#65. Why There is Sex [evolution, genetics]

EV  GE

Red, theory; black, fact

The flower Coronilla varia L.

Sex as an evolvability adaptation

There are always two games in town: reproduction and evolution. Since we live on an unstable planet where the environment can change capriciously, species here have been selected for rapid evolvability per se to enable them to adapt to the occasional rapid environment changes and not go extinct. Apparently, mutations, the starting point for evolutionary adaptation, become more common when the organism is stressed, and stress may partly be a forecast of loss of fertility due to a developing genome-environment mismatch. Bacteria exhibit the large mutation of transformation under stress conditions, and three types of stress all increased the meiotic recombination rate of fruit flies (Stress-induced recombination and the mechanism of evolvability. Zhong W, Priest NK. Behavioral ecology and sociobiology. 2011;65:493-502). Recombination can involve unequal crossing-over in which changes in gene dose can occur due to gene duplication or deletion. However, since most mutations are deleterious (there are more ways to do something wrong than to do it better) many mutations will also reduce fertility, and at precisely the wrong moment: when a reduction in fertility is impending due to environment change. The answer was to split the population into two halves: the reproduction specialists and the selection specialists, and remix their respective genomes at each generation.

The roles of the two sexes

Females obviously do the heavy lifting of reproduction, and males seem to be the gene testers. So if a guy gets a bad gene, so long, and the luckier guy next to him then gets two wives. The phenomenon of greater male variability (Greater male than female variability in regional brain structure across the lifespan. Wierenga LM, Doucet GE, Dima D, Agartz I, Aghajani M, Akudjedu TN, Albajes‐Eizagirre A, Alnæs D, Alpert KI, Andreassen OA, Anticevic A. Karolinska Schizophrenia Project (KaSP) Consortium. Hum. Brain Mapp., doi:10.1002/hbm.25204, and I have never seen so many authors on a paper: 160.) suggests that mutations have more penetrance in males, as befits the male role of cannon fodder/selectees. What the male brings to the marriage bed, then, is field-tested genetic information. This system allows many mutations to be field tested with minimal loss of whole-population fertility, because it is the females who are the limiting factor in population fertility.

Chromosomal mechanisms of greater male variability

Chromosomal diploidy may be a system for sheltering females from mutations, assuming that the default process is for the phenotype that develops to be the average of the phenotypes individually specified by the paternal and maternal chromosome sets. Averaging tends to mute the extremes. The males, however, may set up a winner-take-all competition between homologous chromosomes early in development, with inactivation of one of them chosen at random. The molecular machinery for this may be similar to that of random x-inactivation in females. The result will be greater penetrance of mutations through to the phenotype and thus greater male variability. 

Quantitative prediction

This reasoning predicts that on a given trait, male variability (as standard deviation) will be 41% greater than the female variability, a testable prediction. 41% = [SQRT(2) -1] × 100. Already in my reading I have found a figure of 30%, which is suggestive. 

Mechanistic reconciliation with Mendel's laws

The postulated chromosome inactivation process may feature an exemption mechanism that operates on genes present in only one copy per parent. The effect will be to double the penetrance of dominant alleles at that gene. 

#62. Storming South [evolution, evolutionary psychology]

EP   EV

Red, theory; black, fact

This is a theory of the final stages of human evolution, when the large brain expansion occurred.

H. sapiens appears to have arisen from Homo erectus over the last 0.8 million years due to climate instability in the apparent origin area, namely East Africa. During this time, Europe was glaciated every 0.1 million years because of the astrophysical Milankovitch cycle, a rhythm in the amount of eccentricity in the Earth's orbit due to the influence of the planet Jupiter.

However, consider the hominins who had settled in Europe (or Asia, it doesn't matter for this argument) during the interglacial periods (remember that H. erectus was a great disperser) and when the ice began advancing again, were now facing much worse cooling and drying than in Africa, and thus much greater selection pressures. At least during the last continental glaciation, the ice cap only extended to the Baltic Sea at the maximum, but long before the ice arrives, the land is tundra, which can support only a very thin human population. In any given glaciation, the number of souls per hectare the land could support was relentlessly declining in northern Europe/Asia, and eventually the residents had to get out and settle on land further south, almost certainly over the dead bodies of the former owners. This would have selected early Europeans or Asians for warlike tendencies and warfaring skills, which explains a lot of human history. 

Our large brains

However, our large brains seem to be great at something else besides warfaring: that is, environment modification. It's clear that the first thing someone living in the path of a 2-km wall of ice needs is to keep from freezing to death, and this would have been the first really good reason to modify environments. Unlike chipping a stone axe, environment modification involves fabricating something bigger than the fabricator. Even a parka has to be bigger than you or you can't get into it. This plausibly would have required a larger brain to enable a qualitatively new ability: making something you can't see all at once when it is at working distance.

Our rhythmic evolution

After parkas, early northerners might have evolved enough association cortex on the next glaciation cycle to build something a little bigger, like a tent or a lean-to. On the next cycle, they might have been able to pull off a decent longhouse made of wattle. On the next, a castle surrounded by cultivated lands and drainage ditches. These structures would have delayed the moment of decision when you have to go and take on the hominins to the south. This will buy you time to build up your numbers, and I understand that winning battles is very much a numbers game. Therefore, environment modification skill would have been selected for in tandem with making like army ants.

