https://royalsocietypublishing.org/doi/ ... .2019.2208

Cerebral blood flow rates in recent great apes are greater than in Australopithecus species that had equal or larger brains

Brain metabolic rate (MR) is linked mainly to the cost of synaptic activity, so may be a better correlate of cognitive ability than brain size alone. Among primates, the sizes of arterial foramina in recent and fossil skulls can be used to evaluate brain blood flow rate, which is proportional to brain MR. We use this approach to calculate flow rate in the internal carotid arteries (Q˙ICA), which supply most of the primate cerebrum. Q˙ICA is up to two times higher in recent gorillas, chimpanzees and orangutans compared with 3-million-year-old australopithecine human relatives, which had equal or larger brains. The scaling relationships between Q˙ICA and brain volume (Vbr) show exponents of 1.03 across 44 species of living haplorhine primates and 1.41 across 12 species of fossil hominins. Thus, the evolutionary trajectory for brain perfusion is much steeper among ancestral hominins than would be predicted from living primates. Between 4.4-million-year-old Ardipithecus and Homo sapiens, Vbr increased 4.7-fold, but Q˙ICA increased 9.3-fold, indicating an approximate doubling of metabolic intensity of brain tissue. By contrast, Q˙ICA is proportional to Vbr among haplorhine primates, suggesting a constant volume-specific brain MR.

Large brains don't equate to intelligence, else sperm whales would be about 6 times more intelligent than us. A better measurement is brain to body ratio, and it seems to work fairly consistently across the mammals.

Another plausible measurement of the potential for intelligence is the rate of synaptic activity, but that can be harder to measure, particularly for extinct animals.

Measurements of modern humans show that a) although our brains are only approx. 2% of our body weight, they use as much as 20% of energy in the body and ii) that our brains use 70% of their energy on synaptic activity, and the availability of that energy is directly related to the volume of oxygen rich blood, of which 15% of all the blood the heart pumps goes to the brain.

Consequently, the delivery rate of oxygen rich blood presents a ceiling on potential synaptic activity.

This study then shows that modern great apes (hominids) have a potential higher supply of blood entering the brain due to the size of the arterial foramina - the holes in the skull through which the internal carotid arteries pass - than the australopithecines.

The potential cerebral blood flow in A. afarensis was approximately half of a modern gorilla and gorillas have low brain to body ratios, meaning that even though they're the least intelligent of the great apes, they're potentially significantly more intelligent than the australopithecines.

so a gorilla was smarter than Lucy?

Svartalf wrote:so a gorilla was smarter than Lucy?

My way of answering this would be 'potentially smarter'.

As you know from humans, give a child a rich and varied environment and lots of opportunities to learn, then they may reach the full potential of their intelligence while a child of similar potential in a poorly stimulated, disinterested, uneducational environment may end up sub-normal intelligence.

We don't know how complex the australopithecine world was, how rich their social interactions were, but even if there were more interesting ways they could use their minds, their brains could only manage a metabolic rate of synaptic activities at about half the rate of gorilla. That's pretty substantial.

Smart enough to threaten smilodons with a stick?

playing dirty Lak... smilodons and other sabre toothed critters were a feature of Northern Europe and America, while autralopithecines dutifully remained Africans, plus, I guess that a dozen of those squealing apes with big sticks to enhance the obvious deterrent power of their fangs would have caused large cats to think again about preying on that group... even if the sticks were neither long, nor really pointy nor fire hardened yet.

Spearthrower wrote:
We don't know how complex the australopithecine world was, how rich their social interactions were, but even if there were more interesting ways they could use their minds, their brains could only manage a metabolic rate of synaptic activities at about half the rate of gorilla. That's pretty substantial.

I’m resisting making the obvious comment relating to “symbiosis.”

Svartalf wrote:playing dirty Lak... smilodons and other sabre toothed critters were a feature of Northern Europe and America, while autralopithecines dutifully remained Africans,...

True of the smilodons, but not of the sabre toothed felids as a group - the machairodontinae (try spelling that while drunk) - which originated in Africa.

Svartalf wrote: plus, I guess that a dozen of those squealing apes with big sticks to enhance the obvious deterrent power of their fangs would have caused large cats to think again about preying on that group... even if the sticks were neither long, nor really pointy nor fire hardened yet.

Terrifying, isn't it - the hairy hobbit hordes with their slightly sharp sticks!

felltoearth wrote:

Spearthrower wrote:
We don't know how complex the australopithecine world was, how rich their social interactions were, but even if there were more interesting ways they could use their minds, their brains could only manage a metabolic rate of synaptic activities at about half the rate of gorilla. That's pretty substantial.

I’m resisting making the obvious comment relating to “symbiosis.”

