Mammals, including humans, have a distinctively upright posture that has contributed to their evolutionary success. However, the earliest ancestors of mammals had a sprawled posture more similar to reptiles. The transition from a sprawled stance to an upright posture in modern mammals, such as humans, dogs, and horses, marked a crucial moment in evolution. Despite over a century of study, the exact reasons behind this evolutionary leap remained unknown. A new study published in Science Advances by Harvard researchers sheds light on this mystery, revealing that the shift to an upright posture was complex and nonlinear, occurring later than previously thought.
Lead author Dr. Peter Bishop and senior author Professor Stephanie Pierce from Harvard’s Department of Organismic and Evolutionary Biology used advanced biomechanical modeling to analyze the transition from a sprawled to an upright posture in mammals. By studying the biomechanics of five modern species representing different limb postures and then extending their analysis to eight fossil species spanning 300 million years of evolution, the researchers were able to generate simulations to determine the force that hindlimbs could apply on the ground. This provided insights into how limb anatomy evolved over time to support locomotor performance in animals.
The study introduced the concept of feasible force space to the fossil record for the first time, providing a new way to understand how extinct animals moved. The researchers developed computational tools that can assist paleontologists in exploring their own questions and aid engineers in designing bio-inspired robots. They found that locomotor performance in extinct species fluctuated over millions of years, rather than progressing linearly from sprawling to upright. Some extinct species displayed flexibility in their postures, while others showed a reversal towards more sprawled postures before mammals evolved, indicating that traits associated with an upright posture in modern mammals developed later than previously believed.
The findings of the study help reconcile unresolved problems in the fossil record, such as the persistence of asymmetrical hands, feet, and limb joints in mammal ancestors. The research also sheds light on why early mammal ancestors are often found in a sprawled pose in fossils, while modern placental and marsupial fossils are typically found lying on their sides. The study suggests that major evolutionary transitions, like the shift to an upright posture, were complex and potentially influenced by chance events. The researchers speculate that ecological factors, such as the Permian-Triassic mass extinction, may have influenced the evolutionary trajectory of synapsids.
The study highlights the importance of advances in computing power and digital modeling in providing new perspectives on ancient mysteries of evolution. By using these techniques with ancient fossils, scientists can gain a better understanding of how animals evolved and moved in their environments. The research emphasizes that the evolution of mammal posture was not a simple, linear process but rather a complex and multifaceted journey that shaped the animals we see today. Overall, the study contributes to a more comprehensive understanding of mammalian evolution and the factors that influenced the development of unique traits in modern mammals.
