Paleo-Hokum: The Human Tendency to Build Romantacized Versions of the Past

Conservatives often seem gripped by an almost crippling nostalgia for days gone by, idealizing the 1950s as some kind of wholesome social Eden or arguing that the moral strictures concocted by people living 3000 years ago provide a useful template for a how to live in 2014. Occasionally characterized as hallmarks of the conservative disposition, such willfully romanticized, ferociously uncritical views of the past are products of a type of delusional sentimentality wherein one constructs a largely fictitious picture of history and argues that the present should be structured accordingly. The notion that conservatives have a proclivity toward adopting signally imaginative pictures of history is not entirely unfair. Indeed, the Right’s habit of repeatedly attempting to rewrite public school history and science curriculum to better match their ideological sensitivities has been well documented. The flaw in this perspective has nothing to do with its veracity. Rather, it comes from the notion that it is a trait to unique to conservatives.

Artist's rendering of a Paleolithic hunt.

Artist’s rendering of a Paleolithic hunt.

Personally, I am more sympathetic to the perspective that this tendency to construct and subsequently fetishize incongruous versions of history is a more broadly human characteristic. Take for example the recent “Paleodiet/Paleo Lifestyle” trend. Personal experience suggests that the rank-and-file of the Paleodiet movement consists of left-leaning folks. Unfortunately, such anecdotal evidence makes for a rather wobbly foundation upon which to build broader conclusions, and reliable data on the political demographics of the Paleo movement are hard to come by. So, for the sake of inclusivity, let’s just say for now that the Paleo trend seems to be a load of bullshit just about anyone from anywhere on the political spectrum can get behind. Liberals might find the rhetoric more readily palatable, appealing as it does to their instinctive revulsion regarding all things industrialized, capitalistic, or otherwise offensive to a strong sense of equity. Conservatives – especially those huddled out in the feral, intellectual hinterlands of the Far Right – might be somewhat more likely to find Paleo lifestyles unsettling, given their appeal (both implicit and explicit) to the notion that humans have evolved, or their not-so-subtle suggestion that there was a period of history that could be called Paleolithic. After all, reputable scholars have demonstrated that the world is only 6000 or so years old.

In any event, the exact demographics of the Paleo movement are largely immaterial to my overall point. The core of my argument is that the Paleo diet is based on a nonsensical view of the past, and that such views are not entirely monopolized by people sympathetic to Right Wing ideology – rather, the construction of romanticized versions of the past is a broadly human theme.

To be clear at the outset, I do not take issue with whether or not this trend is actually healthy. Its health consequences are more or less incidental to the core philosophy. Those who experience health benefits probably do so because it is beneficial to cut back on high calorie, low nutrient foods, not because they are eating a diet that more closely resembles the one that humans have “evolved to eat”. And therein lies my primary grievance: it’s not that a Paleo lifestyle is somehow deleterious, it’s that it is based on both a distorted picture of the past and a shoddy understanding of the process of evolution.

The Paleodiet is essentially a hodge-podge of pseudoscience and outright fantasy, concocted more out of imagination than a real understanding of the evolutionary history of modern Homo sapiens. It would hardly be unfair to cast it as a modern manifestation of the primitive Eden Jean-Jacques Rousseau invented in the 18th century. Rousseau argued that humanity resided in a state of simple, gentle savagery, until the disease of avarice tore us from our position of grace. Such a rosy view – especially absent empirical evidence – appropriately warrants a hearty dose of incredulity. Nonetheless, Rousseau’s ideas about humanity’s utopian ancestry have proven immensely influential, particularly in the humanities, where researchers have seized upon his narrative as a reflex against the abject barbarity of European colonialism and the pervasive racism that clouded anthropological and sociological thought throughout the 19th and early 20th centuries. It is hard to see some of the more spurious ethnographic work produced by subsequent generations of scholars as anything less than ideologically motivated reactions to colonialism, racism, and “social Darwinism” (a misnomer among misnomers), fashioned in the image of Rousseau’s initial musings.

