The birds that populate our world today are descended from a group of dinosaurs. What whim of evolution had to happen for small birds and birds like chickens to descend from the huge Tyrannosaurus rex ? The key could be in 50 million years in which the dinosaurs were shrinking in size.
The scientific community affirms that modern birds descend from theropods : dinosaurs that walked on two legs and had three fingers on their extremities. However, we do not know the exact evolutionary process by which dinosaurs became birds .
Cold blooded or warm blooded?
Enrico Rezende, a researcher at the Center for Applied Ecology and Sustainability (CAPES) of the Catholic University of Chile, led a study in which it was explained that dinosaurs spent millions and millions of generations decreasing in size:
“The decrease in size observed in dinosaurs would be explained by the transition that, around the same time, led them to change the way they regulate their body temperature in order to survive, going from ectothermy to endothermy.”
That is, they went from being cold-blooded to warm-blooded animals . Ectothermic animals, such as reptiles or fish, cannot generate internal heat, so they rely on their environment to maintain optimum temperature. Therefore, an ectotherm will have to take shelter and will hardly be able to have activity during cold times. In contrast, endothermic animals, those with warm blood such as birds and mammals, can regulate and maintain their body temperature. This allows them to be active even at low temperatures, but at the cost of considerable energy expenditure.
To balance the high energy costs, smaller endothermic animals have more options to thrive . This transition has been key to the success of the small birds and mammals that spread across the planet.
“We believe that what could have happened is that the decrease in size in these dinosaurs, which ended in the appearance of birds, was the energy shortcut that evolution found to make this transition at the lowest possible cost.”
In the study published in the journal “Science Advances” they were responsible for analyzing the phylogeny from large theropods such as Tyrannosaurus rex , to modern birds.
“We combined a heat transfer model with theropod body size data to reconstruct the evolution of metabolic rates.”
The results show that as the rate of metabolism increases (which is indicative of a transition to warm blood), the size of the species decreases.
“We believe that what could have happened is that the decrease in size in these dinosaurs, which ended in the appearance of birds, was the energy shortcut that evolution found to make this transition at the lowest possible cost.”
This is the logical process by which theropods went from weighing hundreds of kilos to the 0.8 of Archeopteryx , one of the earliest known birds . It seems that the process had its highlight in the transition between the Early and Middle Jurassic, about 180 million years ago, until about 160 million years ago, when Aurornis xui lived, one of the oldest representatives of the aviales. The process continued until 66 million years ago, shortly before the extinction of the dinosaurs, when Asteriornis maastrichtensis , considered the first modern bird, is placed chronologically.
The theory is open to new interpretations and investigations that support what is suggested. The next step could be to reconstruct the paleoclimatic conditions in which the fossils analyzed in the study lived. If it can be confirmed that those considered cold-blooded lived in hot climates, it would add validity to the investigation. If, on the contrary, the fossils of supposed ectothermic animals inhabited cold climates, it would be a contradiction to what Rezende and his team exposed. In fact, a recent study argues that most theropods may have been warm-blooded .
This is one of the most controversial and oldest topics in paleontology. As with the key process by which dinosaurs evolved to modern birds, we will have to continue to be attentive to new research that will illuminate the path of knowledge of these prehistoric beings.
References:
Rezende, E. et al. 2020. Shrinking dinosaurs and the evolution of endothermy in birds. Science Advances 6, 1. DOI: 10.1126/sciadv.aaw4486.
Wiemann, J. et al. 2022. Fossil biomolecules reveal an avian metabolism in the ancestral dinosaur. Nature 606, 522-526. DOI: 10.1038/s41586-022-04770-6.
