I don’t think heat dissipation is actually a limiting factor for humans as things stand right now. Looking at the heat dissipation capabilities of a human brain from three perspectives (maximum possible heat dissipation by sweat glands across the whole body, maximum actual amount of sustained power output by a human in practice, maximum heat transfer from the brain to arterial blood with current-human levels of arterial bloodflow), none of them look to me to be close to the 20w the human brain consumes.
Based on sweat production of athletic people reaching 2L per hour, that gives an estimate of ~1kW of sustained cooling capacity for an entire human
5 watts per kg seems to be pretty close to the maximum power output well-trained humans can actually output in practice for a full hour, so that suggests that a 70 kg human has at least 350 watts of sustained cooling capacity (and probably more, because the limiting factor does not seem to be overheating).
Bloodflow to the brain is about 45L / h, and brains tolerate temperature ranges of 3-4ºC, so working backwards from that we get that a 160W brain would reach temperatures of about 3ºC higher than arterial blood assuming that arterial bloodflow was the primary heat remover. Probably add in 20-100 watts to account for sweat dissipation on the head. And also the carotid artery is less than a cm in diameter, so bloodflow to the brain could probably be substantially increased if there were evolutionary pressure in that direction.
Brains in practice produce about 20W of heat, so it seems likely to me that energy consumption could probably increase by at least one order of magnitude without causing the brain to cook itself, if there was strong enough selection pressure to use that much energy (probably not two orders of magnitude though).
Getting rid of the energy constraint would help though. Proof of concept: ten humans take more energy to run than one human does, and can do more thinking than one human.
I do also find it quite likely that skull size is probably the most tightly binding constraint for humans—we have smaller and very differently tuned neurons compared to other mammals, and I expect that the drive for smaller neurons in particular is downstream of space being very much at a premium, even more so than energy.
Further evidence for the “space, rather than energy expenditure or cooling, is the main binding constraint” hypothesis is the existence of Fontanelles—human brains continue to grow after birth and the skull is not entirely solid in order to allow for that—a skull that does not fully protect the brain seems like a very expensive adaptation, so it’s probably buying something quite valuable.
I note in passing that the elephant brain is not only much larger, but also has many more neurons than any human brain. Since I’ve no reason to believe the elephant brain is maximally efficient, making the same claim for our brains should require much more evidence than I’m seeing.
That’s if you’re counting the cerebellum, which doesn’t seem to contribute much to intelligence, but is important for controlling the complicated musculature of a trunk and large body.
By cortical neuron count, humans have about 18 billion, while elephants have less than 6, comparable to a chimpanzee. (source)
Elephants are undeniably intelligent as animals go, but not at human level.
Even blue whales barely approach human level by cortical neuron count, although some cetaceans (notably orcas) exceed it.
The brain is perhaps 1 to 2 OOM larger than the physical limits for a computer of equivalent power, but is constrained to its somewhat larger than minimal size due in part to thermodynamic cooling considerations.
I conclude something more like “the brain consumes perhaps 1 to 2 OOM less energy than the biological limits of energy density for something of its size, but is constrained to its somewhat lower than maximal energy density due in part to energy availability considerations” but I suspect that this is more of a figure/ground type of disagreement about which things are salient to look at vs a factual disagreement.
That said @jacob_cannell is likely to be much more informed in this space than I am—if the thermodynamic cooling considerations actually bind much more tightly than I thought, I’d be interested to know that (although not necessarily immediately, I expect that he’s dealing with rather a lot of demands on his time that are downstream of kicking the hornet’s nest here).
I don’t think heat dissipation is actually a limiting factor for humans as things stand right now. Looking at the heat dissipation capabilities of a human brain from three perspectives (maximum possible heat dissipation by sweat glands across the whole body, maximum actual amount of sustained power output by a human in practice, maximum heat transfer from the brain to arterial blood with current-human levels of arterial bloodflow), none of them look to me to be close to the 20w the human brain consumes.
Based on sweat production of athletic people reaching 2L per hour, that gives an estimate of ~1kW of sustained cooling capacity for an entire human
5 watts per kg seems to be pretty close to the maximum power output well-trained humans can actually output in practice for a full hour, so that suggests that a 70 kg human has at least 350 watts of sustained cooling capacity (and probably more, because the limiting factor does not seem to be overheating).
Bloodflow to the brain is about 45L / h, and brains tolerate temperature ranges of 3-4ºC, so working backwards from that we get that a 160W brain would reach temperatures of about 3ºC higher than arterial blood assuming that arterial bloodflow was the primary heat remover. Probably add in 20-100 watts to account for sweat dissipation on the head. And also the carotid artery is less than a cm in diameter, so bloodflow to the brain could probably be substantially increased if there were evolutionary pressure in that direction.
Brains in practice produce about 20W of heat, so it seems likely to me that energy consumption could probably increase by at least one order of magnitude without causing the brain to cook itself, if there was strong enough selection pressure to use that much energy (probably not two orders of magnitude though).
Getting rid of the energy constraint would help though. Proof of concept: ten humans take more energy to run than one human does, and can do more thinking than one human.
I do also find it quite likely that skull size is probably the most tightly binding constraint for humans—we have smaller and very differently tuned neurons compared to other mammals, and I expect that the drive for smaller neurons in particular is downstream of space being very much at a premium, even more so than energy.
Further evidence for the “space, rather than energy expenditure or cooling, is the main binding constraint” hypothesis is the existence of Fontanelles—human brains continue to grow after birth and the skull is not entirely solid in order to allow for that—a skull that does not fully protect the brain seems like a very expensive adaptation, so it’s probably buying something quite valuable.
I note in passing that the elephant brain is not only much larger, but also has many more neurons than any human brain. Since I’ve no reason to believe the elephant brain is maximally efficient, making the same claim for our brains should require much more evidence than I’m seeing.
That’s if you’re counting the cerebellum, which doesn’t seem to contribute much to intelligence, but is important for controlling the complicated musculature of a trunk and large body.
By cortical neuron count, humans have about 18 billion, while elephants have less than 6, comparable to a chimpanzee. (source)
Elephants are undeniably intelligent as animals go, but not at human level.
Even blue whales barely approach human level by cortical neuron count, although some cetaceans (notably orcas) exceed it.
jacob_cannell’s post here https://www.lesswrong.com/posts/xwBuoE9p8GE7RAuhd/brain-efficiency-much-more-than-you-wanted-to-know#Space argues that:
Does that seem about right to you?
I conclude something more like “the brain consumes perhaps 1 to 2 OOM less energy than the biological limits of energy density for something of its size, but is constrained to its somewhat lower than maximal energy density due in part to energy availability considerations” but I suspect that this is more of a figure/ground type of disagreement about which things are salient to look at vs a factual disagreement.
That said @jacob_cannell is likely to be much more informed in this space than I am—if the thermodynamic cooling considerations actually bind much more tightly than I thought, I’d be interested to know that (although not necessarily immediately, I expect that he’s dealing with rather a lot of demands on his time that are downstream of kicking the hornet’s nest here).