To study evolutionary morphings, scientists often look at teeth, which are well preserved and thus well represented in the fossil record. But scientists can do more than observe the traces left behind by extinct species. They can actually experimentally reproduce evolutionary shifts that occurred millions of years ago. Typically, these shifts are induced in embryonic development. As it turns out, embryonic development may be so tuned as to recapitulate evolution. In fact, in a recent study, scientists obtained teeth similar to those that enabled the gnawings of extinct species, creatures that separated from the ancestors of modern mice as far back as the Triassic.
This study was conducted by researchers representing the University of Helsinki and the Universitat Autònoma de Barcelona (UAB). They investigated whether gradual alterations of tooth development could produce gradual changes in the phenotype, and whether these changes could reflect known evolutionary transitions.
The details of the study appeared July 30 in Nature, in an article entitled, “Replaying evolutionary transitions from the dental fossil record.” The article described how the researchers used mice that carried a spontaneously occurring null mutation in ectodysplasin (Eda). This mutation, the researchers said, was chosen because the effects of Eda on tooth morphology are relatively subtle, simplifying dental morphology without causing a complete loss of teeth. Yet the Eda mutation also alters many morphological characteristics, and is thus highly informative.
“By identifying development-based character interdependencies, we show how to predict morphological patterns of teeth among mammalian species,” wrote the authors of the Nature article. “[In addition,] in vivo inhibition of sonic hedgehog signaling in Eda null teeth enabled us to reproduce characters deep in the rodent ancestry.”
The researchers induced teeth to form that had different degrees of complexity in their crowns. The more primitive changes observed coincide with those which took place in animals of the Triassic period, some two hundred million years ago. The development of more posterior patterns coincides with the different stages of evolution found in rodents which became extinct already in the Palaeocene Epoch, some 60 million years ago. Researchers thus experimentally reproduced the transitions observed in the fossil registry of mammal teeth.
The scientists were also able to contrast the shape of these teeth with a computer-generated prediction model created by Isaac Salazar-Ciudad, Ph.D., a researcher at the UAB and at the University of Helsinki. The model reproduces how the tooth changes from a group of equal cells to a complex three-dimensional structure, with the full shape of a molar tooth, calculating the position of each cell. The model is capable of predicting the changes in the morphology of the tooth when a gene is modified, and therefore offers an explanation of the mechanisms that cause these specific changes to occur in the shape of teeth throughout evolution.
“Evolution has been explained as the ability of individuals to adapt to their environment in different ways,” Dr. Salazar-Ciudad stated. “But we do not know why or how individuals differ morphologically. The research helps to understand evolution, in each generation, as a game between the possible variations in form and natural selection.”