This problem of birthing big-brained babies in slim-hipped mothers is known as the ‘obstetrical dilemma’ and until recently was the accepted account of why human infants are born so early relative to other primates. However, anthropologist Holly Dunsworth at the University of Rhode Island has argued that another reason why our infants are born so early is that mothers would starve if the gestation period was any longer.19 Pregnancy is incredibly demanding on the mother in terms of the energy required to support both herself and the rapidly growing foetus. In primates and across other mammals, there is a reliable relationship between the relative size of the newborn compared to the mother that indicates that each species’s delivery date represents the point where the energy demands of the foetus begin to exceed what the mother can safely provide.20 Bigger foetuses require more energy. Dunsworth argues that pelvic size is not the only problem, but rather feeding babies without starving the mother is why humans are born prematurely.
What is undeniable is that human childbirth is not easy. One of the more intriguing ideas about the evolution of humans and their growing brains is that the difficulty and dangers posed by childbirth could have led to the development of assisted deliveries and ultimately contributed to the evolution of human domestication.21 Humans needed help in order to give birth, which means that the onset of midwifery may have contributed to the social development of our species. No other animal has assisted childbirths and this unique feature which appeared early in our history may have been significant in shifting our species towards greater prosocial interactions. Other primates give birth relatively quickly in trees or bushes by themselves. It is possible for humans to give birth alone, and many do, but it is not the norm and especially not for first-time mothers, who typically experience longer, more painful labours. Assisted childbirth is part of our domestication. Having other members of the group present would have helped to protect against predators and reduce the stress of the situation by offering reassurance as well as provide physical assistance in actually delivering the baby.
Assisted childbirth could have been an early behaviour that fostered the right conditions for compassion, altruism, trust and other social exchanges that would become the behavioural foundations of our cultural domestication. Even if helping a mother to deliver entailed nothing more than being present to obscure, distract or confuse a potential opportunistic predator, these behaviours could have been the basis for reciprocal relationships with others in the group. Moreover, the stress and relief associated with a potentially dangerous birth could have triggered emotions that foster motivations to shape behaviours. Those who sought and offered assistance could have passed on such traits to their own offspring, thus increasing the likelihood of this cooperative behaviour becoming an established social pattern in the species.
In the same way that domesticated dogs seek assistance, when faced with a problem, our earliest ancestors began to look to others for help. Childbirth as shared emotional experience in the evolution of social behaviour may be highly speculative, but for anyone who has witnessed a birth for the first time, the extent of the experience is unexpected, surprisingly emotional and often beyond reason and control, suggesting that it triggers behaviours that lie deep in the history of our species to help others.
Brain size and behaviour
Considering all the problems that giving birth to big brains seems to entail, we are still left with the question, ‘Why did our ancestors evolve much larger brains about 2 million years ago?’ One possibility that is consistent with the argument we began with is that a larger brain enabled animals to move around and keep track of where they have been.22 If you look at the animal kingdom, different patterns of feeding are related to different brain sizes. Primates who eat mostly fruits and nuts have larger brains than those primates who eat only leaves. Leaves are readily available in predictable locations and so require less foraging. Primates who live mostly on leaves have to consume much larger volumes of these low nutritional foods that then have to be broken down by enzymes in the stomach. This is why leaf-eating primates have much larger guts for fermenting the material. It also explains why they have to spend most of their day sitting around and simply eating and digesting.
In contrast, fruits and nuts are more nutritious but they are also sparse, more seasonal and more unpredictable. Coming down from the trees and learning to walk upright meant that foraging over greater distances by our ancestors would become the typical behavioural pattern. Bigger brains would have been necessary to find higher value nutritional foods that would have been necessary to maintain a bigger brain.
This is why fruit-eating primates have to travel much further to satisfy their dietary needs. They also have much smaller guts and proportionally larger brains. Their habitats are more extensive and require greater navigational skills so they are generally more active. Take spider monkeys and howler monkeys, two closely related species that live in the tropical rainforests of South America. The diet of the spider monkey is 90 per cent fruit and nuts, whereas howler monkeys live mostly on the rainforest’s canopy leaves. This difference in diet and the need to forage could explain why the spider monkey’s brain is proportionally twice the size of the howler monkey’s, with a corresponding greater level of problem-solving abilities.
But our early ancestors were not simply foraging for nuts and berries – they were beginning to process food and carcasses with rudimentary stone tools. Animals with large brains are better tool users and humans are experts who far exceed any of the tool-making skills of other animals. Even making the earliest simple stone tools required special skills that are uniquely human. The anatomy of the human hand and the brain mechanisms that coordinate dexterity enabled our ancestors to hold a flint in one hand and knap it into the right shape with the other – a skill so far not observed in non-human primates.23 Animals also tend to fashion tools from what is immediately available and abandon them soon after, whereas our ancestors hung on to their manufactured tools, carrying them around for future use. That requires a level of knowledge, expertise and intelligent planning to develop technology unprecedented in the animal kingdom – one notable exception being the sea otter that is said to carry a stone in its pouch that it uses for cracking seashells!
As unique as human tool use is, a significant increase in brain size occurred between 2 and 1.5 million years ago, and yet the oldest stone tools are between 3 and 2 million years old, predating the expansion of the hominid brain.24 There have been considerable developments in the sophistication of the tools following the expansion of the brain, but the invention of tool technology itself probably did not seem to depend on the significant increase in brain size.
Another class of explanation is required to explain the need to develop larger brains but one that can include changing patterns of both food exploration and hunting. Early humans not only foraged but they also increasingly hunted, which meant that they had to travel further and they had to collaborate. They had to understand each other and cooperate to satisfy mutual goals. They had to navigate a social environment as much as a physical one and this social environment would soon get crowded.