From dinosaurs to birds, why did evolution slow down?

There are some important node events in the history of life evolution, such as the emergence of jaws, fish landing on land, reptiles flying in the sky, etc. In the past, scientists took it for granted that when such characteristics or morphological functions with "evolutionary innovation" just appeared in a certain group, the evolution rate of the group would become faster and the diversity would increase. The reason is that these "innovations" can help organisms quickly enter new ecological niches.

Wang Min, a researcher at the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, has been engaged in the study of early bird evolution. However, according to the Mesozoic bird fossil specimens he observed, the diversity of early birds that had just taken to the sky did not seem to be high. So he re-verified this "common sense" in evolutionary biology, and the results were indeed very different from previous cognition. On June 5, the relevant research results were published in Nature Ecology & Evolution.

The evolution from dinosaurs to birds is one of the most shocking events in the history of vertebrate evolution. This process involved a large number of structural changes in the skeletal system, muscular system, and epidermal derivatives, and most of these changes are related to the origin of flight.

"The most critical change is the change in body shape represented by the length of limb bones." Wang Min introduced that, for example, from the overall trend, in the evolutionary lineage, theropod dinosaurs that are more closely related to birds have relatively longer forelimbs.

According to previous speculation, the limb bones of early birds would have evolved at a faster rate and their overall diversity should have been higher. However, the researchers systematically compared the limb bone diversity of three groups: birds, non-avian paraavian dinosaurs, and non-paraavian theropod dinosaurs, and found that the overall diversity of limb bones of early birds was the lowest, while non-paraavian theropod dinosaurs were the highest, and this difference was mainly reflected in the forelimbs.

The researchers also calculated the evolutionary rates of the limb bones of Mesozoic theropod dinosaurs, including birds, and found a similar pattern. The overall evolutionary rate of the forelimbs and hindlimbs slowed down as they approached the origin of birds, and this trend was most obvious in the forelimbs.

Zhu's Craton bird of prey restoration map drawn by Zhao Chuang

How to explain the poor diversity of limb bone morphology and the slowdown in evolutionary rate among early birds?

Wang Min believes that this phenomenon is mainly due to the "drag" of bird forelimbs. "In the early evolution of birds, flight had a strong natural selection effect. Therefore, the forelimbs of primitive birds could only undergo limited changes within the framework of aerodynamics."

"With the emergence of many flight-related advanced features, such as the joints of muscles, ligaments, and bones related to the completion of fine flight movements, the changes in bird forelimbs were able to break through the 'bottleneck' and eventually evolve into the diverse forelimb structures presented by modern birds."

Wang Min gave an example. Later, there were some highly specialized birds, such as fish birds and dusk birds, which were able to "dive" and fly in the ocean. This unique lifestyle was closely related to the evolution of their subtle muscles, ligaments, and bone joints. They were better able to break through the limitations of limb bones than early birds.

"When an organism acquires some important characteristics or functions, it will have a huge promoting effect on the development of the entire species. But this research tells us that we still need to make a fine distinction, as the impact of this promoting effect is different at different stages of evolution," said Wang Min.