Oligoribonucleotides are potentially capable of Darwinian evolution - they may replicate and can express an independent chemical phenotype, as embodied in modern enzymatic cofactors. Using quantitative chemical kinetics on a sporadically fed ribonucleotide pool, unreliable supplies of unstable activated ribonucleotides A and B at low concentrations recurrently yield a replicating AB polymer with a potential chemical phenotype. Self-complementary replication in the pool occurs during a minority (here ≈ 35 %) of synthetic episodes that exploit coincidental overlaps between 4, 5 or 6 spikes of arbitrarily arriving substrates. Such uniquely productive synthetic episodes, in which near-ideal reaction sequences recur at random, account for most AB oligonucleotide synthesis, and therefore underlie the emergence of net replication under realistic primordial conditions. Because overlapping substrate spikes are unexpectedly frequent, and in addition, complex spike sequences appear disproportionately, a sporadically fed pool can host unexpectedly complex syntheses. Thus, primordial substrate fluctuations are not necessarily a barrier to Darwinism, but instead can facilitate early evolution.