You are here


Human Characteristics: Brains

Bigger Brains: Complex Brains for a Complex World

Endocasts of Homo erectus, (left) and Homo sapiens (right)
Endocasts of Homo erectus (left) and Homo sapiens (right) illustrate rapid increase in brain size.

As early humans faced new environmental challenges and evolved bigger bodies, they evolved larger and more complex brains.

Large, complex brains can process and store a lot of information. That was a big advantage to early humans in their social interactions and encounters with unfamiliar habitats.

Over the course of human evolution, brain size tripled. The modern human brain is the largest and most complex of any living primate.

Brain size increases slowly

From 6–2 million years ago

During this time period, early humans began to walk upright and make simple tools. Brain size increased, but only slightly.

Brain and body size increase

From 2 million–800,000 years ago

During this time period early humans spread around the globe, encountering many new environments on different continents. These challenges, along with an increase in body size, led to an increase in brain size.

Brain size increases rapidly

From 800,000–200,000 years ago

Human brain size evolved most rapidly during a time of dramatic climate change. Larger, more complex brains enabled early humans of this time period to interact with each other and with their surroundings in new and different ways. As the environment became more unpredictable, bigger brains helped our ancestors survive.

Why the sudden increase in brain size?

Graphs showing changes in climate and changes in braincase volume
Graphs showing changes in climate and changes in braincase volume. Courtesty of Karen Carr Studios

The top graph shows how Earth’s climate has fluctuated over the past 3 million years. Notice how much the fluctuations increased between 800,000 and 200,000 years ago. To construct this graph, scientists studied fossils of tiny organisms found in ocean sediment cores.

The bottom graph shows how brain size increased over the past 3 million years—especially between 800,000 and 200,000 years ago. A large brain capable of processing new information was a big advantage during times of dramatic climate change. To construct this graph, scientists measured the brain cavities of more than 160 early human skulls.

Evidence of ancient brains

Endocasts are replicas of the insides of early and modern human braincases. They represent the size and shape of the brains that once occupied the braincases.

 How are endocasts made?

Brains do not fossilize. They deteriorate, leaving a cavity inside the braincase.

Sometimes sediments fill the cavity and harden, making a natural endocast. Scientists also make artificial endocasts to study, like the ones above.

To obtain an accurate measure of brain size, scientists remove an endocast from the braincase and record its volume, or use CT scanning to measure the inside of the braincase.

Compare Your Brain with a Chimpanzee’s

Humans are primates, and chimpanzees are our closest living relatives.

The brains of the earliest humans were similar in size to those of chimpanzees. But over time, human and chimpanzee brains evolved in several different and important ways.

Speed of Growth

chart of Chimpanzee brain growth rapidly before birth.
Chimpanzee brains grow rapidly before birth. Growth levels off very soon after birth. Image courtesy of Karen Carr Studio.
chart of Human brain growth showing growth rapidly before birth through the first year and into childhood
Human brains grow rapidly before birth through the first year and into childhood. Image courtesy of Karen Carr Studios


A chimpanzee’s temporal cortex has less white matter, reflecting fewer connections between nerve cells.

Humans have far more white matter in the temporal cortex, reflecting more connections between nerve cells and a greater ability to process information.


Average weight of adult chimpanzee brain: 384 g (0.85 lb)

Average weight of modern human brain: 1,352 g (2.98 lb)

Benefits and Costs of a Big Brain

graphic of silhouettes of two men and text: Brain Size 2% and Brain's Energy Needs 20%
Brain weight versus energy requirements. Image courtesy of Karen Carr Studios


The modern human brain can:

- store many decades worth of information;

- collect and process information, then deliver output, in split seconds;

- solve problems and create abstract ideas and images.

It can also do much more.


- A big brain gobbles up energy. Your brain is 2 percent of your body’s weight but uses 20 percent of your oxygen supply and gets 20 percent of your blood flow.

- Large brains mean large heads, making childbirth more difficult and painful for human mothers than for other primates.

Page last updated: January 16, 2019