some recent papers on brain Tp & CC

Marc Verhaegen

Cerebral cortical processing time is elongated in human brain evolution
Kosuke Itoh cs 2022 Scientific Reports 12,1103

An increase in nr of neurons is presumed to underlie the enhancement of cognitive abilities in brain evolution.
The evolution of human cognition is then expected to have accompanied a prolongation of net neural-processing time, due to the accumulation of processing time of individual neurons over an expanded nr of neurons.

Here, we confirmed this prediction,
we quantified the amount of prolongation in vivo, using non-invasive measurements of brain responses to sounds in unanesthetized human & non-human primates.
Latencies of the N1 component of auditory-evoked potentials recorded from the scalp were c 40, 50, 60, and 100 ms for marmoset, rhesus, Pan & Hs resp.
The prominent increase in human N1 latency could not be explained by the physical lengthening of the auditory pathway:
it therefore reflected an extended dwell time for auditory cortical processing.
A longer time-window for auditory cortical processing is advantageous for analyzing time-varying acoustic stimuli, e.g. for speech perception.

A novel hypothesis concerning human brain evolution then emerges:
the increase in cortical neuronal nr widened the time-scale of sensory cortical processing:
the benefits outweighed the disadvantage of slow cognition & reaction.


Primate brain size is predicted by diet but not sociality
Alex R DeCasien, Scott A. Williams & James P Higham 2017
Nature Ecology & Evolution 1,0112

The social brain hypothesis posits:
- social complexity is the primary driver of primate cognitive complexity,
- social pressures ultimately led to the evolution of the large human brain.
This has been supported by studies indicating positive relationships between relative brain and/or neocortex size & group size,
but reported effects of different social & mating systems are highly conflicting.

Here, we use a much larger sample of primates, more recent phylogenies & updated statistical techniques:
after controlling for body size & phylogeny, we show:
CC is predicted by diet, rather than multiple measures of sociality:
frugivores exhibit larger brains than folivores.
Our results call into question the current emphasis on social rather than ecological explanations for the evolution of large brains in primates.
We evoke a range of ecological & developmental hypotheses centred on frugivory, incl.
- spatial information storage,
- extractive foraging,
- overcoming metabolic constraints.

Primates, esp. anthropoids, have rel. large CCs vs other mammals.
These observations have led researchers to propose various explanations for the evolution of increased CC in primates:
numerous comparative analyses have been undertaken to identify social and/or ecological variables that explain interspecific variation in overall brain size, or of specific brain regions.


Brain size and thermoregulation during the evolution of the genus Homo
Daniel E Naya, Hugo Naya & Enrique P Lessa 2016
Comp Biochem Physiol A Mol Integr Physiol 191:66-73.
doi 10.1016/j.cbpa.2015.09.017

Several hypotheses have been proposed to explain the evolution of an energetically costly brain in the genus Homo.
Some of these are based on the correlation between climatic factors & CC recorded for Homo the last millions of years.

Here we propose a complementary climatic hypothesis, based on the mechanistic connection between Tp, thermo-regulation & size of internal organs in endothermic spp.
Has global cooling during the last 3.2 Ma imposed an increased energy expenditure for thermoregulation?
could this in hominids represent a driver for the evolution of an expanded brain?
at least, could it imply the relaxation of a negative selection pressure acting upon this costly organ?

1) We assess variation in the energetic costs of thermoregulation & brain maintenance for the last 3.2 Ma.
2) We evaluate the relationship between Earth Tp & brain maintenance cost for the same period, taking into account the effects of body mass & fossil age.
We found:
1) the energetic cost ass.x brain enlargement represents an important fraction (between 47.5 & 82.5 %) of the increase in energy needed for thermoregulation,
2) fossil age is a better predictor of brain maintenance cost than Earth Tp: was (at least) another factor correlated with time more relevant than ambient Tp in CC evolution?
3) there is a significant negative correlation between the energetic cost of brain & Earth Tp, even after accounting for the effect of body mass & fossil age.
Thus, our results expand the current energetic framework for the study of CC evolution in our lineage:
a fall in Earth Tp during the last millions of years may have facilitated brain enlargement.


The Possible Role of Body Temperature in Modulating Brain and Body Sizes in Hominin Evolution
Manasvi Lingam 2022
Front. Psychol. doi org/10.3389/fpsyg.2021.774683

Many models have posited that the concomitant evolution of large brains & body sizes in hominins was constrained by metabolic costs.
In such studies, the impact of body Tp has arguably not been sufficiently addressed, although the rates of most physiological processes are Tp-dependent.
Hence, the potential role of body Tp in regulating the nr of neurons & body size is investigated by means of a heuristic quantitative model.
It is suggested:
modest deviations in body Tp (a couple of °C) might allow for substantive changes in brain & body parameters.
In particular, a higher body Tp may prove amenable to
- an increased nr of neurons,
- a higher brain-to-body mass ratio,
- fewer hours expended on feeding activities,
the converse could apply when the Tp is lowered.
Future studies should
- endeavor to explore & incorporate the effects of body Tp in metabolic theories of hominin evolution,
- also integrate other factors: foraging efficiency, diet, fire control.


Brain size and neuron numbers drive differences in yawn duration across mammals and birds
Jorg JM Massen cs 2021
Communications Biology 4,503

Recent studies indicate: yawning evolved as a brain-cooling mechanism.