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Jean-Jacques Rousseau, 1712-1778

The Paleodiet is heir to this tradition of romanticizing the primitive. As pioneered by Rousseau it is – make no mistake – a liberal tradition. It sets itself the task of vilifying the aspects of modernity some people find uncomfortable in light of their ideological disposition by casting them as “unnatural” (more on that shortly). Humans, the argument goes, are the behavioral and physiological progeny of selective pressures that would have been prevalent during the millennia our ancestors spent as bands of nomadic foragers, scratching a living out of the Pleistocene landscape. Our fall from grace began with plant and animal domestication, spiraling toward the cacophonous dénouement that is a modern world  populated by Big Macs, Hot Pockets, microwave burritos, and white bread.

In a way, the basic claim is a truism. Modern humans are the product of millions of years of evolution. The vast majority of our genetic architecture was in place well before the invention of agriculture. Similarly, the majority of the selective pressures that would have been significant in shaping the suite of behaviors that distinguish us from our primate relatives would have related to the environment(s) hominids inhabited during the five or so million years of the Plio-Pleistocene. During that time, our hominid ancestors were almost certainly foragers, and definitely didn’t have access to Big Macs. Shouldn’t we be adapted to eat a diet of fruit, grass fed meat, and tubers, rather than corn fed, hormone enriched beef product slathered in special sauce, sandwiched between two gluten-rich buns?

Though such a perspective might have some intuitive appeal, it rests on a number of fallacious assumptions. First, it bespeaks a teleological view of the evolutionary process. Humans, according to the Paleo philosophy, evolved to subsist on a diet typical to the average Paleolithic forager. Once we had reached that stage, selection ceased to operate and our evolution came to a halt. For several reasons, this is a clumsy way of looking at the process of evolution. For one, organisms – human included – don’t evolve to do anything. Genetic variation is generated in a manner blind to the challenges that will arbitrate its proliferation or eradication. Additionally, such arguments suggest that evolution has a stopping point – that once creatures have become appropriately adapted to their environment, they cease to evolve. The notion that we – or any other creature – evolved toward some end point and stopped once we got there is simply wrong. Some might argue that this is an overly simplistic caricature of the Paleo movement’s core arguments. The more sophisticated Paleo adherents probably don’t think humans have ceased to evolve. Instead, they recognize that evolution typically occurs gradually, so humans haven’t had time to adapt to a post-agricultural, and – more importantly – post-industrial diet.

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Hieroglyphic depiction of agricultural practices.

In a sense, this is true, but it exposes more unfounded assumptions. Most evolutionary change is a product of the gradual accumulation of advantageous mutations. Populations evolve by slow, subtle steps, so the notion that humans might not have experienced a lot of evolutionary change since the advent of agriculture, some 10000 to 12000 years ago, is not completely unreasonable. However, there is evidence that some human populations have experienced non-negligible genetic change since the Neolithic Revolution (i.e. the widespread adoption of agriculture as a primary means of subsistence). Adult lactase persistence, for instance, is associated with a pair of single-nucleotide polymorphisms that arose – or were at least selected-for – after certain human populations began to consistently engage in ungulate husbandry. The ability some people have to metabolize milk into adulthood is a product of recent evolutionary change, probably due to the fitness gains associated with prolonged access to a new source of caloric energy. So while evolutionary change tends to occur quite slowly, it can still happen rapidly enough to make us better suited to modern diets than the staunchest Paleo advocates would have us believe.

Consider also that plant and animal domestication did not occur as suddenly as is popularly conceived. The long dance of coevolution that is domestication began long before the Neolithic Revolution, as humans began to interact with their ecological neighbors in more and more complex ways. The apparent suddenness with which agriculture became ubiquitous in places like Mesopotamia and Mesoamerica is a result of a cascade of innovations at the tail end of a longer process of give and take. Prior to building irrigation systems and tilling fields, humans were likely setting up camp near useful perennials and annuals, actively encouraging their growth through more subtle types of landscape modification. Later, individuals began to engage in broadcast sewing, actively distributing the seeds of useful plants around productive and accessible stretches of land. Modern wheat, corn, cows, and chickens may be recent innovations, but we’ve been eating their ancestors for centuries.

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Furthermore, the very notion that humans are specifically adapted to a certain diet is fallacious on at least two fronts. First, it ignores the extraordinary range of phenotypic plasticity Homo sapiens displays. We are a species marked by a remarkable range of behavioral and physiological flexibility. To suggest we are adapted, even in the broadest possible sense, to a particularly well bounded lifestyle ignores one of the primary components of our nature. Paleoecological evidence indicates the Plio-Pleistocene was a time of considerable environmental variability. Under such circumstances, a rigid diet regime would have been hard to maintain. Indeed, the overall trend of hominid dietary evolution seems to be one of increasing generality, with successive generations becoming better and better suited to eating a wider and wider variety of plants and animals.