It does show how JJ managed to make his slightly sharp stick in only 30 minutes of hard mental labour.

I don't know about the independent evolutionary history of the rest of the great ape species (that will be a very interesting subject to read about), but we're supposedly more related to Australopithecus than to the extant great apes so it seems surprising that all of us living great apes would have this feature, despite our independent geologically recent histories.
Doesn't this suggest our brain's high metabolic rate evolved independently from the other great apes high metabolic rate, or is it suggesting we're more distant to Australopithecus?

The_Piper wrote:I don't know about the independent evolutionary history of the rest of the great ape species (that will be a very interesting subject to read about), but we're supposedly more related to Australopithecus than to the extant great apes so it seems surprising that all of us living great apes would have this feature, despite our independent geologically recent histories.

Doesn't this suggest our brain's high metabolic rate evolved independently from the other great apes high metabolic rate, or is it suggesting we're more distant to Australopithecus?

We all but certainly are... to not be more closely related to the australopithecines than to chimpanzees would mean that 5.5mya - or whenever the chimpanzee and human lineage finally and permanently split, one successor branch led to the australopithecines and eventually died out, while another branch led to Homo. I don't think anyone believes that's likely, although it's certainly still possible that australopithecines are not our immediate ancestors but a sister lineage.

However, I'm not sure if it was clear, but this isn't a particular trait shared among the great apes - all mammals have very similar morphology of arterial foramina - basically just holes in the skull for the arteries to get to the brain. It's the size of them that's relevant here, but it's not saying that we all share the same size holes, as it were.

Rather, the implication is that there was a general trajectory among the great apes for increased metabolic rate in the brain, suggesting all great apes were relying on their brains more to navigate their environments - which is consistent with the increasing size of the brain (particularly frontal lobe) over time in primates, even more pronounced in apes, and even more pronounced in the great apes, and this trajectory is ancient. So even though the ancestors of all modern great apes split from us many millions of years ago, they continued on that trajectory independently of us, continuing to evolutionarily invest in brain metabolic rate. Our particular lineage ended up really focusing on this, but apparently not so much until after the australopithecines (assuming they were our ancestors).

Essentially, we (great apes) have all just continued moving along that trajectory started by our shared ancestors, and the difference reported in this paper between modern great apes and australopithecines is simply due to the 2-3 million year gap.

I've stayed up all night, so it's quite possible that what I've just written there doesn't help at all and has made things even more confusing! If so, let me know and I will try again once I've regained sapiens' level synaptic activity in my brain.

That makes sense to me, thanks Spearthrower.

Spearthrower wrote:https://royalsocietypublishing.org/doi/10.1098/rspb.2019.2208

Cerebral blood flow rates in recent great apes are greater than in Australopithecus species that had equal or larger brains

Brain metabolic rate (MR) is linked mainly to the cost of synaptic activity, so may be a better correlate of cognitive ability than brain size alone. Among primates, the sizes of arterial foramina in recent and fossil skulls can be used to evaluate brain blood flow rate, which is proportional to brain MR. We use this approach to calculate flow rate in the internal carotid arteries (Q˙ICA), which supply most of the primate cerebrum. Q˙ICA is up to two times higher in recent gorillas, chimpanzees and orangutans compared with 3-million-year-old australopithecine human relatives, which had equal or larger brains. The scaling relationships between Q˙ICA and brain volume (Vbr) show exponents of 1.03 across 44 species of living haplorhine primates and 1.41 across 12 species of fossil hominins. Thus, the evolutionary trajectory for brain perfusion is much steeper among ancestral hominins than would be predicted from living primates. Between 4.4-million-year-old Ardipithecus and Homo sapiens, Vbr increased 4.7-fold, but Q˙ICA increased 9.3-fold, indicating an approximate doubling of metabolic intensity of brain tissue. By contrast, Q˙ICA is proportional to Vbr among haplorhine primates, suggesting a constant volume-specific brain MR.

Large brains don't equate to intelligence, else sperm whales would be about 6 times more intelligent than us. A better measurement is brain to body ratio, and it seems to work fairly consistently across the mammals.

Another plausible measurement of the potential for intelligence is the rate of synaptic activity, but that can be harder to measure, particularly for extinct animals.

Measurements of modern humans show that a) although our brains are only approx. 2% of our body weight, they use as much as 20% of energy in the body and ii) that our brains use 70% of their energy on synaptic activity, and the availability of that energy is directly related to the volume of oxygen rich blood, of which 15% of all the blood the heart pumps goes to the brain.

Consequently, the delivery rate of oxygen rich blood presents a ceiling on potential synaptic activity.

This study then shows that modern great apes (hominids) have a potential higher supply of blood entering the brain due to the size of the arterial foramina - the holes in the skull through which the internal carotid arteries pass - than the australopithecines.

