Dear Evil Engineer: Could I transplant a human brain into the body of a bear?

Dear Evil Engineer,

Last week, a gang of kids and their dog broke into my high-security evil facility. They were eventually caught and fed to the sharks, but not before causing chaos among my guards. Four fell into the shark pool, two were killed by friendly fire, one perished from a ferocious chihuahua bite, and one fell a great height from a ramp and barely survived. That guard is being kept alive in an iron lung in my cellar, but everything below the neck resembles steak tartare and it seems he’ll never crush a skull in his bare fists ever again.

I am reluctant to say goodbye to this guard. It’s not easy recruiting guards so utterly without human conscience. Would it be possible to transfer his brain into a fresh body – perhaps the body of a bear?

Yours,

A canny villain

Dear villain,

While I must applaud you for your out-of-the-box thinking, I’m afraid that brain transplants are not nearly possible at this point, even when both donor and recipient are happily of the same species.

There has, however, been some fascinating research involving introducing a limited amount of human neural material into animals.
Organoids are blobs of tissue grown from stem cells which can show features of real organs. Brain organoids – considered a game-changer for neuroscience – were once all grown in petri dishes, but in recent years there has been a cautious move towards implanting them in animals.

In October 2022, Stanford University researchers published a paper in Nature describing how this could influence the animals’ behaviour. The scientists grew brain organoids inside embryonic rat brains, and the organoids integrated with the rat brain to grow and comprise about one-sixth of the total mass. When the scientists stroked the rats’ whiskers, the human cells responded, and when they stimulated the human cells, the rats were prompted to get up and drink water.

These rats were still very much rats which – despite their technically part-human brains – did not show any hint of human behaviour. Suggestions that part-human brains might give rise to human behaviour were prompted, however, when researchers from the Kunming Institute of Zoology created transgenic macaques with extra copies of a gene thought to play a part in shaping human intelligence. The transgenic macaques performed better in a memory test and showed gradual brain development not dissimilar to that of a human child. Of course, neither the rats nor the macaques received a human brain in the sense you are referring.

What you’re really asking about is a brain transplant: a hypothetical procedure in which a brain is transferred from one body to another. It has much in common with the similarly hypothetical head transplant (AKA ‘whole-body transplant’), which also aims to give someone a functional new body while preserving their mind.

In 1970, neurosurgeon Robert J White momentarily seemed to have performed the impossible when he attached the head of one rhesus monkey to the headless body of another. The monkey was paralysed from the neck down – there being no technology for reconnecting the spinal cord – but it showed blood flow to the brain, normal EEG readings, and could eat and respond to external stimuli. After a few days, however, it succumbed to immunorejection and died.

White’s work has been continued by the controversial neurosurgeon Sergio Canavero, who has claimed that it is already possible to perform by far the trickiest part of a head transplant: reconnecting the severed spinal cords. His proposed procedure involves the spinal cords being severed using an ultra-sharp blade and subsequently fused using polyethylene glycol (a polymer which has been shown to rapidly repair damaged nerve membranes in spinal cord injuries in animal models). Canavero said that this would make it possible to locate, length-adjust and fuse the stumps within two minutes.

Through the 2010s, Canavero issued sundry updates on his experiments using animal models, culminating in a 2017 announcement that he had overseen a trial procedure using human cadavers. This work has been quite secretive and conducted entirely in China, with Canavero explaining that no US or European institute would accept it. “Western bioethicists need to stop patronising the world,” he once commented, in an inspiring moment of pride and resistance for our community.

Canavero’s work has been met with not so much scepticism as outright contempt from the wider scientific community. An independent review into the feasibility of human head transplants concluded in 2018 that the procedure is still a “remote and futuristic endeavour” with critical obstacles including spinal cord reattachment – a colossal challenge which really cannot be overstated – and immunorejection.

A brain transplant is a head transplant with extra steps. The brain is too easily damaged to scoop from one skull and place it in another, and doing so would require disconnecting and reconnecting many delicate cranial nerves. A brain xenotransplant is a brain transplant with extra steps. While there is plenty of exciting ongoing research into xenotransplantation, work remains in early stages and a brain xenotransplant would be the most complex procedure of them all. I do not often describe things as impossible, but I also cannot help but wonder about the basic problem of how to fit a grown man’s brain (1,350g) within the cavity intended for, say, a polar bear’s brain (500g).

Do not be discouraged. You could always try designing a brain-computer interface that would allow your injured guard to control a robotic bear remotely – not such a far-fetched idea! – or you could simply purchase a bear. Bears are, I have been informed, utterly without human conscience.

Yours,

The Evil Engineer