Compared to other primates, human neonates are deemed altricial due to their relative underdevelopment at birth. But when looking at things more broadly, other mammals are more altricial than humans. It has been suggested that enhanced brain plasticity and obstetrical or metabolic restrictions led to humans’ secondary evolution of altricial development.
To explore this association, UCL scientists used comparative data from 140 placental mammals to measure how altricity evolved in humans and other species. They also estimated how changes in brain size and gestation length influenced the timing of neurodevelopment during hominin evolution.
Scientists found that human brains develop at a rate typical for similar primates when they are born. Still, after birth, human brains become larger and more complex than other species, creating a false belief that newborn humans are underdeveloped or “altricial.”
This new research has modified the current body of knowledge regarding the evolution of human brain development. Compared to other primates, humans appear much more defenseless at an early age—not because their brains are relatively immature, but because they have a long way to go.
Measuring the difference between a species’ birth and adult brain sizes allows scientists to study how their brains evolve. Compared to other primates, humans have a brain that is comparatively smaller at birth than it is as an adult, giving the impression that they are less developed at birth. This new research, however, demonstrates that this metric is misleading because human brain growth is broadly comparable to that of other primates, including chimpanzees, bonobos, gorillas, and orangutans, according to additional assessments.
This new research is changing what we used to think about how our brains grow. Before, people believed that when humans were born, their brains were not as developed as those of other primates. Babies are quite helpless and can’t control their muscles well. It was believed that this happened because, during evolution, there was a compromise: babies’ heads needed to be small enough to fit through their mother’s birth canal. This meant that babies’ brains had to continue developing outside the womb.
According to this old idea, because humans start with less developed brains, their brains can change more in the early years of life and can be easily influenced by things in the environment. It was thought that starting with less developed brains at birth helps make human brains more adaptable and, in the end, more intelligent.
Instead, the scientists discovered that the prolonged growth period of human brains, taking longer than in other species is not because humans are born significantly less developed. Instead, it’s because human brains experience significant growth later in life. The researchers clarified that their findings do not undermine the importance of brain plasticity in human evolution. However, they suggest that this increased adaptability is unlikely to result from humans being born less developed than other primates.
To explore the evolutionary development of human brains, the scientists examined the brain growth of 140 diverse mammal species, including modern primates, rodents, carnivores, and fossils of early humans and related ancestral hominins. They compared factors such as the duration of fetal gestation in modern mammals, the proportional size of newborn brains and bodies to their adult size, and the overall brain size of newborns and adults. This analysis aimed to gain insights into the evolution of human brains.
Although there are significant differences in the initial stages of brain development among various mammal species, primates, including humans and their hominin ancestors, exhibit relatively consistent patterns. Humans are not born at markedly lower levels of development compared to contemporary primates. Likewise, the duration of human gestation is not shorter than expected when compared to other primates.
Journal Reference:
- Gómez-Robles, A., Nicolaou, C., Smaers, J.B. et al. The evolution of human altriciality and brain development in comparative context. Nat Ecol Evol (2023). DOI: 10.1038/s41559-023-02253-z