Skip to main content

Verified by Psychology Today

Environment

Bon Appétit! Our Brains Are Big and This May Be Why

Controversial paper presents new ideas on brain size and ancestral food options.

Banksy's Caveman by Lord Jim/Creative Commons
Source: Banksy's Caveman by Lord Jim/Creative Commons

Key Points:

● New research paints a picture in which the population of large mammals declined resulting in an increase in human brain size.

● Evolution, the theory argues, favored large brain humans who could successfully hunt smaller, faster animals for food.

● Brain size has grown significantly over the past 2 million years, but there is controversy over why this is the case. Some say the increase was the result of many small environmental changes over time. Others argue there might have been one major change, like this one.

Sometime between 2.6 million years ago and right before the development of agriculture about 10,000 years ago something remarkable happened to human brains: They got much bigger. But why? A newly published theory argues that human brains grew significantly larger as the huge mammals around them disappeared and our ancestors had to develop skills for hunting smaller, swifter prey.

The argument hinges on the assumption that early humans specialized in taking down massive animals, such as elephants. When the population of large animals declined this triggered evolutionary pressure that favored humans with a capacity for snagging faster, smaller animals. Enter the bigger brain humans.

All animals, including humans, have heritable variations that are selectively favored or disfavored in accordance with their reproductive success. The authors of this new study suggest that our ancestors who had greater brain volume probably had more brainpower, and therefore had more success at securing these small—but hard to catch—fatty protein meals. Over time, natural selection would have favored those who possess slightly beefier brains.

How Much Did Our Brains Grow?

Two million years ago the average brain size was 650 cubic centimeters. Between that time and the agricultural revolution 10,000 years ago brain size more than doubled to 1,500 cubic centimeters. This tremendous growth is unparalleled in the world of human anatomy. Height, hand size, or heart volume did not more than double in the same time span. What could have been so crucial as to motivate this degree of growth over evolutionary time? Because brains, just like every other anatomical structure or system, are products of Darwinian natural selection there must have been something changing in the environment that created selection pressures favoring big brains.

Not Everyone Agrees With This Theory
Among scientists who study human origins, the controversy is not about whether brain size increased over some long period of time in evolutionary history, but what the driving force was behind the change. Was it one major change in the environment like the current paper suggests or was it in response to a lot of little pressures over time, as many scholars in the field see it?

Scientists who are critical of the theory point to a couple of problems with the assumptions. The authors argue that the decline in prey size is a unifying explanation not just for brain expansion, but for other biological and cultural changes. One issue here is that we are not certain that early humans hunted megaherbivores at all. Scientists have found human cut marks on large mammal bones, it is not clear if humans killed them or scavenged them.

In addition, the growth in human brain size was not linear, nor were the declines in large animals. Therefore, it is difficult to draw a straightforward relationship between animal size and brain size.

The Energetic Cost of Large Brains
All organisms face decisions about how to allocate their time and energy. There are a limited number of hours in a week to dedicate to securing food, boosting resources, maintaining social alliances, and satisfying family needs, and we only have so much energy to invest in doing all of these things. In terms of evolutionary growth, the brain is a costly organ. Even though it makes up only 2 percent of a person’s body weight, it uses around 20 percent of the body’s oxygen at rest. The average brain weighs just under three pounds, quite large compared with other great apes like the chimpanzee (.85 pounds), gorilla (one pound), and the orangutan (one pound).

At birth, we all have approximately 80-100 billion neurons and this number remains relatively stable through life. That is, we don’t really lose or grow more brain cells as we age. The important changes that take place within the brain are the connections between neurons. The synaptic pathways that allow for communication between one brain cell and another can number in the trillions and differ dramatically from one person to the next depending on life experiences and genetics. A small sample of brain tissue the size of a grain of sand contains 100,000 neurons and 1 billion synapses.

Why Isn't the Human Brain Even Larger?
At the most fundamental level, our brains are designed to process sensory information from the outside world. Data critical to human activity is taken in by sensory receptors specializing in vision, auditory, olfactory, touch, and taste—the five senses—and processed in various parts of the brain. We use this information to help us navigate the physical, social, and intrapersonal worlds around us. Why, if this is so vital, do we not have brains that are two, three, or even four times the volume of our current system? The simple answer is that everything is a trade-off. If we had brains four times the size of what we have now, selection pressures would have forced us to give up something else. In other words, big brains offer an advantage but there is a cost to all that computing power.

©2021 Kevin Bennett Ph.D. All rights reserved.

References

Ben-Dor, M., & Barkai, R. (2021). Prey Size Decline as a Unifying Ecological Selecting Agent in Pleistocene Human Evolution. Quaternary, 4(1), 7. MDPI AG. Retrieved from http://dx.doi.org/10.3390/quat4010007

advertisement
More from Kevin Bennett Ph.D.
More from Psychology Today