Juvenalization Cycle / W.H. Calvin, U of WA



by William H. Calvin / Link at bottom of post.

Even those with a working knowledge of evolution often make the mistake of assuming that big brains would naturally evolve by slow increments: we assume that a bigger brain is a smarter brain. And since a smarter brain is surely a better brain, then it is not surprising that, analogous to compound interest, we should have bootstrapped ourselves up to a much bigger brain. After all, some people naturally have somewhat bigger heads than others, so all it takes is some natural selection for the obviously useful variant.

There is something very wrong with this commonplace explanation: it ignores the enormous natural selection against bigger heads. Maybe bigger brains are indeed better for something, but it would have been bought at an enormous price, extorted over and over again at each little increment along the way to a brain four times larger than that of our presumed ancestors, the australopithecines.

Actually, it isn’t clear that bigger brains are even necessary; an ape-sized brain reorganized to facilitate language and plan-ahead might work equally well. Yet the truly horrendous problem with bigger-heads-are-better should have been obvious long before anyone got around to noticing that someone’s hat size didn’t correlate with how smart he was: big heads cause a lot of trouble at childbirth. Big heads not only kill themselves but, moreover, others carrying similar gene combinations: their mothers. Thus all potential siblings (and occasionally some of the still-dependent prior children of that mother as well), many likely to carry those same gene combinations, will also be eliminated from the surviving gene pool.

It is hard to imagine any form of natural selection that is more powerfully negative; modern genetic diseases such as hemophilia pale by comparison. Big heads are a candidate for the worst genetic disease of all time. By all rights, any straightforward tendency toward bigger heads should have been promptly squelched.

Those who nonetheless argue bigger-is-smarter-is-better should realize that a small increment in intelligence would have had to be overwhelmingly better ever to establish a somewhat larger brain. The next increment would have had to be overwhelmingly better than the previous miracle, and so on. While perhaps anything is possible given a long enough time and compound interest, bigger-brain cleverness per se seems unlikely as a source for the fastest encephalization on record, fourfold in a mere 2.5 million years.

It makes you wonder how bigger-brains-are-better ever became established in the first place as the dominant explanation for human evolution. If women had been the scientists doing the theorizing, I suspect that we would have long ago abandoned the notion and gone in search of a better idea.

Big heads, however, nonetheless happened. And so there is presumably some way around this problem. Something else must have been under frequent selection pressure, with big heads as an unwanted side effect that was dragged along. This suggests that big heads were achieved by some decoupled backdoor route, rather than via straightforward selection for variants in brain size. And indeed big heads come as part of a package, a panoply of linked features called juvenilization (or paedomorphosis or, in even older literature, fetalization) that has been a repeated theme of vertebrate evolution.

BIG HEADS, RELATIVE TO BODY SIZE, are most readily achieved by early puberty. We know that brain size, as such, isn’t the determinant of cleverness, since elephants and dolphins aren’t the leaders in that department. Furthermore, among modern humans a large brain is no sign of genius; despite centuries of looking for a correlation, geniuses keep coming in a variety of head sizes.

It’s relative brain size that counts, given that brain size tends to scale up with adult body size across mammals. But despite our realization that we need to normalize brain size in some manner, there is really no rational reason for talking about the brain/body ratio: losing some weight around your waist, and thereby increasing your brain/body ratio, might be a wise move but it won’t make you smarter more generally. And if women are smarter than men, it probably isn’t because of their larger brain/body ratio (that they typically owe to earlier sexual maturity than men, and to their lower levels of testosterone).

So it is hard to imagine why brain size would be under natural selection for its advantages — especially when the disadvantages of an increased brain/body ratio are so immediate and so horrendous. For it is the bigger head relative to the smaller body that gets us into so much trouble: If hip size had increased commensurately, no birth canal bottleneck would have developed.

Yet it is precisely brain/body ratio that increases with juvenilization. And so an adult woman has to give birth with (by the standards of earlier generations) the narrow-hipped body of an adolescent girl. True, hip size in women does increase with childbearing; true, short adult women cannot find something that fits in the children’s section of a clothing store, thanks to the hip size disproportion. But whatever the hip size compensation has been, it has been insufficient: it cannot explain the fourfold larger brain of modern humans compared to apes and the australopithecines. So if the boom time physiology of the ice ages produced juvenilizations, selection against big heads would surely have followed.

