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MysteryTropical forest biochemistry, the driving force in human evolution.
The evolution of the large human brain remains one of biology’s greatest unsolvedmysteries. Primates generally have a relatively large brain to body ratio, apes, theextinct hominids and particularly humans have taken this trait to extreme. No theoryto date has come close to explaining this phenomena.
Some of the key elements
The relatively large brain to body ratio exhibited by primates generally The continued expansion of the brain from apes through the extinct hominids tohumans.
The rapid and accelerating expansion of the brain in the later phase of humanevolution.
The abrupt stall in neural expansion c 200,000 years ago and the subsequentshrinkage.
Associated traits that have also proved difficult to explain, i.e. retention of juvenilecharacteristics, fertility/menstruation, nakedness, gut morphology, fuel hungry brain,dentation etc.
Attempts to explain the large brain and other unusual traits have centred on adaptiveselection i.e. identifying environmental/social pressures that may have led to suchunusual physiological adaptations. The savannah was virtually accepted as the environment that must have driven thesetraits though it could not account for what was already a significant mystery i.e. therelatively large ape brain that had already evolved in the forest. However withoutsolving that unique problem a totally new one was invented, how to explain the extralarge brain of the hominids. That two such clearly related and unusual phenomenawould have two utterly distinct causes seems unlikely. The savannah model has in recent times been significantly discounted, subsequentpollen analysis at famous ‘savannah’ hominid fossil sites has clearly indicated that thehabitats were wooded or forested. In addition Dr Michael Crawford a biochemist fromthe Institute of Brain Chemistry and Human Nutrition at the University of NorthLondon has pointed out that there are insufficient fatty acids (specifically DHA, thebrain is composed of 80% DHA) available on the savannah to grow a large brain. Dr Crawford and others support a coastal dwelling scenario whereby human ancestorsmoved from the forest to live by the coast as this environment provides an abundantsupply of DHA. The implication being that the expansion of the brain was limited bythe lack of a ready supply of DHA or the rate the body can convert omega 3 fattyacids to DHA and simply providing a ready source was all that was required to inducerapid expansion.
In recent correspondence with Dr Crawford I pointed out that an ape can grow a c200cubic centimetre brain in around 8 months primarily by converting omega 3 fattyacids available in a forest diet to DHA. By simply extending the rapid growth windows a much larger brain is feasible via omega 3 conversion without recourse toadditional ready made DHA. Human brain growth occurs over a much longer periodthan apes. Longer gestation and a very significant period of postnatal brain growth isa unique feature of human development. Breast milk contains significant amounts ofDHA depending on the fatty acid composition of the diet being eaten and can bedrawn from reserves laid down before pregnancy. Extending the breastfeeding periodmay have been an essential part of the neural expansion formula ironing out the highdemand for DHA during early development.
Another contender is the aquatic ape theory, it suggests many human traits could beexplained by a long period of living in a semi-aquatic environment. No mechanism isproposed to explain how the association was responsible for producing such a largebrain.
The reality is that no coherent model exists and there is no consensus to explain theunique features of the primate brain particularly the apes, extinct hominids andhumans.
Perhaps it is reasonable to consider that evolution of such rare traits may haverequired a novel mechanism to produce them. Genetic mutation and selectiveadaptation seems to account very well for virtually all traits in all organisms. In thecase of primate/human evolution a subtle variation may have been at work alongsideclassic selection. One well known anthropologist, Dr Colin Groves has suggested that the large brainmay be a fortuitous consequence of neoteny (retention of juvenile features). Thiscontradicts previous ideas where neoteny has been presumed to be a consequence ofthe expanding brain. In recent correspondence he stands by his proposal though he hasnot proposed a mechanism to account for this.
A common factor central to juvenility and brain development is our own endocrinesystem and the hormones it produces, particularly the sex steroids. They play a majorrole in governing windows of development and directly influence the structuraldevelopment of the brain. In addition steroids are directly involved in the transcriptionof DNA, they are part of the reading mechanism dictating how the code is translatedinto bio-chemical structure. Anything that alters the action of steroids will inevitablyalter all of the above.
I have proposed that the powerful steroid modulating chemicals that are abundant in atypical primate diet were responsible for modifying the growth and development ofthe brain and the window that such growth and development occurs.
The chemicals in question are particularly rich in fruit and flowers and typicallyinhibit the activity of sex steroids such as testosterone and estrogens. In addition theyhave mild to moderate neuroactive properties (Monoamine oxidase inhibitors).
Flavonoids are increasingly the subject of research in part because they demonstratesuch powerful endocrine altering properties.
Any animal consuming these chemicals in quantity will be affected. A tropical forest environment has the capacity to provide these chemicals 24/7 forevolutionary time scales. In effect eating a diet rich in fruit/flowers significantly alters your endocrine system.
This creates a blanket effect, all aspects of growth development and physiology willbe modified though any part of the physiology that is particularly steroid sensitivewill exhibit the most significant response i.e. developmental windows, developingneural tissue, fertility cycles etc.
