Frugal to the point of vacuity

What does it take to get Carl Zimmer to review your research in the New York Times?

I suppose it helps to be at Harvard. It also helps to have a combination of subjects — evolution and the human brain — that Zimmer has written about in the past. It helps to have a paper with lots of very pretty diagrams — the authors' hypothesis is professionally illustrated. It's also a good idea to have a vast sweeping explanation for the exceptionalism of the human brain. In this case, they call it the Tethering Hypothesis, and it's supposed to explain how humans evolved all these remarkable cognitive abilities.

The human cerebral cortex is vastly expanded relative to other primates and disproportionately occupied by distributed association regions. Here we offer a hypothesis about how association networks evolved their prominence and came to possess circuit properties vital to human cognition. The rapid expansion of the cortical mantle may have untethered large portions of the cortex from strong constraints of molecular gradients and early activity cascades that lead to sensory hierarchies. What fill the gaps between these hierarchies are densely interconnected networks that widely span the cortex and mature late into development. Limitations of the tethering hypothesis are discussed as well as its broad implications for understanding critical features of the human brain as a byproduct of size scaling.

You know what you don't need? Data, or a hypothesis that makes sense.

The paper is largely a review of neuroanatomy, describing features of the human brain that we've known about for a long, long time…except now we can illustrate them with lovely color diagrams and fMRI scans. Here's an illustration of the problem in human evolution:


There are areas of the brain that we know what they do: in red, for instance, is the primary somatosensory cortex, which is a map of muscles and sensory areas on our skins, while blue is the primary visual cortex, which is where information from our eyes is processed. In between these known areas are great beige unknowns — regions of the brain called association cortex, which integrate information from various other regions in complex ways. Our primary somatosensory and visual cortices aren't much bigger than those of a chimpanzee, which makes logical sense, since there isn't much difference in surface area or visual acuity between us, and most of the growth has occurred in the association cortex.

All well and good. The question is, what made our association cortex expand in our evolution, and how is that expansion related to specific human intellectual capacities? Those are good questions, and I'd be curious to see them answered. Too bad this paper doesn't.

One problem is that it is a review paper and really doesn't test anything — it catalogs some existing knowledge about brain organization and then throws out this Tethering Hypothesis to explain it all, which it doesn't. I do like the fact that it suggests that most of our abilities are spandrels, not explainable as adaptations, and that what it proposes is that novel abilities arose from regions of the brain that were not constrained by ancestral functional requirements. I just don't see how their mechanism explains that.

Here's one short paragraph from the paper that neatly summarizes their hypothesis.

The idea of some form of radiation outward from core organizing centers is appealing because the hominin cerebral cortex vastly expanded in a short time. It seems implausible that molecular gradients could emerge fast enough to specify new cortical areas, although developmental expression patterns have clearly been modified. Building from Rosa and colleagues’ ideas about visual cortex organization, we propose a more general tethering hypothesis to explain how new features of cortical organization might have emerged during the rapid evolutionary expansion of the cerebral mantle. The word ‘tether’ is used to emphasize that the expanding cortical plate is tethered to gradients that initially evolved in a cortex with a far smaller surface area. Much as taffy, being pulled apart, thins until it breaks in the middle, the expanding cortical zones far from the strong constraints of developmental gradients and sensory input may become untethered from the canonical sensory–motor hierarchies.

OK, that begs the question: why did the hominin cerebral cortex expand in the first place? They keep talking about this "expanding cortical plate", but not why it was expanding or why it necessitates new organizing centers. The taffy metaphor is also telling; why are they talking about things being pulled apart, when expansion of the brain is not caused by external forces pulling on it, but on internal forces of growth generating more tissue between known cortical zones?

I'm also put on edge by the phrase "It seems implausible that…", especially when applied to something that doesn't seem implausible at all. Why balk at a timescale of several million years to evolve a use for a bit of extra brain matter?

But even worse, nervous systems growing bigger is what I study. My Ph.D. research was on connectivity in the developing spinal cord of zebrafish, for instance.

