Social Mammals Evolve Faster Than Solitary Ones

In a pioneering new study, published in Science, researchers have developed a new model of mammal evolution, showing that social mammals evolve faster than solitary ones and herbivores evolve faster than carnivores.

Extinction of the dinosaurs shaped mammal evolution

The team of researchers from the Natural History Museum, UK, used 3D scans of 322 placental mammal skulls in an international collaboration from over 20 museum collections.

Analysis of these specimens, including both extinct (no longer existing) and extant (still in existence) species, has allowed the researchers to generate a new model of mammal evolution. The findings provide a rare opportunity to look across time and evolution to track the rise of mammal species after the extinction of the dinosaurs.

The earliest mammals lived at the same time as some of the dinosaurs, though they were not very diverse and were incredibly limited in size – the largest mammals of the Mesozoic Era (approximately 252 million years ago) could only grow to the size of a small dog.

Nevertheless, the current study demonstrates that the diversity of placental mammal species soared after the extinction of the dinosaurs. Within a few 100,000 years of the mass extinction, the fossil record indicates the appearance of some of the earliest ancestors of today’s mammals.

The study also shows that evolution quickly slowed down after this initial explosion in diversity, occasionally followed by smaller evolution events that have gradually diminished over time – possibly initiated by periods of climate change and global cooling.

Professor Anjali Goswami, honorary professor of paleobiology at University College London and senior author of the study, explains, “This research will transform how we understand the incredible radiation of placental mammals, a group that includes our own species, and how that critical period after the last mass extinction 66 million years ago has shaped over evolution ever since.”

Goswami and colleagues aimed to use this study to better understand how different species may respond to rapid environmental changes, investigating the characteristics of mammals that rapidly evolve.

What influences evolutionary speed?

Some of the key factors that influenced the speed of evolution were habitat choice, social behavior, diet, the level of care required by offspring and time of activity.

Social mammals that live in groups were found to evolve much faster than solitary mammals, which live as single individuals. For example, some ungulates (mammals with hooves) evolved antlers and horns for fighting and social display, including ungulates such as deer, antelopes and bison.

“I conducted most of the analyses for this paper while isolating at home for several months at the start of the COVID-19 pandemic, so seeing the results of evolution in social versus solitary mammals really struck home,” Goswami adds.

Additionally, the researchers found herbivores to evolve faster than carnivores as they must monitor plants and the environment more closely. Other fast evolvers include marine mammals such as whales, seals and manatees.

The degree of parental care required by offspring was also a major factor that influenced the speed of evolution. Animals such as horses and antelopes, which are precocial – i.e., born relatively mature and mobile – evolve much faster than altricial species such as primates, which have long infancy periods and require a significant amount of parental care.

Picturing the earliest placental mammals

Data from these skulls also allowed the researchers to reconstruct what the skulls of some of the earliest placental mammal ancestors might have looked like. The identification of these ancestral placental mammals, which would have lived in the late Cretaceous period 100–66 million years ago, has been a major debate among scientists.

Reconstructions show that the earliest members of all the main groups of placental mammals probably looked very alike, irrespective of whether they are the ancestors of rodents or elephants. As a result, this presents major challenges for the identification of the earliest fossils of placental mammals but also gives an indication of the slight differences in these fossils that scientists must recognize.

Reconstruction of the ancestor of placental mammals. Credit: A.Goswami/NHM.

Goswami summarizes, “Museum collections are a unique asset as they allow us to predict the future by looking into the past. Approximately one-third of the samples used in this study came from the collections here at the Museum, including a beautiful 3D scan of Hope, the blue whale hanging in the Museum’s Hintze Hall. These data are invaluable in helping us understand how past events have shaped mammal evolution over the Cenozoic era, and which features will help mammals survive the environmental challenges that lie ahead.”

Reference: Goswami A, Noirault E, Coombs EJ, et al. Attenuated evolution of mammals through the Cenozoic. Science. 2022;378(6618):377-383. doi: 10.1126/science.abm7525.

This article is a rework of a press release issued by the Natural History Museum. Material has been edited for length and content.