Image: Etsuko Nogami, Primate Research Institute, Kyoto University
Despite sharing 98 percent of our DNA with chimpanzees, humans have much bigger brains and are, as a species, much more intelligent. Now a new study sheds light on why: Unlike chimps, humans undergo a massive explosion in white matter growth, or the connections between brain cells, in the first two years of life.
The new results, published today (Dec. 18) in the Proceedings of the Royal Society B, partly explain why humans are so much brainier than our nearest living relatives. But they also reveal why the first two years of life play such a key role in human development.
"What's really unique about us is that our brains experience rapid establishment of connectivity in the first two years of life," said Chet Sherwood, an evolutionary neuroscientist at George Washington University, who was not involved in the study. "That probably helps to explain why those first few years of human life are so critical to set us on the course to language acquisition, cultural knowledge and all those things that make us human."
Chimpanzees
While past studies have shown that human brains go through a rapid expansion in connectivity, it wasn't clear that was unique amongst great apes (a group that includes chimps, gorillas, orangutans and humans). To prove it was the signature of humanity's superior intelligence, researchers would need to prove it was different from that in our closest living relatives.
However, a U.S. moratorium on acquiring new chimpanzees for medical research meant that people like Sherwood, who is trying to understand chimpanzee brain development, had to study decades-old baby chimpanzee brains that were lying around in veterinary pathologists' labs, Sherwood told LiveScience. [Images: Baby Chimpanzees Welcomed]
But in Japan, those limitations didn't go into place till later, allowing the researchers to do live magnetic resonance imaging (MRI) brain scans of three baby chimps as they grew to 6 years of age. They then compared the data with existing brain-imaging scans for six macaques and 28 Japanese children.
The researchers found that chimpanzees and humans both had much more brain development in early life than macaques.
"The increase in total cerebral volume during early infancy and the juvenile stage in chimpanzees and humans was approximately three times greater than that in macaques," the researchers wrote in the journal article.
But human brains expanded much more dramatically than chimpanzee brains during the first few years of life; most of that human-brain expansion was driven by explosive growth in the connections between brain cells, which manifests itself in an expansion in white matter. Chimpanzee brain volumes ballooned about half that of humans' expansion during that time period.
The findings, while not unexpected, are unique because the researchers followed the same individual chimpanzees over time; past studies have instead pieced together brain development from scans on several apes of different ages, Sherwood said.
The explosion in white matter may also explain why experiences during the first few years of life can greatly affect children's IQ, social life and long-term response to stress.
"That opens an opportunity for environment and social experience to influence the molding of connectivity," Sherwood said.
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8 Comments
Add Comment"Despite sharing 98 percent of our DNA with chimpanzees, humans have much bigger brains and are, as a species, much more intelligent."
Reply | Report Abuse | Link to thisThe phrasing is curious since, qualifying that humans are more intelligent _as_a_species_ seems to imply that some individual humans are NOT more intelligent than chimpanzees.
That aside, as I understand the 2% of DNA that is not shared could determine which portion of the 98% is expressed in the organism. It might even possible that none of the expressed DNA is shared between the two species...
On the other hand, it's almost as though chimpanzees are the same animal as humans on a continuum. Same genetic template, except with different gene expressions. Just a matter of switches turned on and off to change the resulting organism. Ultimate mass manufacturing. Very philosophical to think about.
Reply | Report Abuse | Link to thisAn intellectually sophisticated chimpanzee may be as "smart" as a three year old and since there are some human adults with disorders that cause them to have an intelligence less than that of a three year old human, some chimpanzees can possess more processing power than some humans.
Reply | Report Abuse | Link to thisFor the second point, most multicellular DNA is used to express the creation, dismantling, transportation, and manipulation of the macromolecules necessary for organisms that need oxygen to survive. Multicellular organisms also share a number of genes that are responsible for the creation of a cell membrane, the creation and role of ribosomes, and other vital strutures/processes. Since a gene is simply a number of base pairs in a particular sequence, a gene that produces a specific protein is identical to the same gene in another organism. So, much of what it takes to make a chimpanzee is identical to what it takes to make a human, but the 2% difference is enough for anatomical differences- including the brain, pelvic structure, quantity of hair, and opposable thumbs.
jtdwyer said
Reply | Report Abuse | Link to this"The phrasing is curious since, qualifying that humans are more intelligent _as_a_species_ seems to imply that some individual humans are NOT more intelligent than chimpanzees."
Thinking back on some of the people I have ran across, I would say chimps are smarter than some of them.
Yes, and don't forget the comatose - they really illustrate your point. These examples, as a species, are merely statistical outliers, aren't they?
Reply | Report Abuse | Link to thisI'll concede that my statement "It might even possible that none of the expressed DNA is shared between the two species" went too far to make a point, but the point is that DNA is not a program for producing an organism, as implied by the statement that chimpanzees have 98% in common with humans. Rather, DNA is more like a reference that is dynamically interpreted by the processes of gene expression. Please see:
http://en.wikipedia.org/wiki/Gene_expression
"Gene regulation gives the cell control over structure and function, and is the basis for cellular differentiation, morphogenesis and the versatility and adaptability of any organism. Gene regulation may also serve as a substrate for evolutionary change, since control of the timing, location, and amount of gene expression can have a profound effect on the functions (actions) of the gene in a cell or in a multicellular organism."
"The genetic code stored in DNA is "interpreted" by gene expression, and the properties of the expression give rise to the organism's phenotype. Such phenotypes are often expressed by the synthesis of proteins that control the organism's shape, or that act as enzymes catalysing specific metabolic pathways characterising the organism."
I don't agree with your statement:
"Since a gene is simply a number of base pairs in a particular sequence, a gene that produces a specific protein is identical to the same gene in another organism."
The results differ greatly depending on when, where and how genes are expressed.
DNA that is 98% identical does not produce organisms that are anywhere near 98% identical, even if they both have many similar features and characteristics.
i wonder if anyone has ever tried creating a human chimp hybrid. it seems like it would be possible.
Reply | Report Abuse | Link to thisMorally I think it raises too many problems and the anti-scientific types would attack it and anything that it might show. Just be satisfied with Ligers, Mules, and any other existing cross between species.
Reply | Report Abuse | Link to thisI deleted my comments about the Speaker of the House before posting this comment.
"I wonder if anyone has ever tried creating a human chimp hybrid. It seems like it would be possible. "
Reply | Report Abuse | Link to thisRussians tried it in a very primitive way almost a century ago. Luckily they did not succeed.
It seems that the future will ask us some difficult ethical questions.