- Larger brains have greater thermolytic needs,
- brain Tp is determined in part by heat production from neuronal activity:
do animals with larger CC & more neurons yawn longer, to produce comparable cooling effects?

We performed the largest study on yawning ever conducted: 1291 yawns of 101 spp (55 mammals, 46 birds).
Phylogenetically controlled analyses revealed robust positive correlations between yawn duration &
1) CC,
2) total neuron nr,
3) cortical/pallial neuron nr in mammals & birds,
which cannot be attributed solely to allometric scaling rules.

These relationships were similar across clades,
mammals exhibited considerably longer yawns than birds of comparable brain & body mass.
These findings provide further evidence suggesting:
yawning is a thermo-regulatory adaptation, conserved across amniote evolution.


Quantifying patterns of endocranial heat distribution:
Brain geometry and thermoregulation.
Jose Manuel de la Cuetara 2012 Am J hum Biol

The mechanisms involved in brain thermo-regulation are still poorly known,
many disagreements still exist concerning the selective cooling capacity of the brain volume.
This issue has also been discussed in human evolution & paleo-neurology, speculating on possible changes ass.x hominid encephalization.
The vascular system is supposed to be the main component responsible for thermo-regulation,
but brain geometry also plays an important role in the pattern of heat distribution.
In fossils, the only neuro-anatomical evidence available for quantitative analyses is the endocranial form, molded by the brain morphology.

Here, we present a quantitative method, based on numerical simulations to quantify & localize variation in heat dissipation patterns ass.x endocranial morphological changes,
we present a case-study on modern humans & chimps.
Thermic maps provide a graphic tool to visualize heat loading on the endocranial surface.
The distribution of the values (thermic spectrum) supplies a quantification, which can help describe & compare the patterns of heat distribution within & between groups.
Absolute values are largely influenced by size differences.
Normalized values suggest further differences ass.x brain shape.
Simulation & numerical modeling are useful to provide a descriptive & quantitative approach to endocranial thermoregulation,
they supply a quantitative tool to investigate ontogenic & phylogenetic changes.
This is particularly relevant in paleo-neurology, considering the large shape & size differences described for fossil hominid brains.


The pattern of brain-size change in the early evolution of cetaceans
David A Waugh & JGM Thewissen 2021
PLoS doi org/10.1371/journal.pone.0257803

Most authors have identified 2 rapid increases in rel.brain size (encephalization quotient EQ) in cetacean evolution:
1) at the origin of the modern suborders odonto- & mysticetes, around the Eo-Oligocene transition,
2) at the origin of the delphinoid odontocetes mid-Miocene.
We explore how methods used to estimate brain & body mass alter this perceived timing & rate of cetacean EQ evolution.
We provide new data on modern mammals (mysti-, odontocetes & terrestrial artiodactyls):
- brain mass & endocranial volume scale allometrically,
- endocranial volume is not a direct proxy for brain mass:
inconsistencies in the methods used to estimate body size across the Eo-Oligocene boundary have caused a spurious pattern in earlier relative brain size studies.

Instead, we employ a single method:
we use occipital condyle width as a skeletal proxy for body mass, using a new data-set of extant cetaceans.
We suggest:
cetacean rel. brain size is most accurately portrayed using EQs based on the scaling coefficients as observed in the closely related terrestrial artiodactyls.
Finally, we include additional data for an Eocene whale, raising the sample size of Eocene archaeocetes to 7.
Our analysis of fossil cetacean EQ is different from previous works, which had shown a sudden increase in EQ at the Eo-Oligocene origin of odontocetes.
Instead, our data show:
- brain size increased at the origin of basilosaurids, 5 My before the Eocene-Oligocene transition,
- we do not observe a significant increase in rel.brain size at the origin of odontocetes.


Evolution of the Human Brain:
the key roles of DHA (omega-3 fatty acid) and Δ6-desaturase gene
Didier Majou 2018 d.majou@...
OCL 25, A401 doi org/10.1051/ocl/2017059

The process of hominization involves an increase in brain size.
The development of hominids’ cognitive capital up to the emergence of Hs was due to interactive, iterative & integrative co-evolution, allowing positive selection.
Although this depends on many factors, here we show 3 categories that stand out:
- gene mutations,
- food resources,
- cognitive & behavioral stimulation.
Australopithecus benefited from
- the inactivation of the GULO & uricase genes,
- bipedalism causing the cognitive capital of the Homo genus to develop advantageously.
This evolution depended on 2 factors:
1) a triggering factor: gradual climate change. Homo started to regularly consume meat in addition to plants & insects,
2) a stimulating factor: mutations in the FADS2 gene, which encodes Δ6-desaturase, a key-enzyme for the synthesis of DHA & sapienic acid.
The polymorphism of this gene appears to have been essential in allowing the Homo genus to adapt to its food & for its evolution.
It provides an undeniable advantage in terms of the productivity of fat synthesis (DHA), and may partly explain positive selection.
With the advent of cooking & new mutations producing even more FADS2, the brain reached its maximum size in H.neanderthalensis, in a food eco-system that provided favorable quantities of α-Linolenic acid & DHA,
but the Würm glaciation upset this equilibrium, revealing its fragility as regards to the brain & fertility.
Hs, benefiting from new variants of the FADS2 gene, were able to adapt to this harsh environment,
Neanderthal man was unable to do so, and became extinct.

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