'Big Brain' Gene Found in Humans, Not Chimps

A single gene may have paved the way for the rise of human intelligence by dramatically increasing the number of brain cells found in a key brain region.

A single gene may have paved the way for the rise of human intelligence by dramatically increasing the number of brain cells found in a key brain region.

This gene seems to be uniquely human: It is found in modern-day humans, Neanderthals and another branch of extinct humans called Denisovans, but not in chimpanzees.

By allowing the brain region called the neocortex to contain many more neurons, the tiny snippet of DNA may have laid the foundation for the human brain's massive expansion.

"It is so cool that one tiny gene alone may suffice to affect the phenotype of the stem cells, which contributed the most to the expansion of the neocortex," said study lead author Marta Florio, a doctoral candidate in molecular and cellular biology and genetics at the Max Planck Institute of Molecular Cell Biology and Genetics in Dresden, Germany. Still, it's likely this gene is just one of many genetic changes that make human cognition special, Florio said. [The Top 10 Things That Make Humans Special]

An expanding brain The evolution from primitive apes to humans with complex language and culture has taken millions of years. Some 3.8 million ago, Australopithecus afarensis, the species typified by the iconic early human ancestor fossil Lucy, had a brain that was less than 30 cubic inches (500 cubic centimeters) in volume, or about a third the size of the modern human brain. By about 1.8 million years ago, Homo erectus was equipped with a brain that was roughly twice as big as that of Australopithecus. H. erectus also showed evidence of tool and fire use and more complex social groups.

Once anatomically modern humans, and their lost cousins the Neanderthals and Denisovans, arrived on the scene, the brain had expanded to roughly 85 cubic inches (1.4 liters) in volume. Most of this growth occurred in a brain region called the neocortex.

"The neocortex is so interesting because that's the seat of cognitive abilities, which, in a way, make us human - like language and logical thinking," Florio told Live Science.

The neocortex is so large because it is jam-packed with neurons, or brain cells. But what genetic changes ushered in this explosion of neurons?

Single gene To understand that question, Florio, along with her thesis advisor, Dr. Wieland Huttner, a neurobiologist also at the Max Planck Institute, were studying one type of neural progenitor cell, a stem cell that divides and then forms brain cells during embryonic development. In mice, these cells divide once, and then make neurons. But in humans, these same types of cells divide many times over before forming a huge number of neurons.

Florio isolated this pool of cells, and then analyzed the genes that were turned on in both mice and humans at a stage of peak brain development. (The researchers looked at this process in both 13-week gestation human fetuses whose tissue had been donated by women after abortions and in mice at 14 days gestation.)

The researchers found that a particular gene, called ARHGAP11B, was turned on and highly activated in the human neural progenitor cells, but wasn't present at all in mouse cells. This tiny snippet of DNA, just 804 letters, or bases, long, was once part of a much longer gene, but somehow this fragment was duplicated and the duplicated fragment was inserted into the human genome.

Then the team inserted and expressed (turned on) this DNA snippet in the brains of mice. Though mice normally have a tiny, smooth neocortex, the mice with the gene insertion grew what looked like larger neocortices; these amped-up brain regions contained loads of neurons and some even began forming the characteristic folds, or convolutions, found in the human brain, a geometry that packs a lot of dense brain tissue into a small amount of space. (The researchers did not check to see if the mice actually got smarter, though that is a potential avenue of future research, Florio said). [The 10 Biggest Mysteries of the Mind]

Unique gene Building on past work by Evan Eichler and colleagues at the University of Washington, the team also looked at the genomes of several other species, and confirmed that Neanderthals and Denisovans had this gene, but chimpanzees and mice do not.

That suggests the gene emerged soon after humans split off from chimpanzees, and that it paved the way for the rapid expansion of the human brain.

Still, this genetic change is unlikely to fully explain human smarts, Huttner said. Both humans and Neanderthals had large brains, but human's unique intelligence may have more to do with how brain cells form and prune neural networks over time, he said.

Though the gene creates many more neurons to work with, "how those neurons wire up to allow us to fly to the moon, but not the Neanderthal, that is more likely to be a function of genes expressed in neurons," as opposed to genes expressed in progenitor cells, Huttner told Live Science.

The gene was described today (Feb. 26) in the journal Science.

Originally published on Live Science.

3D Images: Exploring the Human Brain Denisovan Gallery: Tracing the Genetics of Human Ancestors Inside the Brain: A Photo Journey Through Time Copyright 2015 LiveScience, a Purch company. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

New research suggests that a single gene may be responsible for the large number of neurons found uniquely in the human brain.

Back in the Beginning

To put a human face on our ancestors, scientists from the Senckenberg Research Institute used sophisticated methods to form 27 model heads based on tiny bone fragments, teeth and skulls collected from across the globe. The heads are on display for the first time together at the Senckenberg Natural History Museum in Frankfurt, Germany. This model is Sahelanthropus tchadensis, also nicknamed "Toumai," who lived 6.8 million years ago. Parts of its jaw bone and teeth were found nine years ago in the Djurab desert in Chad. It's one of the oldest hominid specimens ever found.

