24-25 Sep 2010
I woke up around 7:00 PM, although I still didn't get much sleep. I didn't really get anything done today (sometimes it's good to take a break), except for writing this:
Back in 2007, Delaware's Republican Senate candidate Christine O'Donnell was featured on The O'Reilly Factor during a segment about the ethics of cloning animal embryos for medical research. In the ensuing shouting match, O'Donnell made the rather extraordinary claim that American scientists had already created "mice with fully functioning human brains". It was pretty surprising, and raised the question of how anyone could actually believe that such a thing is even possible - physiologically, neurologically, or just plain logically. How, exactly, could a mouse have a human brain?
Taken at face value, the idea is obviously ludicrous. Human brains can't fit inside mice, and certainly nobody is creating proportionally larger mice whose skulls could accommodate a human brain. Likewise, human brains can't be made small enough for a mouse. There's a reason our brains are this large: it's necessary to implement human-level abilities, such as intelligence, memory, conscious awareness and self-reflection. The average human brain weighs about 1.5 kilograms; the brain of a mouse, about 400 milligrams. You can't remove 400mg of a human brain and expect it to function as an adequate substitute for the entire organ - it doesn't have that much redundancy. There simply isn't enough "storage space" in a mouse brain to fully implement the cognitive processes that make us human. Biologically, this isn't even remotely conceivable.
However, it's possible that she may have meant something else by "human brains", as straightforwardly stupid as it sounded. The potential for different interpretations lies in the question: What do you believe makes a brain "human"? There actually is a bit of a grey area here, and this is why people might label entirely different things as "human brains". A number of experiments have been performed with mice and human tissue or genes. In 2005, Dr. Irving Weissman injected human embryonic stem cells into the brains of developing mouse fetuses, and these stem cells differentiated into human brain cells which were incorporated into the mouse's brain.
The new work, which started with human embryonic stem cells instead of cells that had already become brain cells, showed that those human cells developed into all the major kinds of cells normally found in mammalian brains, namely neurons and nerve-nurturing glial cells. It also showed that the neurons are biologically active and make what appear to be good connections, or synapses, with adjacent mouse cells.
Still, less than 1% of the mouse's brain cells were human. Weissman hopes to one day create mice with brains composed entirely of human cells, but this hasn't happened yet. In another experiment, researchers at Johns Hopkins University inserted a gene associated with schizophrenia into the forebrains of mice, resulting in marked anatomical and behavioral differences. At the University of British Columbia, scientists accidentally created highly aggressive mice by deleting a certain gene that regulates brain development. However, the altered mice once again exhibited normal behavior when given a human version of the missing gene, abnormalities in which have been associated with bipolar disorder.
So, can the brains of these mice be regarded as "human"? Not really. Structurally, even a mouse with a brain composed of 100% human cells would still have the anatomical arrangement of a mouse's brain - the human cells would simply fulfill the same functions as the mouse cells would. And even if it didn't, and instead developed differently, there still wouldn't be enough human brain cells present to enable anything resembling a human mind. There's no risk of inadvertently creating a mouse with a human consciousness trapped inside. Human brain cells don't possess anatomical features different from those of other animals which give us the human-level abilities that they lack. Our greater mental capacity results from the sheer quantity of brain cells, as well as their arrangement. Theoretically, a person with a human brain made of 100% mouse cells that perform the exact same function would still be just as human, without losing their human intelligence and awareness. For the same reason, a mouse with a "human" mouse brain won't unexpectedly acquire the same mental abilities as humans.
The description of "mice with fully functioning human brains", while perhaps bearing some very distant resemblance to the truth (there are mice with brains that are partially "human", and they are "fully functioning"), is more likely to give a highly inaccurate impression of the reality of the situation. Instead, it portrays something like a mouse with an actual human mind that experiences the same rich inner life, thoughts, feelings and awareness as any person. That's not what's happening here - and it almost certainly never will.
(And if I've gotten any of this wrong, please inform the hell out of me!)