A four-step history of the mammalian jaw

The mammalian jaw has a story that every biology textbook tells: the bones that chew in mammals are descended from the bones that bit in their reptile-like ancestors, and the middle ear bones of mammals, the hammer and anvil, are jaw bones that changed jobs. It is one of the most famous transformations in vertebrate evolution, a neat example of how evolution repurposes what it already has. A paper in the 13 November issue of Nature argues that the story, as usually told, is too neat. Using fossils from Sichuan and Yunnan, the authors reconstruct the evolution of the jaw in a four-stage sequence, and show that the transformation was not a single straight line but a set of repeated, parallel solutions.

The new fossils anchor the sequence. The study draws on specimens from two famous localities: a multituberculate-like fossil from Zigong in Sichuan and a morganucodontan from Lufeng in Yunnan, both of which preserve the jaw joint region in unusual detail. From these and related material, the team proposes four stages in the evolution of the jaw joint, tracking how the bones of the jaw progressively reorganised, with the quadrate and articular bones moving toward the roles they would eventually play in the ear. The sequence is not merely chronological; it is anatomical, a step-by-step account of how chewing apparatuses changed shape and function.

The striking part is the multiplicity. The authors find that the transition to the modern mammalian jaw arrangement happened several times, in different groups, along somewhat different paths, a pattern of convergent evolution in one of the most intimate parts of the skeleton. The jaw joint, one of the defining features of mammals, turns out to have been reinvented more than once, and the classic mammalian arrangement is the surviving branch of a bush, not the end point of a single lineage. This is exactly the kind of result that reshapes how the textbook story should be told.

The fossils also matter for what they record about the timing. The Zigong and Lufeng localities are famous for their Early Jurassic faunas, a period when the earliest mammals were diversifying, and the specimens fit into a wider picture of a world in which mammal-like lineages were experimenting with jaw structures in parallel. The study is a reminder that the big transitions in evolution are usually visible only in the aggregate, across continents and lineages, and that the clean narratives in textbooks are reconstructions imposed on messier evidence.

There is a satisfying symmetry in the story. The jaw joint is the part of the skeleton that lets mammals chew, and chewing is part of what let mammals diversify; and the same bones, in their second career as ear bones, give mammals the hearing that made much of their behaviour possible. A structure that did double duty across deep time is now the subject of a study showing it was invented more than once. The ear bones in every mammal alive today are the descendants of jaws that several lineages converged on, and the fossils from Sichuan and Yunnan are among the places where that history can still be read.