“A chimpanzee’s ability to learn is drastically reduced upon reaching maturity. But baby chimps will eagerly mimic a human caretaker – sticking out their tongues, opening their mouth wide, or making their best effort at a kissy face.” (Geoff, 2009)A newborn creature will spend much time exploring its environment. As it comes to know its surroundings, it no longer has to acquire new information at the same rate. It loses its ability to learn.
We humans are different. We never grow up. As adults, we retain this infant-like mental plasticity, much in the same way as a child’s tolerance for milk persists into adulthood in dairy-farming societies.
Geneticists are now a step closer to understanding this evolutionary change. A team led by David Kingsley of Stanford has shown that ancestral humans lost a key piece of DNA that switches on GADD45G, a gene that stifles growth of brain tissue. In non-humans, this gene regulator slows down the growth of brain tissue:
The GADD45G regulator was active in layers of the brain where cells that ultimately form the cortex are born. Specifically, in mice and chimps, GADD45G suppresses the development of brain regions which in humans are involved in higher cognitive functions like conscious thought and language."
Completely losing GADD45G would be like losing the brakes," says Kingsley. That happens in pituitary tumours when the regulator fails and cells grow without restraint, but in healthy humans the regulatory change would have only decreased activity in specific brain areas, causing them to grow larger. (Coghlan, 2011; see also McLean et al., 2011)
This kind of genetic change may largely explain how the brain progressively expanded in ancestral humans. And this evolution did not stop with the advent of Homo sapiens. Human populations today vary at several gene loci that regulate brain growth: ASPM, MCPH1, CDK5RAP2, and CENPJ. At these loci, the most recent alleles likewise seem to favor brain growth by allowing each cortical column of neurons to expand outward over a longer period of time. Interestingly, the main result does not seem to be higher IQ, but rather some other, still unknown, enhancement of mental capacity (Frost, 2008; Montgomery & Mundy, 2010; Rimol et al., 2010; Wang et al., 2008).
Differences in mental capacity should thus steadily increase from infancy to adulthood. This is an important point. If young children perform equally well on a mental task, it is often assumed that later differences must be due to differences in the learning environment.
References
Coghlan, A. (2011). Key to humanity is in missing DNA, New Scientist, March 9http://www.newscientist.com/article/mg20928033.500-key-to-humanity-is-in-missing-dna.html
Frost, P. (2008). The spread of alphabetical writing may have favored the latest variant of the ASPM gene, Medical Hypotheses, 70, 17-20.
Geoff. (2009). Chimpanzees and Neoteny, March 30.
http://www.gmilburn.ca/2009/03/30/chimpanzees-and-neoteny/
http://www.gmilburn.ca/2009/03/30/chimpanzees-and-neoteny/
McLean, C., Reno, P., Pollen, A., Bassan, A., Capellini, T., Guenther, C., Indjeian, V., Lim, X., Menke, D., Schaar, B., Wenger, A., Bejerano, G., & Kingsley, D. (2011). Human-specific loss of regulatory DNA and the evolution of human-specific traits, Nature, 471 (7337), 216-219 DOI: 10.1038/nature09774
Montgomery, S.H. and N.I. Mundy. (2010). Brain Evolution : Microcephaly genes weigh in, Current Biology, 20(5), R244
Rimol, L.M., I. Agartz, S. Djurovic, A.A. Brown, J.C. Roddey, A.K. Kähler, M. Mattingsdal, L. Athanasiu, A.H. Joyner, N.J. Schork, et al. for the Alzheimer’s Disease Neuroimaging Initiative (2010). Sex-dependent association of common variants of microcephaly genes with brain structure. Proceedings of the National Academy of Science. USA, 107, 384–388.
Wang, J.K., Li, Y., and Su, B. (2008). A common SNP of MCPH1 is associated with cranial volume variation in Chinese population. Human Molecular Genetics, 17, 1329–1335.
12 comments:
Is it true that neoteny was first suggested by Ashley Montagu and Steven J Gould? There is genetic evidence for the retained plasticity theory but I think humans are, as John Manning says, oestrogenized rather than neotenic.