WOULD THAT REVERSE the trend that produced juvenilization? There might have been some other way of compensating, of having your cake and eating it too. Back-and-forth need not imply maneuvering along a one-dimensional track; when there are many degrees of freedom in a developmental system, an advance on one track may be partly compensated by a retreat on another. It’s similar to the way a cook can raise the oven temperature but shorten baking time; time and temperature are two of the major themes the chef varies (along with ingredients and the order of mixing) in looking for better versions of a dish.

What are the typical “variations on a theme” in human development that might have been involved? Variation in head size is not, as such, a major theme. The major themes are robust-to-gracile, short-to-tall, time of puberty, rate of somatic development, plus various behavioral traits such as bold/cautious, etc. And so it is useful to discuss such prominent themes, rather than postulate random changes in this or that. ** Body size varies and short-average-tall is also partially heritable.       ** Early in this century, the pioneer anthropologist Franz Boas noted that there was a considerable variation in the rate (slow, average, fast) at which infants and children add on to height and weight; he called it the “tempo of growth.” ** Another heritable variant, not merely part of somatic development, is the time of sexual maturity: mothers with early menarche tend to have daughters who reach sexual maturity early too.

These themes are somewhat interdependent (time of puberty affects height, for example) but let us discuss them as if heritable genes (note the plural) for stature existed, as if somatic development rate genes existed, and as if sexual development rate genes were separate too. And that nutrition influenced them all.       Now suppose early puberty had happened to an early hominid — for whatever reason (boom-time physiology, or some advantage of juvenilized adults such as behavioral plasticity, or even throwing skill). What next?

A single phenotypic trait — height, for example — may be influenced by a number of different genes; conversely, a single gene [whose alternative versions are called alleles] may influence the development of various traits. Furthermore, a particular favorable value of a trait may be attained, by different members of a species, through different allele combinations. We cannot assume in a human population that all persons of a given height have the same combination of alleles for controlling height. There may be a substantial number of alternative genetic patterns that, holding environment constant, would produce people of the same height. Herbert A. Simon, 1983

WE’RE NOT SHORTER on average than at least one adolescent specimen of Homo erectus dated to 1.6 million years ago: he was 168 centimeters tall (known in several idiosyncratic countries as 5’6″) and, had he survived to adulthood, would probably have reached 180 centimeters (5’11”). You might expect that we would be considerably shorter, since early puberty tends to reduce body size.

So it seems likely that stature has re-enlarged via some other gene affecting stature. If repeated juvenilizations have occurred in the hominid lineage, we would all be miniature pygmies if some re-enlargement trend had not supervened. While many of the influences on stature — such as the improved diet and fewer childhood diseases of industrialized countries in the last century — only affect the phenotype (body style) and not the genotype (the genes carried by that body and passed on to offspring), stature is nonetheless relatively heritable.

Natural selection likely operated on these variants in the genotype, e.g., bigger bodies for better throwing distances, better nursing of babies during involuntary fasting, better abilities to undertake long migrations to distant patches of food, or better protection from predators. Competition between individuals can presumably enlarge the average species stature, just as harem mating systems and male competition have caused male gorillas to become twice as large as females. And in the context of the temperate zone where someone was occasionally trapped out in a blizzard, the reduced surface-to-volume ratio that goes along with bigger bodies would have increased the survival time in freezing conditions because of lengthening the time it takes to reach a life-threatening internal temperature (body size is indeed larger at high latitudes). Bigger females have bigger birth canals. For these and other reasons, bigger bodies are sometimes better, despite costing more to build and operate.

Rather as a baker might have tried raising the oven temperature but shortening the baking time, we now have a population whose body style (and genome, because natural selection has been operating) is somewhat juvenilized compared to their ancestral population, but whose body size has re-enlarged via another genetic route. What now?

Man, with his remarkable brain, developed the use of fire, but, even apart from considerations of brain power, as F. W. Went has pointed out, only a creature of man’s size could effectively control that fire. It happens that a small campfire is the smallest fire that is reliable and controllable. A still smaller flame is too easily snuffed out and a larger one too easily gets out of control. Prometheus was just large enough to feed the flame and keep from getting burnt. the architect Peter K. Stevens, 1974

THE BIRTH CANAL BOTTLENECK comes next because, without further changes, bigger-headed fetuses are going to start getting stuck during childbirth (if they hadn’t already had trouble at the smaller stature). This in turn will start selection operating on another common variation-on-a-theme, somatic developmental rate — just due to their genes, some children gain height and weight more slowly than others.