The first neurons in the zebrafish embryo emerge at about 18 hours after fertilization, at a time when the nascent spinal cord is about 2mm long, in total. Cells in the hindbrain send axons all that distance (2mm is a long way in an embryo!), and as they grow, they make a little knot of synapses every 40-50µm with cells called primary motoneurons.

That's in the embryo. In the adult, the spinal cord is roughly 4cm long — there's been a 20-fold expansion in size. What do you think happened to that earlier array of cells? Did the system stretch and break?

No, it grew. In the adult, the same hindbrain neurons are still present, and their axons still reach all the way back to the tailtip. And the same motoneurons are still present, they're just spread out more to be separated by 1-1.5mm, and they still retain the same synapses.

I also did research on the earliest neurons to differentiate in the grasshopper nervous system. I studied Q1, a neuron that established one of the commissures in the grasshopper ganglion. That story is a little different: Q1 doesn't seem to have any function in the adult, and in fact looks to be abandoned and gone. But what it does is send the first slender thread across on a specific pathway; it pioneers a route across the nervous system, and then other axons pile on and follow it across. It's like sending a kite string across a chasm, then using the string to pull a rope across, and then using the rope to pull a cable across, and pretty soon you've got a bridge — and it's doing this as the chasm is widening, because like the zebrafish, the grasshopper is also growing substantially during these events.

Growth is an integral process in the development of the nervous system. Without specific evidence that these developmental mechanisms break down during growth (which seems implausible to me…), why would you postulate that a failure of developmental processes was an essential element of human evolution? A tripling of brain size from the human-chimpanzee common ancestor to the modern human seems like a small shift relative to the much larger expansion of the human fetal brain to the adult brain.

What I'd like to see, and did not find in this paper, are comparative developmental studies. When these various cortical regions of the brain are specified or establishing connectivity, how far apart are they in different species? Compare mouse and rat, for instance, or rhesus monkey and human. I suspect that in early embryos, the distances, and their relative differences, will be minuscule, and that signaling centers will be close enough that it will seem silly to argue that broad patterns of connectivity would be unable to form in the biggest brains, leaving gaps that need to be filled in by novel mechanisms and structures.

But again, the paper doesn't look at any of that at all. I found one paragraph that briefly discusses other observations that association cortex matures later than other regions of the brain, and that's about it — it is definitely not sufficient information to argue that association cortex is out of reach of intrinsic signaling gradients in the early brain.

At least the first subtitle in the paper is "A Speculative Hypothesis," which is entirely accurate. I don't see how it justifies the praise it was given in Carl Zimmer's article.

Dr. Sherwood, the George Washington University expert, praised the hypothesis for being “fairly frugal.” The emergence of the human mind might not have been a result of a vast number of mutations that altered the fine structure of the brain. Instead, a simple increase in the growth of neurons could have untethered them from their evolutionary anchors, creating the opportunity for the human mind to emerge.

Oh, wait. When the best thing you can say about a hypothesis is that it is "fairly frugal", that's not much praise at all.

Buckner RL, Krienen FM (2013) The evolution of distributed association networks in the human brain. Trends Cogn Sci 17(12):648-65.

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PZ Myers, I agree that lots of the paper is a review of old stuff. On the other hand, I don't see your background in developmental neuroscience particularly relevant. The principal questions in my mind (not addressed adequately in the paper) involve the connectivity constraints in large brains. This is largely a cortical processing problem, not particularly relevant in other brain structures. For example, several have argued (Trevis) that random connectivity is idea for association in small cortical structures (CA3 hippocampus), but too costly in larger cortical areas because of the wiring costs. If I remember correctly, Sam Wang has done some calculations on wiring costs and brain size (gray matter / white matter ratios). I also don't agree with your criticism of "why the brain got larger". I don't think the authors were addressing that issue. They assumed the brain got larger due to the benefits of computational power. But, assuming the brain got larger, it clearly couldn't simply scale. What non-linear changes would you expect? They come up with the tethering hypothesis, and speculate on the consequences. Overall, I agree that this is not ground-breaking speculation. Moreover, more "ancient" cortical regions (allocortex, all of the cortex in reptiles) are "untethered".

