Appearance of alternative splicing undoubtedly marks a crucial step in eukaryotic evolution. Even though it's not clear when exactly this phenomenon appeared, it must have strongly increased the coding potential of genomes. It clearly appeared as an upgrade to pre-mRNA splicing. RNA splicing came about as a consequence of introns existing in early genomes, which should have been cut out from transcripts. Introns probably derived from transposons, which initially gave them the ability to spread across ancient genomes quickly. The complexity of the splicing system increased through the process of evolution, but the main mechanism remained the same. Approximately at the time of eukaryotic divergence from archaea, in early ancestor of eukaryotic group, snRNA and Sm proteins appeared that associated into ribonucleoproteins (snRNP) and could catalyse RNA splicing. This strongly relaxed the selection pressure on intronic sequences because their ability to be spliced out didn’t depend on their secondary structure anymore. Also, snRNPs were able to interact with newly arisen protein groups – SR and hnRNP which regulated them. This marks the beginning of spliceosome and alternative splicing and both characteristics were already present in the last eukaryotic common ancestor. Since that time, core spliceosomal subunits remained highly conserved between all studied eukaryotes, but alternative splicing regulation diverged and directly influenced the structure and lifestyle of eukaryotic organisms in general.