To explore the role environment plays in influencing galaxy evolution at high redshifts, we study $$2.0 \le z \lt 4.2$$ environments using the FourStar Galaxy Evolution (ZFOURGE) survey. Using galaxies from the COSMOS legacy field with $${\rm{log}}\left( {{{\rm{M}}_{\rm{*}}}/{{\rm{M}}_ \odot }} \right) \geqslant 9.5$$ , we use a seventh nearest neighbour density estimator to quantify galaxy environment, dividing this into bins of low, intermediate and high density. We discover new high density environment candidates across $$2.0 \le z \lt 2.4$$ and $$3.1 \le z \lt 4.2$$ . We analyse the quiescent fraction, stellar mass and specific star formation rate (sSFR) of our galaxies to understand how these vary with redshift and environment. Our results reveal that, across $$2.0 \le z \lt 2.4$$ , the high density environments are the most significant regions, which consist of elevated quiescent fractions, $${\rm{log}}\left( {{{\rm{M}}_{\rm{*}}}/{{\rm{M}}_ \odot }} \right) \geqslant 10.2{\rm{\;massive}}$$ galaxies and suppressed star formation activity. At $$3.1 \le z \lt 4.2$$ , we find that high density regions consist of elevated stellar masses but require more complete samples of quiescent and sSFR data to study the effects of environment in more detail at these higher redshifts. Overall, our results suggest that well-evolved, passive galaxies are already in place in high density environments at $$z \sim 2.4$$ , and that the Butcher-Oemler effect and SFR-density relation may not reverse towards higher redshifts as previously thought.

This thesis investigated the relationship between galaxy evolution and environment when the universe was approximately 3.2 to 1.4 billion years old. This project used data from the FourStar Galaxy Evolution (ZFOURGE) survey, and the previously developed seventh nearest neighbour metric to find and quantify distant galaxy environments. By using various galaxy properties (e.g. quiescence, dustiness, star formation activity), this thesis finds evidence that the local galaxy evolution-environment correlation appears to be present in the early universe at approximately 2.7 billion years old. This thesis presents encouraging results for the future of early galaxy evolution studies.