O-5 - The 20 apparent A-B intermediate genomes are low-coverage and lab-clustered
Twenty genomes contained either T28144C or C8782T but not both (C/C or T/T). Examination showed depth of coverage at or below ~10x at the key positions and lab-specific shared rare mutations — the empirical pattern the authors use to argue these are not genuine circulating intermediates.
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Why this is evidence
Under H-16 a single introduction that diversified into A and B requires an intermediate haplotype (carrying only one of the two lineage-defining mutations) to have genuinely circulated in humans, bridging A to B. O-5 finds the apparent intermediates to be sequencing artifacts, i.e. no genuine intermediate circulated, removing that bridge and favouring H-5, in which the two-mutation gap sits across the human-population boundary via separate introductions.