All the birds that we can observe today have beaks and feathers. They are two of the most distinctive features of these species that survived the mass extinction that wiped out the rest of their relatives: the dinosaurs. The evolution of birds, the most species-rich group of vertebrates on our planet, is one of the most prominent stories in biological research to understand our world. We know that feathers already covered the body of some dinosaurs more than 150 million years ago. However, the beak as we see it in today’s birds arose much later, during the Lower Paleogene, about 60 million years ago . Our current understanding of the evolution of the bird beak has led us to understand how this change from the nose of the dinosaurs to the beak of the birds took place, but we are still debating the reason for this selection in evolutionary history.
The origin of the bird’s beak
We can trace the origin of the beak to the Jurassic. About 160 million years ago, a group of maniraptorans took a different evolutionary branch. The size of these beings was decreasing over many years, which changed their way of life, their diet and, therefore, the way they eat. These ancestors of modern birds began to show modifications to their skulls. The jaws began to lengthen and many of the bones that made up the head became smaller and finer.
This process is known as paedomorphosis , whereby a species maintains ancestral juvenile characters in its formation as an adult individual.
“A good example where we can observe this pedomorphic origin is found in Archeopteryx itself, whose skull seems to be the juvenile version of the skull of a Coelophysis, a more basal theropod but which did not belong to the maniraptoran group.”
This process was joined by another fundamental one: edentulism , that is, the loss of teeth in the jaws. To investigate this process we have genetic studies in which it has been possible to verify that the elongation and fusion of the jaw bones in some species would be related to the absence of teeth. Fossil remains of those hybrid species that connect the chain of evolution from dinosaurs to birds play a fundamental role in this knowledge. It is about the observation of cases such as Hesperornis , Yixianornis or Archaeorhynchus , species that showed an original beak, but still partially toothed.
The creation of a “dino-chicken”
Through the genetic study of current species, research from Yale and Harvard universities managed in 2015 to find out more about the molecular bases that allowed the transition by which the species went from having a toothed nose to toothless beaks. The experiment consisted of genetically modifying a current species to try to go back in time, that is how they managed to get some chicken embryos to develop a snout with teeth similar to that of Archeopteryx .
This type of research allows us to learn more about the molecular pathways that favored the transition, but we are far from reaching a consensus about why the beak (and all its different morphologies) have allowed such an extraordinary diversity of species to thrive.
It is worth wondering when we can trace a beginning of the evolutionary change between snouts and peaks. In 2018, a study marked the theropods Limusaurus and Sapeornis as being 150 and 120 million years old, respectively. These could be counted among the first dinosaurs with ontogenetic edentulism . That is, these animals were born with teeth, but they were losing them as they developed to adulthood. Since then, evolution has been marked by mutations that ended up forming the modern beak about 60 million years ago.
Hypotheses about this evolutionary success
Birds were not the only family of species that experienced this and similar processes. Turtles, pterosaurs, reptiles, and other groups of vertebrates also developed toothless beaks. However, none have reached the level of diversity that the bird’s beak has.
Different hypotheses try to explain the reasons for the appearance of the peak, without any of them being able to go unpunished in the face of objections. Some have seen the decrease in size and the ability to fly as the causes that led to the evolution of the beak. Other theories suggest that the toothless beak would allow a shorter incubation time as it is easier to form a beak than a jaw with teeth, and this acceleration reduced the risk of predation during the defenseless embryonic stage.
“In the end, as happens with so many other adaptations, the beak would emerge as an adaptation to specific ecological scenarios and natural selection knew how to exploit it in each avian lineage at its convenience.”
References:
Bhullar, B. et al. 2015. A molecular mechanism for the origin of a key evolutionary innovation, the bird beak and palate, revealed by an integrative approach to major transitions in vertebrate history. Evolution 69, 7, 1665-1677. DOI: 10.1111/evo.12684.
Garrido Bautista, J. 2021. The origin of the beak of birds. elpulgardelpanda.com.
Wang, S. et al. 2017. Heterochronic truncation of odontogenesis in theropod dinosaurs provides insight into the macroevolution of avian beaks. PNAS 114, 41, 10930-10935. DOI: 10.1073/pnas.1708023114.
