Forty-Seven How humans lost a pair of chromosomes, and why the standard model says we should not have

Abstract

Human chromosome 2 formed when two ancestral ape chromosomes fused. The fusion is shared with Neanderthals and Denisovans, so it was fixed before those populations separated. Every new fusion begins in carriers of 47 chromosomes, who produce unbalanced embryos, and standard population genetics treats that intermediate stage as a barrier. Using Kimura's diffusion theory, checked by simulation and parameterised with human embryo data and ethnographic marriage records, this paper asks what conditions would have let the fusion through. Without meiotic drive, fixation requires groups of about a hundred receiving fewer than one immigrant per generation for tens of thousands of generations. No living ape or documented human society is organised that way: recorded intermarriage runs at six to seventeen times the rate that erases the fusion. Bottlenecks, near-extinction and plausible advantages to the fused state do not close the gap. The same model fits human–chimpanzee sequence divergence only by assuming an ancestral population three times larger than earlier estimates. One route has human evidence: female carriers of related fusions transmit them to 56–59% of balanced offspring. On sex-specific embryo data, that suffices if an unbalanced conception costs less than about a fifth to a quarter of a live birth. That cost has not been measured.