This almost certainly happened when proto-primates began to eat flowers and thenfruit as well as leaves etc. The potential to extend the juvenile phase by inhibiting sexsteroids and in turn allow a longer period of brain growth is perhaps the most obviouseffect.
These effects have never been considered in an evolutionary context. Rather thantrying to explain brain expansion from an adaptive perspective in regard to singletraits it becomes possible to see the brain and other features as a fortuitous by productof a biochemically modified endocrine system. (I have proposed one further step in regard to hominid/human evolution. Givensufficient variation in the effects of flavonoids on the developing neuroendocrinesystem it seems plausible that in some instances the modified endocrine system itselfbegins to add a layer of steroid inhibition. For example, elevating the activity of thepineal gland produces more melatonin and pinoline, both powerful steroid inhibitors.
In such a scenario the scene is set for a classic runaway feedback loop, more steroidinhibition further expansion of the brain and modification of the endocrine systemequals more blanket steroid inhibition etc. As these effects are not locked into theDNA in an adaptive sense they are potentially unstable, lose any part of the positivefeedback loop i.e. the tropical forest flavonoids and it will stall. There is someevidence for such a scenario.) There are undoubtedly a number of variables to consider The genetic predisposition/sensitivity of any given primate lineage.
The variable % of fruit/flowers in a given dietary specialisation.
The variable outcome in any combination of above.
While these and other factors need to be considered the overall effect of these plantchemicals is not in doubt. Their power is sufficient that detrimental as well asbeneficial effects may well have initially occurred. However it is entirely plausiblethat any primates that significantly specialised in fruit/flowers would exhibit thegreatest effects. The question that needs to be addressed now is how can our neuroendocrine systempossibly function without a complex cocktail of powerful steroid modifying chemicalsthat were permanently present during 70 million years of evolution? Aside from proposing that plant chemicals initiated and drove the structural/functionalevolution of our brain I have also proposed that the loss of these chemicals left our uninhibited endocrine system unable to provide an appropriate hormonal environmentfor our brain to develop. Once our connection with the forest was lost our brainstopped expanding and now fails to develop it full function. Due to archaic specialisation between the cerebral hemispheres I have proposed thatthe effects of the loss of these chemicals is lateralised one side being more affectedthan the other. Cerebral dominance and handedness etc are symptoms of thiscondition.
Significant evidence is emerging to support this scenario, Professor Alan Snyder(Director, Centre for the Mind, Australia) Dr Darold Treffert (University ofWisconsin Medical School) and Professor Vilayanur Ramachandran (Director of theCentre for Brain and Cognition) amongst others have increasingly highlighted asomewhat perplexing scenario. The dominant side of our brain is considerably lessfunctional than the non-dominant side, the emerging data is still considered within theframework of adaptive selection i.e. there must be an evolutionary reason for thephenomena. Shamanic techniques (i.e. sleep deprivation) and ethnobotanical use of plantchemicals were an attempt to address the emerging condition. For example thewidespread use of plant DMT combined with MAO inhibitors was simply a crudeattempt to ameliorate a progressive reduction in the production of neural DMT andpinoline in the brain. These and other deficiencies emerged as the human neuro-endocrine system struggled to function normally once the plant hormones were lost. Flavonoids are extremely potent endocrine modulators.
They were an integral part of our endocrine system for tens of millions of years, theirimpact on our general health is only just beginning to be researched. Their effects ongrowth and development in an evolutionary perspective have not been considered. Flavonoids powerfully inhibit the activity of steroids
Flavonoids powerfully inhibit the conversion of steroids (androgens to estrogens)
Flavonoids inhibit monoamine oxidase increasing pineal production of melatonin
Melatonin powerfully inhibits the activity of steroids.
Steroids are central in all aspects of development growth and function. Neuraldevelopment in the uterus is particularly sensitive to steroid activity as are steroidgoverned developmental windows i.e. puberty.
An increasingly specialised fruit diet rich in flavonoids would it seems explain manyof the mysteries surrounding human evolution. The gross nutritional aspects are ofsome relevance i.e. larger fuel hungry brain requiring an ever-greater quantity ofsimple sugars, however it is the hormonal effects that have thus far been ignored. Once equipped with an increasingly large brain and the intelligence it conveys it isfeasible to survive in a range of hostile habitats. No doubt repeated waves of forestmigrants did just that and survived and adapted as distinct species on the savannah orin temperate climates. The orthodox assumption is that the expansion of the brainmust have been driven by selective adaptation in relatively hostile or challengingenvironments. Is there any evidence that the brain continued to expand in theseenvironments or was it the relatively benign tropical forest and its complex hormonemodifying biochemistry that played an essential part in the brain expansion formula?
Developing World Bioethics ISSN 1471-8731 (print); 1471-8847 (online) Volume 4 Number 1 2004 PATENTS AND ACCESS TO DRUGS IN DEVELOPING COUNTRIES: AN ETHICAL ANALYSIS More than a third of the world’s population has no access to essentialdrugs. More than half of this group of people live in the poorest regionsof Africa and Asia. Several factors determine the accessibility of drugs indeveloping