The fossil evidence for this theory

Fossil evidence of all this in Europe or Asia may exist in the form of Neanderthal and Denisovan discoveries, hominins who have been difficult to account for in terms of previous theories of human origins. My scenario can be defended against the fossil evidence for a human origin in East Africa in general terms by citing the well-known incompleteness of the fossil record and its many biases. Moreover, a detailed explanation begins by citing what else is in East Africa: the Suez, a land bridge to both Europe and Asia via the Arabian tectonic block, which was created by plate tectonics near the end of the Miocene, thus antedating both H. sapiens and H. erectus. Not only can hominins disperse through it to other continents during interglacials, but they can come back in, fiercer and brainier than before, when the ice is advancing again, to then deposit their fossil evidence in the Rift Valley region of East Africa. The Eurasian backflow event of 3000 years ago may be a relatively recent example of this. The Isthmus of Suez is low-lying and thus easily drowned by the sea, but the probability of this was minimal at times of continental glaciation, when sea levels are minimal. This argument has similarities with the Beringia theory of how North America was populated. Early hominins may have expanded like a gas into whatever continent they could access. Increasing glaciation/tundrafication of that continent would have recompressed the "gas" southward, causing it to retrace its path, partly back into Africa. 

Pleistocene selection pressures

This process would have been accompanied by great mortality and therefore, potentially, much selection. Moreover, during the period we are considering, temperatures were declining most of the time; the plot of temperature versus time has a saw-tooth pattern, with long declines alternating with short warming events, and it is the declines that would have been the times of natural selection of hominins living at high latitudes.

A limestone block in Canada showing scratches left by stones embedded in the underside of a continental glacier. The rock has also been ground nearly flat by the same process.

#59. Neuromodulators as Peril Specialists [neuroscience, evolution]

NE   EV

Red: theory; black, fact

Solanum dulcamara, a plant with anticholinesterase activity.

“Life is difficulty.” -The Buddha Gautama

My PhD thesis was about a neuromodulator (acetylcholine) acting on mammalian brain. It was tough to decapitate so many rats; I never got used to it.

The basic theory

I conjecture that the primordial function of any type of transmitter substance acting on the g-protein-coupled cell-surface receptors or nuclear receptors of neurons was to coordinate the whole-organism response to some class of perils.

Table 1.

Peril	Substance	Failure mode
Extremes of heat and cold	glutamate and GABA	?
Predator	serotonin	depression
Parasite	histamine	phobia
Rival conspecific	noradrenaline	paranoia
Social isolation

#57. The Drill Sergeants of the Apocalypse [evolutionary psychology, population]

PO     EP

Red, theory; black, fact

Till Eulenspiegel, a classical trickster in European cultures

The trickster type may really be Nature's penetration tester who tests our defences against adversity. 

The type probably emerges in contexts of unequal power (Person A has the shotgun; person B doesn’t). Thus, an abiding fear is the soil out of which tricksterism grows, by the following positive feedback: A successful trick shows up the more powerful persons and shows them in a feckless light, which reduces the fear level of the trickster, which reinforces trick-playing. This is a short-term high that comes at the expense of worse relations with the more powerful persons and thus eventually even greater fear levels for the tricksters, which they try to remedy with still more tricks. An example of an unequal power relationship is between a foreign invader and the defenders. Invasion is such a common event in history that by now, countermeasures will have evolved. Tricksterism is likely to be a tile in the mosaic of any such adaptation. 

Tricksterism also seems to be a form of play. A result from animal ethology is that the play of young animals is a form of learning. The thing learned in playing tricks may be how to manage power inequalities.

A biological precedent for penetration testing?

Evidence for a biological precedent may be the many retroviruses integrated into the human genome. One of these may become active now and then at random and kills the host cell if the anti-viral defences of the latter have become weak due to some somatic mutation. The red team-blue team strategy seems to be too good a trick for nature to miss.

Evolution of the trickster

Modern human populations may have two independent axes of political polarization: oppressor-oppressed and trickster-control valuer. The first may subserve dispersal by generating refugee groups and the second may subserve building. Any built thing must serve in a complex world in which many constraints must be simultaneously observed. Thus, after the initial build, a long period of tweaking must typically follow. The role of the tricksters is to powerfully motivate this tweaking, for example, by cleverly making someone’s shelter fall down, before the complacency of the control-valuer builders leads to disaster. This may have been how engineering was done by an archaic version of Homo sapiens. Tricksterism may have evolved out of a previously evolved capacity for military strategy, which involves essentially putting one over on the enemy. 

Tricksterism today

The tricksters can also make mistakes, causing damage that cannot have a silver lining in any possible world, and moving to correct this is a natural role of the builders. If you are a builder, ask this: “What is the best use of my indignation?” It is to keep to a strict harm-reduction approach. 

Tricksterism can intensify into sadism, in which the protagonist takes pleasure in the victim’s torment and wants to make it last. However, if you make it last, you are giving the victim plenty of time and motivation to figure out solutions, like a patient old instructor giving his pupil his lessons one at a time, as he is ready for them, and this is how the wise victim will construct the situation. Such a victim will end up with information and know-how others will pay for. 

Before the trickster

Our evolutionary forebears may have been champion dispersers for a long time before the ice age forced some of them to become champion builders, initially, of shelters and warm clothing. (Champion environment modifiers may be closer to the mark.) It is an interesting fact that physically, humans exceed all other animals only in long-distance running, which can be read as dispersal ability. Our carelessness with preserving the local environments and our propensity for overpopulation can be read as typical r-selected disperser behavior. The r-selecting niche may have been big game hunting. H. erectus sites indicate consumption of medium and large meat animals. Overhunting would have occurred routinely, due to the slow reproduction rates of large animals and the high hunting efficiency of H. erectus due to tool use, so that dispersal of the hunters to new habitats would likewise have been routine.

Picture credit: Wiki Commons