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Wednesday April 28 7:45 PM ET
Unique Neurons Make People, Apes Smart - Report WASHINGTON (Reuters) - A unique type of brain cell seems to separate humans and apes from lower animals, and may make them smarter, scientists say.
But the neuron clusters come at a price -- they may make us susceptible to diseases such as Alzheimer's, the researchers at the California Institute of Technology and Mount Sinai School of Medicine in New York said.
The large, spindle-shaped cells are found in humans, bonobos, chimpanzees, gorillas and orangutans, but not any other type of animal, Patrick Hof of Mount Sinai and his colleagues report in this week's issue of the Proceedings of the National Academy of Sciences.
Bonobos are a type of chimpanzee recently found to be a distinct species and more closely related to humans than other chimpanzees. And Hof's team also found that bonobos have more of these cells than the other apes.
``This declining concentration matches the degree of relatedness of
these apes to humans,'' John Allman of Caltech said in a statement.
The brain cells could finally offer what many people have searched for -- a defining element that separates humans from the animal world.
Criteria have fallen by the wayside one by one with discoveries that gerbils have a fairly sophisticated language to warn of predators, that birds use tools and that dolphins can respond to the grammatical structure in hand signals.
Several chimpanzees have been taught to use either sign language or computers to communicate and have been shown to use original thought in structuring sentences.
Hof and Allman's team looked at 28 different species of primate, and at humans who died and had autopsies.
They found their spindle cells in the anterior cingulate cortex of the brain, which controls automatic functions such as heart rate but which also has been linked to emotions -- including the emotional response to pain -- and facial expressions.
``Spindle cells were notably absent in the gibbon as well as in New
World monkeys, Old World monkeys and all of the prosimians (such as lemurs),'' they added.
The findings show that, as biologists have long said, chimpanzees and bonobos are the closest relatives of humans.
``The observation that among humans and great apes, chimpanzees have
spindle cell densities comparable to humans -- and it is the bonobo whose spindle cell distribution most closely matches that of the human -- underscores the relatedness of Homo (the human species) and Pan (the chimpanzee species),'' they wrote.
Spindle cells in this area have also been shown to be affected in Alzheimer's disease, the most common cause of dementia, the researchers pointed out. This suggests that the susceptibility to such diseases may have evolved only recently.
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