Zhifei Zeng ’23
Of all vertebrates, mammals have the largest brains in terms of absolute size and relative to body size. Significant encephalization (an increase in brain size relative to body size) has been observed in the placenta of extant mammals. However, until recently it has not yet been determined when mammalian brains began to increase in size and how they evolved to their current state today. A team of researchers from the University of Edinburgh investigated when and how the encephalization and sensory components of modern mammals emerged.
The researchers collected 34 newly discovered mammalian skulls from the Paleocene Epoch (66 to 56 million years ago) as well as the Eocene Epoch (56 million to 34 million years ago). Among them, the brain structure of fossils from the Eocene was found to be most similar to that of modern mammals. The Paleocene Epoch marked the beginning of the mammalian era after the Cretaceous mass extinction of the dinosaurs. The researchers used high-resolution computed tomography (CT) to scan and create 3D models of the skulls and made digital endocasts, which are calculated models of the brain based on the shape of the skull. They estimated the body size of these mammals from the teeth and bones, and subsequently calculated the ratio of brain size to body size. They also calculated the proportion of sensory areas in the brain from the model. They found that in the Paleocene Epoch, most mammals gained weight faster than they gained brain volume. Although their brain size increased relative to the ancestor, the proportion of sensory areas decreased, implying that their brain volume increased mainly to control their enlarged bodies. In contrast, the brains of Eocene mammals showed high encephalization, and growth was concentrated in areas involved in higher senses. This provided mammals with better olfaction, vision, part control, and sensory integration, greatly increasing their adaptive capacity.
The study suggested that the relative brain size of mammals was not a gradual increase; they first increased their body size after the extinction of the dinosaurs, while the proportion of sensory areas of the brain became even smaller. It was not until 10 million years after the extinction of the dinosaurs that mammals began to increase brain size by significantly increasing sensory areas, which may be related to increasing competition for resources. Future studies can further investigate changes in neuronal density, which can help us understand the evolution of cognitive abilities in the mammalian brain.
 O. C. Bertrand, et al., Brawn before brains in placental mammals after the end-Cretaceous extinction. Science 376, 6588 (2022).
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