A second, but not unrelated, point is that we do not have a perfect picture of what prehistoric diets really consisted of. Even if humans were adapted to eating a certain range of forager staples, exactly what those staples were remains cloudy. More than likely, our prehistoric diet was frequently dictated more by what we could physically capture and metabolize, rather than some idealized set of nutritional guidelines. Basic behavioral ecology would suggest most species, including humans, will preferentially target those food items that yield the largest caloric returns relative to costs associated with capture and/or collection. That means – and both archaeological and ethnographic evidence bears this out – that what people ate likely varied from ecosystem to ecosystem. Homo sapiens was a more or less global species before the dawn of institutionalized agriculture. Different populations occupying a wide range of environments seemed to get by just fine by exploiting an expansive variety of food items. In the high Arctic forager populations subsisted (and still do) on a diet massively biased toward animal protein, while groups living in the verdant tropics incorporated (and still do) myriad fruits and nuts, in addition to a greater diversity of animal protein, into their diet. Given this apparent versatility, the argument that there is one diet humans are designed to eat looks somewhat less than convincing.

Paleo Lifestyle rhetoric is littered with appeals to nature, as if we’ve somehow lost our way and become something abhorrent before the eyes the All Knowing Universe. Advocates of this lifestyle have set up an arbitrary demarcation between natural and unnatural, as if the innovations associated with agriculture mark a frontier beyond which humans suddenly began to behave “unnaturally”. This sets up a false dichotomy between the types of behavior we engage in today and the types of behavior our ancestors engaged in the largely invisible past. Eating Hot-Pockets and watching You Tube videos may accurately be considered behavioral novelties in the broad scope of human evolution. But it is not reasonable to infer from said novelty that such behaviors are somehow unnatural.

On a purely philosophical level, the very idea of unnatural behavior subverts the Paleo advocate’s attempts to find an evidentiary basis for their dietary choices, implying as it does a certain level of mysticism. Methodological naturalism – the primary scaffolding around which all scientific research is constructed – denies the very possibility of anything “unnatural” occurring. How could it? Nothing can or ever will transpire in violation of what we might loosely call the Laws of Nature. To suggest otherwise is a direct invocation of the supernatural or at the very least a crude argument for some kind of vitalism, both of which strain scientific credulity and provide sufficient room for motivated advocates to weasel around unwelcome dispositive evidence. Either way, it boils down to a pile of fluffy nonsense.

Brass tacks, the Paleo Lifestyle is based on a romanticized version of the past. Like the conservative fantasies of 1950s suburban utopia or Wild West individualism, it is a canvas onto which people project their dissatisfaction with the present, crying, “if only things were thus…” Perhaps such fantasies provide a star by which people reckon their course, assuaging their fears that there might not be a right way to live. After all, many people probably find the idea that humanity has lost its way more comforting than the idea that it never really had one to begin with.

Human phenotypic plasticity – how we succeed wherever we find ourselves

Increasingly, researchers are beginning to argue that environmental heterogeneity and long-term ecological stochasticity have sculpted a remarkable breadth of phenotypic plasticity as the primary means by which humans have succeeded in replicating their genetic material1,2. Variation in phenotypic response, it is argued, represents an adaptive solution to the problem of environmental unpredictability. Whether or not this is so is a matter to be arbitrated by empirical results – that Homo sapiens have managed to find a place in nearly every ecological niche on the planet, from the freezing Canadian Arctic to the island tropics of the South Pacific, is highly suggestive in this regard. To some degree this phenotypic flexibility is observable in physiological responses like shifting adiposity and skin tone3, though it is likely the expansive behavioral repertoire of humans provides the largest range of adaptive flexibility4. Humans cultivate social networks that ease energy flows and mitigate the reproductive costs experienced by females, who typically bear the brunt of the child-bearing load,  accumulate and modify trans-generational stores of information, use and invent tools to access previously unattainable resources, and play an active role in constructing the niches they inhabit5,6. Of course, genetic variation stills plays a role in human evolution, as illustrated by the changes that facilitate adult lactase persistence in populations with long histories of animal husbandry7. Yet it is becoming more and more evident that the primary aspect of human variation responsible for our adaptive diversity is rooted in the dynamic interplay among genetic, cultural, and ecological systems of inheritance.

