Some may wonder how our ears developed to tell Bach apart from the Beatles. Thanks to a 115 million-year-old fossil co-discovered by James Hopson, a retired professor of organismal biology and anatomy at the University, scientists now have a better understanding of how acute hearing developed in mammalsand how it may have developed twice.
Much of Hopson’s research focused on mammal-like reptiles, a class of prehistoric animals from approximately 150 million years ago that gave rise to modern mammals. In fossils of most mammal-like reptiles, paleontologists note that what are now parts of the middle ear were once attached to the jawbone as auxiliary bones called the angular, the articular and the prearticular.
“In some of the later specimens, we saw that the accessory jaw bones were getting smaller, making ears more complex and more sensitive to sound,” Hopson said in an interview, describing the cooption of the accessory jaw bones to hearing systems. “This suggests that in mammals, a refinement of the hearing system would be to move them to the ear.”
Hopson noted that most early mammals have no troughs in their jawbones.
As mammalian jaws evolved to be simplermammals now have only one jawbone, the dentarythe three accessory jawbones eventually shifted to the ear to become the malleus, which fuses two bones, and the eardrum. The mammalian auditory system is a complex chain of bones from the eardrum to the inner ear. Because of this, scientists believed that this adaptation occurred in one single ancestor, who passed it on to humans and other mammals.
This theory was rocked when Hopson, on a trip to Australia in 2002, was presented with fossilized jaw of an early monotreme found in an earlier dig on the coast Melbourne by a team of researchers, including Hopson’s colleague, Thomas Rich of Museum Victoria in Melbourne. The specimen, Teinlophos trusleri, interested Hopson because the primitive mammal’s jaw had a large groove, or “trough,” which suggested that the smaller jaw bones had not developed into the ear.
“I was just amazed when I recognized the significance of this trough,” Hopson said. “Only this specimen has the trough, which indicates that these bones were still attached to the jaw in this specimen.”
In Hopson’s paper, co-written by Rich and the other Australian researchers for the February 11 issue of Science, the two most detailed specimens were adult or adolescent, with most of their teeth and jawbones fully developed. While the nature of all the bones in the Teinolophos are not known, the structure of the mandibular trough suggests that it housed a rod of accessory jaw bones similar to the angular, articular and prearticular group found in the mammal-like reptiles.
If this is true, it would mean that the two branches of mammalsthe group that gave rise to placentals and marsupials and the precursor group to monotremesevolved their acute hearing systems independently of one another, an example of convergent evolution in the development of mammals. The discovery ultimately suggests that the common ancestor of all mammals lacked the middle ear structure, though to what extent the three bones had evolved in the common mammal ancestor is still unknown.
According to Jerry Coyne, a professor in ecology and evolution at the University, while convergence is not a new idea, evidence showing its occurrence, including the Teinolophos specimen, is significant.
“How evolution can go in different lineages and different directions is the key here,” Coyne said. “The same developmental mechanisms can be used independently. This [Teinolophos] specimen may show constraints on development and the limited number of evolutionary pathways to get to a feature.”
Coyne had nothing but praise for Hopson, calling him “a great contributor to Chicago,” who still managed to rock the science world in his retirement.
As for Hopson, he will continue his research on the Teinolophos, and plans to work on a longer paper for paleontological journals and continue studying the fossils that only begin to untangle the history of complex adaptation in mammals.