The potential cerebral blood flow in A. afarensis was approximately half of a modern gorilla and gorillas have low brain to body ratios, meaning that even though they're the least intelligent of the great apes, they're potentially significantly more intelligent than the australopithecines.

This is interesting, especially as it seems to be claiming that the metabolic rate of modern human brains is very much higher than would be predicted from studies of other modern primates, excluding the great apes. From the article you linked:

The scaling relationships between Q˙ICA and brain volume (Vbr) show exponents of 1.03 across 44 species of living haplorhine primates and 1.41 across 12 species of fossil hominins. Thus, the evolutionary trajectory for brain perfusion is much steeper among ancestral hominins than would be predicted from living primates. Between 4.4-million-year-old Ardipithecus and Homo sapiens, Vbr increased 4.7-fold, but Q˙ICA increased 9.3-fold, indicating an approximate doubling of metabolic intensity of brain tissue.

This is clearly an active area of research, but I haven’t seen this claim elsewhere, from my usual fairly random googling. In particular, there’s a 2015 article with the same lead author here, which as far as I can tell doesn’t say that at all, it says fairly explicitly that the rate of blood flow to human brains is nothing exceptional for a haplorrhine primate. What am I missing or misreading? Is Professor Seymour saying that australopithecines were stupider than would be expected from their brain size and lineage? Quoting from the article, “Scaling of cerebral blood perfusion in primates and marsupials”, by Roger Seymour and others:

Focussing on the Haplorrhini, a tight relationship exists between Q_ ICA and Vbr, with an exponent of 0.95 (Fig. 5). The 95% confidence interval for the exponent is 0.84–1.06. A trend towards larger brains with higher Q_ ICA emerges throughout haplorrhine evolution, from the smaller brained New World monkeys to the large brained apes, including humans, with the highest Q_ ICA. Blood flow to the human brain is not exceptional and falls almost exactly on the regression line for haplorrhine primates in general.

zoon wrote:
This is interesting, especially as it seems to be claiming that the metabolic rate of modern human brains is very much higher than would be predicted from studies of other modern primates, excluding the great apes.

I'm a little confused as to how you would arrive that conclusion. It doesn't suggest that at all and I'm not seeing where it is claimed in the article. Rather, this suggests that, over time, all apes have followed this trajectory, which would explain why a modern ape has gone further along this trajectory than an ape from 3 or 4 million years ago. It's that passage of time and generations, and the continued selection pressure on metabolic rate which is the take-away, I would say.

In terms of plotting the trajectory, modern humans fall very slightly above the predicted figure, but bear in mind that the quotient is taking into account brain size as well.

Spearthrower wrote:
Terrifying, isn't it - the hairy hobbit hordes with their slightly sharp sticks!

I wonder why they didn't invent cheesy sticks. Not enough blood flow?

well, given that you have to have the discovery chain about raising milk producing animals and cheese making covered before you can even think of cheesy sticks, and that those are relatively recent innovations, it can easily be understood why australopithecines did not have those.

laklak wrote:

Spearthrower wrote:
Terrifying, isn't it - the hairy hobbit hordes with their slightly sharp sticks!

I wonder why they didn't invent cheesy sticks. Not enough blood flow?

It's known that they actually did invent cheesy sticks, but the non-traditional methods of research and analysis by means of which this was discovered are not recognized by mainstream academics and rationalists. Those who made this discovery have been unfairly otrastized by hidebound traditionalists.

The evidence for cheese-stick consumption is written in their elbows, jaunty gait, and perfectly adapted symbiosis with spoons.

Spearthrower wrote:

zoon wrote:
This is interesting, especially as it seems to be claiming that the metabolic rate of modern human brains is very much higher than would be predicted from studies of other modern primates, excluding the great apes.

I'm a little confused as to how you would arrive that conclusion. It doesn't suggest that at all and I'm not seeing where it is claimed in the article. Rather, this suggests that, over time, all apes have followed this trajectory, which would explain why a modern ape has gone further along this trajectory than an ape from 3 or 4 million years ago. It's that passage of time and generations, and the continued selection pressure on metabolic rate which is the take-away, I would say.

In terms of plotting the trajectory, modern humans fall very slightly above the predicted figure, but bear in mind that the quotient is taking into account brain size as well.

The results of the two papers taken together still look very odd to me. The 2 papers in question are:

1) the 2019 paper you linked to in the OP, which is linked again here (abstract only)

2) the 2015 paper which I linked to in my post #12 above, which is linked again here (the full paper).

Both papers have the same lead author, Prof R Seymour, and both are looking at the size of the holes in the skull for the carotid arteries as a measure of metabolic activity in the brain. (The earlier 2015 paper discusses how the diameter of the carotid arteries increases with the flow of blood to the cerebrum in different primate species, and why this can be taken as a measure of the brain’s metabolic activity.)