By John Kubie (not verified) on 27 Dec 2013 #permalink

Tethering hypotheses to the holy grail of evolutionary biology via molecular epigenetics and de novo gene creation.

Excerpted from:…

"The epigenetic effects of nutrients and pheromones extend across the life history of organisms, but from 1996 to 2012 the concept of molecular epigenetics and epigenetic effects on hormone-driven adaptive evolution of the human brain and behavior seems to have gone missing. Evolutionary psychologists and other social scientists, for example, refused to tether their hypotheses to a new discipline called ‘neuroevolutionary psychobiology’, to neurogenetics (Zoghbi & Warren, 2010), or to any biologically based discipline whatsoever (see for review Panksepp, Moskal, Panksepp, & Kroes, 2002). More than five decades of progress that directly links molecular epigenetics to behavior has been virtually ignored (Shapiro, 2012), but see Ledón-Rettig, Richards, and Martin (2012)."

My comment: Buckner had the opportunity to tether his review, and Zimmer had the opportunity to tether his comments, to what is currently known about de novo gene creation, which occurs via one signaling pathway that links nutrient uptake to pheromone-controlled reproduction in species from microbes to man. For example, it is easy to start with ecological, social, and neurogenic niche construction and link it from grazing nematodes to predatory nematodes by a nutrient-dependent single amino-acid substitution, which is associated with the development of teeth in the predator and rewiring of the most primitive neuronal system to result in dramatic differences in behavior.

So where's the hypothesis? Dobzhansky (1964) was critical of anyone who was not interested in what I just wrote. Fifty years ago., he wrote: "The notion has gained some currency that the only worthwhile biology is molecular biology. All else is "bird watching" or "butterfly collecting." Bird watching and butterfly collecting are occupations manifestly unworthy of serious scientists!"

Dobzhansky also noted the finding that, in sickle-cell disease " that hemoglobin S differs from A in the substitution of just a single amino acid, valine in place of glutamic acid in the beta chain of the hemoglobin molecule."

Why is Buckner ignoring the fact that amino acid substitutions differentiate cell types in individuals of the same species and different species. These amino acid substitutions explain differences in the olfactory receptor genes of individual humans, and they arise via the de novo experience-driven intercellular signaling and transcription / gene expression capacity of the genome.

Why is Zimmer just now beginning to realize that what has been neuroscientifically known for decades refutes the concept of mutation-initiated natural selection in the context of the development of the human brain? Did anyone seriously think that our conspecifics selected carriers of the sickle-cell variant? If not, how was it naturally selected to remain in the context of the evolution of the human brain?

By James V. Kohl (not verified) on 27 Dec 2013 #permalink

Did Zimmer ignore the fact that a glucose-dependent duplication of the glutamate dehydrogenase gene took place, which led to amino acid substitutions that characterize hominoid variants of new genes that appear to enhance the ability of mitochondria to provide energy to neurons in the developing brain?

But wait, there’s even more of what may be Zimmer’s ignorance. His expert also knows that two additional amino acid substitutions show up in the context of comparisons with other primates. For example, the amino acid sequence of the FOXP2 gene is highly conserved across mammals and the amino acid sequences are identical in rhesus macaques, gorillas, and chimpanzees. Thus, the additional amino acid substitutions in humans appear to be the clearest link to the neuroscientifically established ability to talk and to understand what others are saying. Does Zimmer understand anything about the role of nutrient-dependent amino acid substitutions in any cell type?

Should he? Perhaps not. But someone who understands the science that should be incorporated into science journalism must look at what is know about amino acid substitutions in the differentiation of all cell types of all individuals in all species. Brain imaging may present us with nice pictures, but has nothing to do with developmental disorders or brain development in general.

Amino acid substitutions are important to everything about health and disease.

"SMAC and Omi/HtrA2 are nuclear-encoded proteins residing in mitochondria. The removal of the mitochondrial targeting sequence reveals the IBM comprising four amino acids at the new N-terminus." --

By James V. Kohl (not verified) on 27 Dec 2013 #permalink


By Klasik Mobilya (not verified) on 02 Jan 2014 #permalink

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By Posicionamiento Web (not verified) on 06 Jan 2014 #permalink