An influential group of bioethicists has given the go-ahead to future research that would introduce human neural stem cells into the brains of our primate cousins, but outlined strict ethical boundaries.
At least two American teams of scientists are already creating "chimeras" by inserting human neurons into primates. Such "neural grafting" conjures up images of supersmart primates such as those in the sci-fi horror film "Planet of the Apes."
Could the introduction of human cells into primate brains make them more "humanlike"? How would one tell? And where should the line be drawn?
To answer such questions, Johns Hopkins University assembled a group of 22 philosophers, neuroscientists, primatologists, stem- cell researchers and other experts, including Stanford University law professor Hank Greely.
"We can't say that it is impossible that putting human stem cells into nonhuman primates will lead to some aspect of humanlike consciousness, although it is highly unlikely," said Greely. "And no one is trying to do that.
"These recommendations are an effort to make sure that something like that doesn't happen inadvertently or accidentally," he said.
There are huge therapeutic opportunities in the field of embryonic stem-cell research, according to the group's report, published in Friday's issue of the journal Science.
Some brain diseases, such as Parkinson's, could be remedied by inserting healthy neurons.
Because future therapies may require testing in primates, the panel sought to create responsible research guidelines.
"We may want animal tests before we use treatments in humans," said Greely.
"The closer to humans you get, the better your model - but the better your model, the more you worry about 'humanizing' the animal you put the cells into," he said.
Six factors should be considered before attempting such research, their report concluded. They include: the number of human cells injected, the age of the animal, the species, the size of the animal's brain, the site of injection, and whether the animal's brain was injured or diseased.
The group was not troubled by transplants of a few human cells into healthy adult brains of more distant relatives like monkeys.
But it had far greater concerns about implanting large numbers of human cells into the developing brains of chimps and great apes, whose brain size and genetic makeup are close to humans - and perhaps transferring humanlike behaviors.
Adult animals are not as impressionable as younger ones, and embryos are thought to be the most impressionable of all, said Greely.
And the larger the quantity of human embryonic stem cells inserted, the more powerful the influence they are likely to have, he said.
In April, a National Academy of Sciences report recommended that stem cells shouldn't be implanted into the embryos of primates. Both Canada and the United Kingdom ban such research altogether.
This group agreed with the academy's recommendations, but said there might be a role for placing such cells in adult primates.
"An adult animal already has a functioning brain, and adding new stuff is much less likely to cause major changes than if you add new cells into an eight-cell embryo," explained Greely.
The group also recommended that scientists who do these experiments report any changes in the animals' cognition. No one is expecting the animals to recite Shakespeare. But there are other ways to detect altered minds.
"You'd look for changes in brain structure. And you'd look for changes in behavior - anything that seems human or un-monkeylike," said Greely.