Women are less robust than men and the evolutionary trend is towards less robustness. Women have better verbal intelligence and language is surely a huge part of the human intellectual evolution towards learning and retaining knowledge in adulthood.
Bushmen/Khoisans have infantile physical characteristics to a greater extent than other humans (J.Baker in his weighty tome Race goes into quite a lot of detail about that) Their mental characteristics could be seen as childlike too.
If we are looking at major races the East Asians are the most like children in that they have reduced sexual dimorphism. Europeans do not. Bearing their overall achievements in mind have Europeans lagged behind East Asians. If neotony is the main determinant of mental plasticity and creativity we ought to expect East Asians to be more creative than anyone.
I think that, for all their IQ and mental stamina, East Asians are more rigid in their thinking than Europeans. Europeans are obviously oestrogenized; their colour, face shape and finger ratios leave no doubt about that.
In individuals I would say that there is a tendency for intelligent men to be less masculine.
The neoteny hypothesis of human evolution actually goes back long before Montagu and Gould (not to mention Lorenz). I believe Bolk was the one who first came up with it. It's a good hypothesis as long as you don't go overboard.
The term 'neoteny' may actually cover two distinct phenomena:
Real neoteny - changes in developmental timing to extend the period of mental plasticity and cerebral development.
Apparent neoteny (or infantile mimicry) - imitation of key visual, tactile, and auditory characteristics to elicit care-giving behavior from others and to inhibit their aggressive impulses. This would account for the more child-like facial shape of human adults, especially women, as well as women's higher pitch of voice and softer, smoother, and fairer skin.
I've heard conflicting views (mostly anecdotal) about the aptitudes of Khoisan peoples. In any case, if they were more neotenous than most humans, they would also be more intelligent as adults.
"In individuals I would say that there is a tendency for intelligent men to be less masculine."
I'd wonder if that is true on a purely biological hormonal level, or if it's just that more intelligent men are less 'controlled' by their hormones (or whatever you want to call it) due to their ability to be more controlled by reason. Or if it's a mix of both. The biological element seems true when comparing different population groups (African Americans compared to White Americans compared to Chinese Americans, for instance) but I don't know how true it is within a population.
In individuals I would say that there is a tendency for intelligent men to be less masculine.
The brain is resource intensive as are big muscles and testicles, and I presume they compete with each other for resources. Many intelligent individuals have not undergone purifying selection to create both an efficient brain and an efficient, masculine body.
Since iq has advantages these individuals will make do with the shortcut. Since it is a shortcut, there are more of them, those of developed minds and frail bodies, and hence these less masculine high intelligent persons will be more commonly encountered than other variants.
As well, there is the case that a high iq allows for more clever planning and weaponry in war and conflict, relaxing selection on body structure.
Retention of mental plasticity would be the overall trend in the human lineage. But an complementary or alternative explanation for group differences emerging over time might be puberty. (There are group differences in age of onset.)
"Inuit Greenlanders ... have fewer alleles of the sort that increase androgen receptor activity or facilitate testosterone to DHT conversion."
From what I have read about the Inuit they are the most intelligent of all hunter gatherer peoples
It's a good hypothesis as long as you don't go overboard.
Neoteny seems to explain human variation fairly well, but is a simplification and leaves out elements of the development of human form (for example, the globularisation of the braincase, in which the ancestral form of Neanderthals is shared by human neonates, with humans developing along a different, less neotenous trajectory) which aren't simply the results of neoteny.
I get the impression (not having read any of) Montagu's stuff, that he kind of postulates that all human neoteny in skin, hair and so on has selective value, which I feel is wrong. Rather it seems more likely we are neotenous on such traits simply because the ancestral "adult" traits lacked selective value in the adaptive strategy early humans progressively took on and instead imposed a burden - probably not because they were selected because they "looked" young or as a necessary side effect of selecting for other neotenous characteristic, which are the explanations I seem to get the impression of from what little I have read from Montagu and Gould on the subject.
I think humans are, as John Manning says, oestrogenized rather than neotenic
From what little I can tell online, adult humans have higher testosterone levels and similar levels of estrogens to chimpanzees. However, there may be a) differences in receptor activity that make this irrelevant or b) chimps may have high t levels early in development.
