Researchers at King's College London have begun to uncover the cellular and molecular drivers underlying a key anatomical change during the evolution of mammals.
Non-mammals such as reptiles and birds have one tiny bone making up their middle ear, where we mammals have a chain of three. The extra bones are a repurposing of the non-mammal jaw joint, allowed by the evolution of a new mammalian jaw joint, known as the TMJ. This transition can be followed in the fossil record, and is reflected during embryonic development where the new parts of the middle ear develop connected to the jaw, with the ear and jaw separating when the TMJ is fully formed.
In work published in Nature Ecology and Evolution Professor Abigail Tucker and Dr Neal Anthwal from King's College London's Dental Institute, working with collaborators at the University of Illinois and Chicago, have demonstrated the importance of a group of cells called chondroclasts in this separation. Amazingly, in a mutant mouse where these cells are lost, the middle ear begins to resemble the anatomy of mammal-like reptiles from the fossil record, creatures that lived more than 150 million years ago.
"It's been really intriguing working out how the unique ear of mammals evolved and how the remarkable transformation of jaw joint to ear bones came about. This Nature Ecology and Evolution paper illustrates how developmental biology can be used to understand fossils and vice versa, by adding mechanisms to major events in evolution," says Professor Abigail Tucker of King's College London, and lead researcher of the study.
In a companion paper published in Proceedings of the Royal Society B, they have also demonstrated the importance of the gene family called Tgf-beta in this separation. Furthermore they demonstrate that a cellular process called apoptosis, or programmed cell death, plays a role in the separation of the middle ear from the jaw in marsupial opossums, but not in placental mice, suggesting that the final step of the evolution of the middle ear in mammals happened independently in placental and marsupial mammals.
Together these studies identify the changes in cellular processes driving mammalian evolution, and shed light on an evolutionary change previously observed only in fossils.
Articles: Meckel's cartilage breakdown offers clues to mammalian middle ear evolution, Anthwal, Urban, Luo, Sears and Tucker, Nature Ecology & Evolution, published 6 March 2017.
A new developmental mechanism for the separation of the mammalian middle ear ossicles from the jaw, Anthwal, Luo, Maier, Sadier, Tucker and Sears, Proceedings of the Royal Society B, doi: 10.1098/rspb.2016.2416, published 8 February 2017.