Posted: Nov 25, 2019 5:59 pm
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?