We knew that some more changes were going to be necessary because juvenilization by itself tends to suggest a shorter childhood — indeed, its truncation by early sexual maturity. But the monkey-to-ape and ape-to-human transitions show exactly the opposite: a lengthening of childhood. This paradox is resolved if we assume that a slowing of general body development (selected from that variation-on-a-theme that Boas observed) has been superimposed on juvenilization, moving the earlier menarche back out to its original year and even beyond. It’s the relative rates of somatic and sexual development that control childhood’s tempo and the resulting adult shape.

The main reason to believe that slowing has actually happened is that slowed development is more general than just childhood. Most life phase durations (conception-to-birth, birth-to-weaning, weaning-to-menarche, adult span) have been nearly doubled in going from monkey to ape. And nearly doubled again in going from ape to human. Though human gestation would at first appear to constitute an exception (it is only several weeks longer than in apes), this doubling rule seems to apply there too: human infants do not attain the same developmental landmarks as newborn apes until many months after birth, for a total internal-plus-external “gestation time” about twice that of chimpanzees.

This halving of the rate of the somatic developmental clock throughout pre- and postnatal life also needs explaining; I’m surely not the first to suggest that it was the solution to the childbirth problem presented by that big head that came along with juvenilization. If there had been a way of slowing only prenatal development without concomitant slowing of postnatal development, it might have done the job too — but the more generalized slowing may have been the only variant available.

Because juvenilization makes the adult head relatively larger and the adult pelvis relatively smaller, repeated juvenilizations will eventually run into trouble when the baby’s head can no longer get through the pelvic outlet. The gene combinations that result in early puberty and normal somatic developmental rates will then be edited out, unfortunately via maternal mortality rather than merely unsuccessful fetuses (but therefore at a much faster rate, because of the kin selection practiced by the unsuccessful fetus). The same would be true for faster-than-average somatic development genes. The gene combinations of precocity and slowed somatic developmental rates will get by, provided parturition is not equally delayed.

So long as the surviving mother can cope with raising a relatively fragile premature infant, the gene pool would soon come to be dominated by the genes for slower-than-average somatic development. This escape route for big baby heads would seem to require slowed somatic development superimposed upon the accelerated sexual maturity; our longer life spans after birth may be largely a side effect of the slowing of somatic development needed to work around the birth canal bottleneck.

Thus we get the sequence of 1) juvenilization via faster-than-average sexual development, 2) re-enlarged stature via other taller-than-average genes, and 3) slower-than-average somatic developmental rate. And because of the carryover of slowed development into postnatal life, the usual time scale is stretched; the number of years that it takes to get to puberty may have moved back out beyond what it was before the changes started to take place. Body size is also potentially back to the norm. Only head size is still increased, along with a few other uncorrected side effects such as reduced tooth size, flatter faces, and other such juvenile features.

Eureka? Only if the three-part cycle can be repeated quite a few times. And body style doesn’t backslide.  Heredity is particulate, but development is unitary. Everything in the organism is the result of the interactions of all genes, subject to the environment to which they are exposed. – the evolutionary biologist Theodosius Dobzhansky, 1961

Can this evolutionary process of juvenilization-reenlargement-slowing become a repeating cycle? Or has it run out of steam after the three phases, like most inventions in evolution — run to the end of its growth curve, with no further progress possible? The classic example of a limited growth curve is hairlessness: when something such as swimming success starts selecting for variants with less body hair, there is a limit beyond which further selection cannot operate (as you can only become so naked).

Can our slightly juvenilized (but re-enlarged and slower-growing) hominid, with its slightly larger-than-ape brain and its slightly flatter face and slightly smaller teeth, be subjected to yet another round of selection exactly like the first one? Can boom-time physiology (or some specific advantage of a juvenilized body style) select for juvenilization again? Can the resulting population then re-enlarge? Will the birth canal bottleneck then again select for slower-than-average somatic development rate genes?

WELL, WHY NOT? If any one of the selection pressures is removed, the cycle won’t repeat. If any one of the three processes runs out of growth curve, it’ll stall. If anything is invented that can break the cycle — such as cesarean sections or really big hips — it should stop. Otherwise, it ought to cycle until the disadvantages balance out the advantages. There might be counterpressures (if you are a hyper-robust australopithecine and need big teeth for processing plant food, this counterpressure might have prevented further rounds of juvenilization). Something, for example, happened after a similar juvenilization transition from the Old World monkeys to the apes: they seem to have stabilized for 30 million years rather than repeating the cycle of juvenilization and slowing.