Australopithecus afarensis

With each new discovery, paleoanthropologists have to rewrite the origins of man's ancestors, adding on new branches and tracking when species split. This model was fashioned from pieces of a skull and jaw found among the remains of 17 pre-humans (nine adults, three adolescents and five children) which were discovered in the Afar Region of Ethiopia in 1975. The ape-man species, Australopithecus afarensis, is believed to have lived 3.2 million years ago. Several more bones from this species have been found in Ethiopia, including the famed "Lucy," a nearly complete A. afarensis skeleton found in Hadar.

Australopithecus africanus

Meet "Mrs. Ples," the popular nickname for the most complete skull of an Australopithecus africanus, unearthed in Sterkfontein, South Africa in 1947. It is believed she lived 2.5 million years ago (although the sex of the fossil is not entirely certain). Crystals found on her skull suggest that she died after falling into a chalk pit, which was later filled with sediment. A. africanus has long puzzled scientists because of its massive jaws and teeth, but they now believe the species' skull design was optimal for cracking nuts and seeds.

Paranthropus aethiopicus

The skull of this male adult was found on the western shore of Lake Turkana in Kenya in 1985. The shape of the mouth indicates that he had a strong bite and could chew plants. He is believed to have lived in 2.5 million years ago and is classified as Paranthropus aethiopicus. Much is still unknown about this species because so few reamins of P. aethiopicus have been found.

Paranthropus boisei

Researchers shaped this skull of "Zinj," found in 1959. The adult male lived 1.8 million years ago in the Olduvai Gorge of Tanzania. His scientific name is Paranthropus boisei, though he was originally called Zinjanthropus boisei -- hence the nickname. First discovered by anthropologist Mary Leakey, the well-preserved cranium has a small brain cavity. He would have eaten seeds, plants and roots which he probably dug with sticks or bones.

Homo rudolfensis

This model of a sub-human species -- Homo rudolfensis -- was made from bone fragments found in Koobi Fora, Kenya, in 1972. The adult male is believed to have lived about 1.8 million years ago. He used stone tools and ate meat and plants. H. Rudolfensis' distinctive features include a flatter, broader face and broader postcanine teeth, with more complex crowns and roots. He is also recognized as having a larger cranium than his contemporaries.

Homo ergaster

The almost perfectly preserved skeleton of the "Turkana Boy" is one of the most spectacular discoveries in paleoanthropology. Judging from his anatomy, scientists believe this Homo ergaster was a tall youth about 13 to 15 years old. According to research, the boy died beside a shallow river delta, where he was covered by alluvial sediments. Comparing the shape of the skull and teeth, H. ergaster had a similiar head structure to the Asian Homo erectus.

Homo heidelbergensis

This adult male, Homo heidelbergensis, was discovered in in Sima de los Huesos, Spain in 1993. Judging by the skull and cranium, scientists believe he probably died from a massive infection that caused a facial deformation. The model, shown here, does not include the deformity. This species is believed to be an ancestor of Neanderthals, as seen in the shape of his face. "Miquelon," the nickname of "Atapuerca 5", lived about 500,000 to 350,000 years ago and fossils of this species have been found in Italy, France and Greece.

Homo neanderthalensis

The "Old Man of La Chapelle" was recreated from the skull and jaw of a Homo neanderthalensis male found near La Chapelle-aux-Saints, in France in 1908. He lived 56,000 years ago. His relatively old age, thought to be between 40 to 50 years old, indicates he was well looked after by a clan. The old man's skeleton indicates he suffered from a number of afflictions, including arthritis, and had numerous broken bones. Scientists at first did not realize the age and afflicted state of this specimen when he was first discovered. This led them to incorrectly theorize that male Neanderthals were hunched over when they walked.

Homo floresiensis

The skull and jaw of this female "hobbit" was found in Liang Bua, Flores, Indonesia, in 2003. She was about 1 meter tall (about 3'3") and lived about 18,000 years ago. The discovery of her species, Homo floresiensis, brought into question the belief that Homo sapiens was the only form of mankind for the past 30,000 years. Scientists are still debating whether Homo floresiensis was its own species, or merely a group of diseased modern humans. Evidence is mounting that these small beings were, in fact, a distinct human species.

Homo sapiens

Bones can only tell us so much. Experts often assume or make educated guesses to fill in the gaps in mankind's family tree, and to develop a sense what our ancestors may have looked like. Judging from skull and mandible fragments found in a cave in Israel in 1969, this young female Homo sapien lived between 100,000 and 90,000 years ago. Her bones indicate she was about 20 years old. Her shattered skull was found among the remains of 20 others in a shallow grave.