Finlayson [] has argued that the extent of reticulation now apparent from genomic data is so great that we should abandon specific distinctions, and envisage evolutionary processes as braided rivulets rather than as branches. However, the view that we should switch to braids and abandon phylogenetic trees is also mistaken, because we need both concepts to understand recent human evolution. As Hawks [] recognised:

25 Prüfer K.

et al. The complete genome sequence of a Neanderthal from the Altai Mountains.

26 Currat M.

Excoffier L. Strong reproductive isolation between humans and Neanderthals inferred from observed patterns of introgression.

27 Sankararaman S.

et al. The landscape of Neandertal ancestry in present-day humans.

28 Vernot B.

Akey J.M. Resurrecting surviving Neandertal lineages from modern human genomes.

27 Sankararaman S.

et al. The landscape of Neandertal ancestry in present-day humans.

29 Fu Q.

et al. DNA analysis of an early modern human from Tianyuan Cave, China.

Comparing 1000 recent human genomes reveals approximately 100 changes in amino acid coding shared by living H. sapiens but absent from the Neanderthal and Denisovan genomes [], including ones that are known to influence the development and maintenance of brain cells. Over 3000 further genetic mutations that might have affected the behavioural or morphological evolution of modern humans are also uniquely fixed in our species. Such distinctions are likely to have accumulated during the several hundred millennia that the lineages of H. sapiens and those of Neanderthals and Denisovans were biogeographically separated (tree concept), and evidently did not diffuse into the genomes of Neanderthals and Denisovans known so far, despite any braids of gene flow that might have existed. This brings us on to a fundamental issue: does the relatively low prevalence of Neanderthal and Denisovan genes in H. sapiens reflect the rarity of ancient hybridisation opportunities, or their lack of viability []? Whereas multiregional and assimilation models often imply unconstrained interbreeding between ancient human groups whenever they had the opportunity, the reality is that many factors (demographic, cultural, and biological) could have militated against reproductive success, and we are now learning more about these constraints, including the fact that exchanges between genetically distant populations can have costs as well as benefits []. New analyses suggest that, when modern humans and Neanderthals met and mixed, they were at the edge of biological incompatibility, such that there was reduced male fertility and rapid natural selection to remove the Neanderthal-derived variants that caused this sterility []. Further evidence of the swift elimination of much of the introgressed DNA comes from the partial genome of the Tianyuan skeleton from China, dated to approximately 40 000 years ago, because this shows a no greater component of Neanderthal-derived DNA than recent Asian samples [].