth evolutionary behaviour at low redshifts leading to a main sequence of star formation that can be explained by deterministic models using accretion history as an input. In contrast, at high redshift (>6), most of the galaxies are bursty. At high redshift, instead of exhibiting a main sequence in SFR - Mh plane, they bunch-up around a halo mass of ≈ 109 Mȯ and SFR ≈0.1 Mȯ yr−1. As a consequence, the reionization of the Universe is led by low mass haloes hosting brighter galaxies that are undergoing intense bursts. Furthermore, the bursts in the infant galaxies lead to a poorly mixed interstellar medium in which the stars can form from gas enriched predominantly by a single nucleosynthetic channel. The lower mass subset of the stars formed in first galaxies resemble the carbon enhanced metal poor stars in our Galaxy while the higher mass ones reionized the Universe.