A single alternative splicing event might have had far-reaching consequences in the evolution of the nervous system in vertebrates, Gueroussov et al. report. The new findings show that one exon of the transcript encoding polypyrimidine tract-binding protein 1 (PTBP1) is skipped in mammals, whereas in other vertebrates this exon is present. PTBP1 binds to many RNAs to regulate alternative splicing on a large scale. Experiments in mammalian cells showed that the absence of exon 9 (exon 8 in some species) represses the activity of PTBP1 so that a mammalian-specific alternative splicing programme is activated during neuronal differentiation. Deletion of exon 9 leads to the appearance of mammalian-like features in chicken cells. These findings underscore the importance of differences in alternative splicing programming in vertebrate evolution.