Other bloggers I have read on the net have suggested that creativity is highest in high t individuals, once g is factored in, and I have not seen a within population study that shows IQ to vary inversely with t.
The three greatest scientists - Newton, Einstein and James Clerk Maxwell - show little sigh of being particularly masculine. If anything there is a tendency to schizophrenia spectrum disorders in the most brilliant scientists
"Newton's character and life was one made of long flashes of brilliance and followed by unexplainable eccentric behavior". Newton never married and he spent a lot of time studying the bible in order to predict the future (which was also George Price's obsession).
Maxwell was peculiarly shy (classmates called him 'daftie') and intensely religious, he married a woman several years older (pretty unusual in those days) and never fathered a child.
Einstein was more normal but he fathered a schizophrenic son (as did James Watson).
Some people (John Manning) have suggested that insensitive androgen receptors increase neural speed and that schizophrenia spectrum disorders are connected with the balance of, and sensitivity to, sex hormones.
"In individuals I would say that there is a tendency for intelligent men to be less masculine."
"but I think humans are, as John Manning says, oestrogenized rather than neotenic."
And what about intelligent women ? Are they more feminine, as they ought to be if human intelligence was related to oestrogenization ?
It seems it's not the case, and intelligent women actually tend to be less feminine.
But is reduction of sexual dimorphism actually a case of neoteny, or a case of hyposexualization ? In other terms, are intelligent people of either gender, more "childlike", or more "androgynous" ?
Now, maybe hyposexualized individuals perform better in intellectual work, because their lower sexual urges helps them to keep being concentrated in their tasks ?
"It seems it's not the case, and intelligent women actually tend to be less feminine."
How'd you reach this conclusion, anon?
The discussion is still fascinating, if not entirely OT with Frost's post (to which digression I have contributed, as the anonymous responsible for "Neoteny seems to explain human variation fairly well...").
As to relative androgen and estrogen levels, levels are hard to measure, but using 2D:4D as a proxy it does not appear from cursory googling that more female 2D:4D has any association with overall intelligence within human populations, although the verbal:numerical/spatial pattern is as expected by Tod. More male 2D:4D does appear to have associations with musical ability, at least in terms of playing instruments (possibly high spatial ability and relative verbal disfluency are more likely to lead to an instrumental musical career?).
As to insensitivity to androgen receptors overall contributing to IQ increase, it would be interesting to see if AIS women (XYs with total androgen insensitivity who develop like typical XXs) have higher intelligence than control males. This - http://www.ncbi.nlm.nih.gov/pubmed/2060143 - indicates that AIS women have impaired spatial ability (relative even to XX non-AIS females and their own [possibly partially AIS] female siblings!) but no improvement in verbal ability, make of which what you will. Google does not reveal any associations, positive or negative, between AIS or schizophrenia, other than that cases do exist in which the conditions coincide.
(apologies if this is a repeat, my earlier post appears to have been swallowed)
Androgens are tough to measure, but female 2D:4D, which has only neonatal hormone associations, has no positive associations with academic performance, although the verbal and spatial ability correlations are as expected. Very male 2D:4D appears to have associations with musical ability amongst males.
As to androgen sensitivity and intelligence, one way to look into this would be to look into AIS women (XYs with androgen insensitivity who develop along XX lines). This is the only such study I can see online - http://www.ncbi.nlm.nih.gov/pubmed/2060143 - which suggests that AIS women have impaired spatial ability (even relative to their own female siblings), but no verbal advantage, make of which what you will, particularly given the low sample sizes involved and the fact that control males in the sample showed no verbal performance defecit. As to associations between AIS and schizophrenia, I can find no evidence of any, positive or negative, other than that the conditions can coincide.
Conversely, congenital adrenogenital syndrome which occurs in XX females and high levels of masculinization, appears to have a mixed picture of increased and decreased IQ.
It has been recently suggested that domesticated dogs evolved from adolescent wolves, 'frozen adolescence'.
In the same way, might women have evolved as 'frozen adolesecent' men, 'domesticated' men in fact?
This makes homosexuality an evolutionary intermediate step towards sexual differentiation, and also explains pederasty from a social darwinist perspective - as evolutionary overshoot.
Post a Comment