January 20, 2010 | Leave a Comment
A professor recently wrote me that she introduced the ideas described in my blog to her class on Neanderthals and Human Evolutionary Theory. Her email asked or suggested several questions or expressed her class’s confusion in the following areas:
Are you proposing that testosterone levels are driving evolution of mammals in general or primates specifically?
The evidence that testosterone is driving evolution mostly comes from anomalies emerging in neuropsychology around progeny maturation changes that result from environmental influences upon a pregnant mother and other studies in the neuropsychological literature.
An interesting primate study was as follows…
“In a 5-year longitudinal study, we examined the effect of disrupting the neonatal activity of the pituitary–testicular axis on the sexual development of male rhesus monkeys. Animals in a social group under natural lighting conditions were treated with a GnRH antagonist (antide), antide and androgen, or both vehicles, from birth until 4 months of age. In antide-treated neonates, serum LH and testosterone were near or below the limits of detection throughout the neonatal period. Antide + androgen-treated neonates had subnormal serum LH, but above normal testosterone concentrations during the treatment period. From 6 to 36 months of age, serum LH and testosterone were near or below the limits of detection. Ten of 12 control animals reached puberty during the breeding season of their 4th year, compared with ﬁve of 10 antide- and three of eight antide + androgen-treated animals. Although matriline rank was balanced across treatment groups at birth, a disruption within the social group during year 2 resulted in a marginally lower social ranking of the two treated groups compared with the controls. More high (78%) than low (22%) ranking animals reached puberty during year 4. During the breeding season of that year, serum LH, testosterone and testicular volume were positively correlated with social rank. Thus the lower social rank of treated animals may have contributed to the subnormal numbers of these animals reaching puberty during year 4. However, of those animals achieving puberty during year 4, the pattern of peripubertal changes in serum testosterone and testicular volume differed between control and antide-treated animals. The results appear to suggest that the disruption of normal activity of the neonatal pituitary–testicular axis retarded sexual development, but that social rank is a key regulatory factor in setting the timing of sexual maturation in male rhesus monkeys. The effect of neonatal treatment with antide and low social rank on sexual development could not be reversed by neo-natal exposure to greater than normal concentrations of androgen.” Abstract from Sexual maturation in male rhesus monkeys: importance of neonatal testosterone exposure and social rank by Mann, Akinbami, Gould, Paul and Wallen.
It seems that mammals in general may be so affected, but I have not explored this.
How specifically is testosterone expression selected for?
I see this as largely a social structure question. Any number of fluctuating environmental situations can encourage differing social structures. For example, if a primate society experiences dispersed food sources gathered by females often foraging out of sight of males, then male control of female procreation may be less effective than a promiscuous social structure evidencing lower male testosterone levels and less hierarchical posturing. See “What is Neoteny?”
How do changes in the timing of testosterone influence the evolution of mammalian life history?
This is not a question that I have researched, but only asked. I don’t know.
How has Darwin’s theory of natural selection, inherently based upon interactions with the environment and organisms, biased a current ability to note the effects of the environment upon evolution?
The version of Darwin’s theory of natural selection in widest use today is the Neo-Darwinian interpretation of Darwin’s work, with Dawkins as the most vocal representative. Fern Elsdon-Baker, in The Selfish Genius, describes the rather odd situation that we are in with Darwin’s pluralistic perspective not being the general understanding of how evolution works; instead, it is Wallace’s rather orthodox interpretation (though Wallace believed deity intervened to make our brain). What this largely boils down to is: How random, exactly, is the variation that emerges in progeny?
Neo-Darwinism makes clear that Lamarckian principles are dead and that evolutionary developmental biological ideas (that the environment can compel changes in ontogeny) are a special case. The current theorizing environment does not support the idea that the environment can affect evolution by influencing the parents in their lives to produce progeny with features that reflect the parents’ experience. Darwin discussed this issue at length, providing numerous examples in his The Variation of Animals and Plants under Domestication. This two-volume work was basically a list of anomalies that did not fit his theory of natural selection.
Whereas the environment is noted as important because it decides which features encourage an individual to live long enough to procreate, in most current theory the environment is not noted as important in its ability to influence the kinds of individuals that are created by the parents’ experience. My emphasis is that the environment influences the rate and timing of maturation, adjusting ontogeny, encouraging the emergence of a host of features.
How does your theory contrast with Darwin’s presentation of sexual selection and differential selective pressures between males and females in The Descent of Man?
In 1998 I was reading Darwin’s The Descent of Man, Gould’s Ontogeny and Phylogeny, Campbell’s The Masks of God and Geschwind and Galaburda’s Cerebral Lateralization at the same time. The four books kind of blended in my mind. I was looking for support for the thesis that we evolved within matrifocal societies featuring males with neurological structures similar to the contemporary autistic. I hypothesized that these matrifocal societies were evolving big brains and cooperation by females. In mate selection, females were mostly picking males for being evocative dancers.
After coming up with the idea that human evolution was compelled by dance, following an R. A. Fisher sexual selection feedback loop thesis, I came across Geoffrey Miller’s 1994 work presenting a more detailed exposition of that thesis, except Miller said it was art in general that compelled human evolution. Miller’s 2000 The Mating Mind is the published account of his ideas.
I see no difference between Darwin’s The Descent of Man thesis and my work, except I’m hypothetically providing far more detail on how exactly the dynamic of sexual selection is engaged. Darwin did not yet have endocrinology, neuropsychology or even anthropology. To me, the work of Gould, Campbell, and Geschwind feels like the manifestation in other disciplines of Darwin’s sexual selection thesis. Sexual selection, without an understanding of how social structure informs the direction evolution takes, only provides part of the picture of species transformation. Sexual selection and insight into how neoteny and acceleration compel specific evolutionary trajectories create an opportunity to view the kind of physical, neurological and behavioral transformations that accompany the particular features that the female is choosing. We can use human sexual selection to understand the neurological repercussions of particular social structures, thus creating an opportunity to view not just our species but particular neurological variations within our species as related to social structure and sexual selection.
I think I’m just answering this question in a way that makes more questions. I can send you a more detailed explanation of this thesis if you like, though it’s kind of long.
What is the empirical support for testosterone managing rate and estrogen the timing of maturation, influencing evolution?
This was her last question, the most difficult. This lies at the foundation of most of what I write about. I answered in six pages, citing a number of different studies, but there was no study that even asked these questions. My conclusions are based upon what I infer.
Back in 1998, when a lot of this was coming together, I read a paper cited by Geschwind and Galaburda in Cerebral Lateralization that noted that a mother’s testosterone levels at six weeks before birth determined her child’s maturation rates. I’ve been looking for the paper for almost a year, having noted that I found the paper in Cerebral Lateralization, but I failed to note the specific paper. I recently reread Cerebral Lateralization to find the reference, but I couldn’t find it. Nithya and Rosanna, who have helped me on this project, haven’t found reference to such a paper. Did I dream it?