The Human Brain
Weighing only about three pounds when fully grown, your brain stores your every memory, generates your every thought and feeling and allows you to manage your world. More than any other part of the body, the human brain--and its capacity for symbolic thought--sets us apart from all other species.
Although the human brain reached its current size some 150,000 years ago, the first evidence of symbolic thought didn't appear until tens of thousands of years later. Our symbolic awakening occurred when modern humans began to use their brains differently.
Artwork © Debbie Maizels, Zoobotanica
The brain of this early hominid species was about the same size as a chimpanzee's brain.
1.9-1.6 million years ago
1.8 million to 40,000 years ago
Neanderthal fossil skulls show us that the brains of these hominids were larger than ours, yet they did not have symbolic thought.
150,000 years ago to today
The Brain: Size Isn't Everything
What makes the human brain unique? Although our brains are among the largest in the animal kingdom, elephants and many whales and dolphins have even bigger brains. Nor are human brains the largest relative to body size--many birds have brains that make up more than 8% of their body weight, compared to only 2.5% for humans. The key to our brain's unique capacities is not its size, but more likely its inner wiring.
Great White Shark
Great white sharks are sensitive to the tiniest amounts of blood in water and can smell prey from hundreds of meters away. So it's no surprise that the long, stalklike olfactory bulb, which detects scents, is the most prominent part of the shark's brain. The cerebrum-responsible for more complex information exchange--is quite small compared to those in mammals.
Like sharks, dogs understand the world best by smelling it. But unlike a shark brain, a dog brain has a neocortex, a large, wrinkled mass unique to mammal brains that supports their greater intelligence. Although a dog's neocortex is much smaller than a human's, the odor-processing region is about four times larger.
Chimpanzee brains look a lot like human brains in general anatomy, and they share some of the human brain's language-related structures. Both brains have an enlarged planum temporale (PT) on the left side, and an enlarged planum parietale (PP) on the right. The PT helps humans process and understand language, while the PP plays a role in reading.
Compared with the brains of our closest living relatives, chimpanzees and bonobos, the human brain is much larger overall. It also has a larger angular gyrus. This region weaves together information from your different senses--what a dog looks like, sounds like and smells like, for instance--and links it to an abstract symbol: the word dog.
Dolphins have some of the largest brains of any animal. A dolphin's neocortex--the wrinkled surface of the brain that enables more complex thought--is also larger than a human's. But the human neocortex is made up of many more layers, making for more efficient use of brain mass.
What Goes on In Your Brain?
The brain is staggeringly complex, made up of some 100 billion neurons--cells that create and transmit messages--plus billions of other kinds of cells. Networks of neurons exchange information among dozens of brain areas specialized for different tasks. Researchers suspect that something about the organization of these networks may be the key to our uniquely human brain.
© AMNH Exhibitions
© AMNH Exhibitions
Molding a Human Brain
How did our brains evolve? Much as our other body parts did: many thousands of changes in human DNA over millions of years altered the size, shape and inner wiring of the brain. Even a single change in a gene--a stretch of DNA--can reshape the brain by altering its early development. Researchers are only beginning to discover which DNA changes allowed the uniquely human brain to emerge.
A Chain Reaction
As the human fetus develops, the brain grows dramatically in both size and complexity. "Regulator" genes guide this development program. Each regulator gene affects many other genes, switching them on or off. Those genes may affect still other genes. So a slight change in one regulator gene can have a cascading effect, dramatically altering brain development. Changes in the fetal environment--such as nutrition or doses of hormones--also can shift the course of brain development.
Caution: genes at work
The green areas in this illustration of a six-week-old human fetus show where PAX-6, a regulator gene, is switched on in the developing brain. The PAX-6 gene shapes this brain region by switching on genes that help develop its unique functions.
The surface of the enlarging brain's cerebrum remains smooth for weeks after this time.
By this point the brain's surface has begun developing wrinkles that increase its surface area and mental capacity.
A newborn's brain contains nearly 100 billion neurons, meaning that the developing fetus grows an average of 250,000 new neurons each minute.
A Big Brain Gene?
Some babies born with unusually small brains have a defect in a gene involved in brain growth. One such gene, called ASPM, evolved rapidly during the last six million years--the same period when human brain size increased dramatically. So ASPM could be one of many genes that help create the human brain's large size and intelligence.
Inside your head
The human brain enables our species to perform many complex tasks involving symbolic thought, such as using language, making music, creating art and developing elaborate tools and technologies.