Might body size have counterpressures? There are situations (islands with dwarf elephants, for example) where small body size is common. But, at least in the temperate zones, our typical adult stature seems better than a pygmy-like stature; while bigger-is-better may not extend to 250 centimeters stature, it may apply in the 100-200 centimeter range if one judges from the latitude data on aboriginal populations — and so another juvenilization episode would again reposition stature on the lower half of its growth curve, ready to re-enlarge again seemingly forever. The ever-more-helpless infant may well have required some prerequisites: the kangaroo’s pouch may suffice for its helpless infant, but we probably adapted with the aid of bipedal locomotion for infant carrying and the two-parent family for provisioning mother and child.

So we are left with whether there were even earlier sexual maturity gene versions around (apparently so, judging from the heritability of early menarche). So what is the juvenilization advantage that comes under selection? Whatever that feature is, it must have had a very long growth curve — where more and more was always better and better — for it to have been used repeatedly during the last 2.5 million years. That’s a big order. What’s so good about juvenilization that has such characteristics?

Reproductive races in boom times might work, if repositioning by slowed somatic development suffices. But it really ought to backslide readily in hard times. I’ve got another candidate for why juvenilization was so useful, one with a spectacular growth curve. It is just what I’d recommend to an ambitious ape, wanting the brain capacity for language.

The scientist [J.B.S.] Haldane, brooding upon the future, has speculated that we will even further prolong our childhood and retard maturity if brain advance continues…. [But ultramodern man has] happened already. Back there in the past, ten thousand years ago. The man of the future, with the [even bigger] brain, and the small teeth…. Those who contend that because of present human cranial size, and the limitations of the human pelvis, man’s brain is no longer capable of further expansion, are mistaken. Cranial capacities of almost a third more than the modern average have been attained among the Boskop people [of southern Africa] and even in rare individuals among other, less [juvenilized] races. Loren Eiseley, The Immense Journey, 1957

Home Page ||


2 thoughts on “Juvenalization Cycle / W.H. Calvin, U of WA

  1. There is also the matter of our vertebraes. We have the same amount whether we are 250 cm tall or 100 cm tall. I am 180 – supposedly 5’11”. That is a lot of body mass to support. Heavier heads would require stronger necks. Stronger necks means more muscle and mass in that region. But that would also require a type of body that could carry that neck. The Neanderthal bodytype seems way more efficient for a big brain than my type – tall and potentially lean.

    Then we have the question of energy. Both my children would have died if all they had to depend on was my milk. There simply was not enough. Just keeping me alive is so much more expensive energy-wise than keeping someone 30cms shorter alive. Bigger brains would mean more energy to them and less to our bodies and we can’t have weaker bodies with bigger brains and heads.

    All of us have big heads in our family, but we certainly aren’t all that smart (except my husband – he is fairly intelligent). Giving birth to those gigantic heads wasn’t all that fun and had long-term consequences for me. So, like the above, big heads+current birth-canals=bad.


    • That’s why I like W. Calvin; he looks at the downside, the costs of being human and what it takes to make the human body work – and that it’s “weird.” No pages and pages of “data” claiming we’re the Masters of the Universe (males, not us!), vastly intelligent and socially perfect, ready to colonize space. Most humans are Asian, SE Asian and small: large people are the minority, and that’s very recent. When the Ancient Romans called the Germans “giants” it was because the Roman male was about 5’4″-6″ and the Germans 5’9″ – The Neanderthal male was about 5’6″ but and insanely strong. N’s were genetically identical to H. sapiens, despite misreporting to the contrary. I think they were the Giants that appear in so much mythology; later people interpreted Giant to mean tall. Modern humans have been shrinking in stature and brain size since the advent of agriculture. Many Americans are enormous, women included (obesity aside) and that’s happened in my lifetime! I suspect it’s the hormones and additives in foods- something environmental, because it happened so fast.

      I remember learning about childbirth and I was literally terrified! My reaction was, How can this be normal? A woman can die having a baby? What’s the point? The baby kills the mother and itself if the head is too big. Something’s wrong here! In other animals, the babies might die, but the mother lives to have more. (Unless it’s a domestic animal that’s been screwed up by humans.) I think modern human childbirth IS abnormal and messed up by human domestication – since agriculture.

      Liked by 1 person

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s