In the 2015 paper there is a graph, figure 5, in which the blood flow to the brain (cubic centimetres per second, y-axis) is plotted against the brain volume (millilitres then litres, x-axis) for a number of haplorhine species, including each of the great ape genera and humans. (Both axes are logarithmic, which I don’t claim to understand, but it has the effect of creating straight lines, and it’s still possible to read off approximate values for both variables for each dot on the graph.) The dots for the different species cluster reasonably closely around the diagonal regression line, and all the great apes are at least close to the dotted 95% confidence belts. Homo, at the top right, is almost exactly on the line.

According to the 2019 paper which you linked to, the rate of blood flow in the carotid arteries of australopithecines is about half the rate in modern chimpanzees, which have similar sized brains. Quoting from the abstract, which you also quoted in the OP:

We use this approach to calculate flow rate in the internal carotid arteries (Q˙ICA), which supply most of the primate cerebrum. Q˙ICA is up to two times higher in recent gorillas, chimpanzees and orangutans compared with 3-million-year-old australopithecine human relatives, which had equal or larger brains.

On the figure 5 graph of the 2015 paper (if I’m reading the logarithms correctly), the dot for chimpanzees (Pan) is at a height which says the blood flow through chimpanzee carotid arteries is approximately 2 cubic centimetres per second. If there was a dot for australopithecines, it would have the same x value as the Pan dot (assuming roughly the same brain volume for australopithecines and chimpanzees), but the y value would be at 1 cubic centimetre per second (if the blood flow for australopithecines is half that for chimpanzees, as stated in the 2019 abstract). In other words, the new dot for australopithecines would be directly below the dot for Pan and level with the 1 on the y-axis (a little to the right of the midpoint of the vertical line between the “P” of Pongo and the “s” of Hylobates). This point is well away from the regression line: if correct, the claim is that australopithecines had a very noticeably lower blood flow to the brain than would be expected for haplorhines in general, and great apes in particular. Which looks extremely fishy to me, I suspect that small holes in 3-million-year-old fossils may not be exactly the size they were in life? Alternatively, I’m missing something which you, or perhaps some mathematician on the forum, can explain to me (this does seem more likely, as both articles, as far as I can tell, are in reputable journals)?

Edited to add: if all the dots on the figure 5 graph are wiped out except the blue ones for the great apes including Homo, and then the dot for australopithecines is put in, there is a new regression line with a much steeper angle, which is the claim in the 2019 article? In that case, the claim in the 2019 article is that early hominins were stupider than would be expected for haplorhines of their brain size, and all the great apes have since been catching up independently?

Which looks extremely fishy to me, I suspect that small holes in 3-million-year-old fossils may not be exactly the size they were in life?

Well, those holes didn't get smaller after the organism died.

This point is well away from the regression line: if correct, the claim is that australopithecines had a very noticeably lower blood flow to the brain than would be expected for haplorhines in general, and great apes in particular.

That is indeed the claim (well, not the bit about haplorhines in general, as I don't know how you arrived at that), but it's also a fact insomuch as australopithecines must have had a noticeably lower level of blood flow to the brain as seen by the relative size of their arterial foramina.

I think the point you're missing, however, is the 3 million year gap, and consequent morphological evolution, between all these modern species and australopithecines.

Spearthrower wrote:

Which looks extremely fishy to me, I suspect that small holes in 3-million-year-old fossils may not be exactly the size they were in life?

Well, those holes didn't get smaller after the organism died.

This point is well away from the regression line: if correct, the claim is that australopithecines had a very noticeably lower blood flow to the brain than would be expected for haplorhines in general, and great apes in particular.

That is indeed the claim (well, not the bit about haplorhines in general, as I don't know how you arrived at that), but it's also a fact insomuch as australopithecines must have had a noticeably lower level of blood flow to the brain as seen by the relative size of their arterial foramina.

I think the point you're missing, however, is the 3 million year gap, and consequent morphological evolution, between all these modern species and australopithecines.

The graph below is taken from the 2015 paper which I linked to in my posts #12 and #18 above, which is linked again here. The graph is of blood flow through the carotid arteries, plotted against brain volume, for a number of haplorhine species. I have added a purple dot for Australopithecus, using the statement from the abstract of the 2019 paper in your OP, that the brain volume of Australopithecus was about the same as that of chimpanzee (Pan), while the blood flow through the carotid arteries was about half that of Pan. The dot for Australopithecus is some distance away from the regression line for haplorhine primates (the solid diagonal line). It appears that the blood flow through the carotid arteries of Australopithecus was about the same as that of a modern mandrill, although its brain volume was well over twice that of a modern mandrill.