Once thought to be something of an evolutionary novelty, phenotypic plasticity is now understood to be extremely common and widespread. In environments characterized by consistent spatial and/or temporal heterogeneity, phenotypic plasticity can evolve as an adaptive response to variation in selection pressures8,9. However, there is some debate concerning what exactly constitutes adaptive phenotypic plasticity and what selective pressures drive its evolution. Additionally, the apparent utility of plasticity in securing positive fitness outcomes across variable environments raises the question of why it is not a ubiquitous feature of all organisms across all environments. Adaptive plasticity can be identified where a single genotype expresses a range of phenotypes across a range of environments, provided that the range of phenotypes is shown to be heritable8. This, of course, leads a small definitional problem: establishing the boundary at which a single genotype begins and another ends is an arbitrary matter. At this point, it is unclear how well certain aspects of human variation meets this particular criterion. Phenotypic traits like adiposity and skin pigmentation show some level of heritability10,11,12 (not a qualitative assertion – there is no reason to assume this should have sociological consequences, because race, as it is popularly construed, is a social construct). If this is so, these traits would be removed from the running as plastic responses in the strictest sense, since it is a rigid – rather than a plastic – response that is inherited. However, if we take the entire human genotype and the range of phenotype it produces the fact that only 6% of identified genetic variation is between sub-groups seems to indicate that, as a species, humans are pretty damn plastic after all13. Again, we return to the problem of definition: what counts as a single genotype?

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Barsh, G.S. 2003

Selecting for Plasticity

Phenotypes are said to be plastic when a single genotype is seen to express multiple phenotypic states (reaction norms) across heterogeneous environments due to cues associated with the variable biotic and abiotic inputs present in said environments9. While one might be intuitively inclined to assume that this plasticity of phenotypic expression is a consequence of the pressures associated with environmental heterogeneity selecting for trait plasticity, this might not necessarily be the case. The observed plasticity could be a byproduct of some other trait, itself not directly connected to fitness outcomes8,14. For the phenotypic plasticity to be considered an adaptive trait, it must be heritable and consistently associated with positive fitness outcomes. Consequently, adaptively plastic traits should evolve in environments characterized by long-term regularity in heterogeneity. That is, relative to the lifetime of any individual, the environment(s) experienced should exhibit some degree of variability or stochasticity, but with respect to the lifetimes of multiple generations of the same organism, the heterogeneity must be encountered consistently.  Conceptually, adaptive phenotypic plasticity is the product of the same forces that produce adaptation in any other trait, with the addition of multi-generational stability in environmental heterogeneity as a condition defining selection for adaptive plasticity.

The selective conditions that produce adaptive plasticity are those that consistently (over multiple generations) sort phenotypes (and the underlying genotypes) in such a way that plastic phenotypes show a higher fitness than alternatives. Put more simply, environments where adaptive challenges vary over differing temporal and spatial scales select for phenotypic plasticity. If this is so, we arrive at the question of whether or not the environments in which humans evolved meet these criteria. Growing evidence from paleo-climatological, archaeological, and paleontological research indicates that the environments experienced by our hominid ancestors throughout much of the Plio-Pleistocene – roughly the last 5 million years – were characterized by just the sort of climatic variability that would have made the more malleable bipedal apes the more fit1,2,3,15. So, with regard to the proper evolution history – statistically significant pressures associated with a fluctuating climate – humans seem to measure up the strictures associated with the evolution of phenotypic plasticity.

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Donges, J.F. et al. 2011 (24)

Demonstrating Plasticity

Though sometimes difficult to construct, experimental demonstrations of adaptive plasticity have been carried out. For example, density-dependent variation in stem elongation in response to light cues in the plant species Impatiens capensis16 and changes in morphology associated with the absence of predation cues in the fish species Poecilia reticulate17 have been put forth as experimentally supported demonstrations of adaptive phenotypic plasticity. The existence – or absence – of adaptive phenotypic plasticity is often examined through the use of reciprocal transplant (also known as common garden) experiments. In such an experiment, an organism from one habitat is placed in a different habitat in order to identify whether or not it responds plastically to the new conditions. If a plant has a plastic response to light cues, it should express different phenotypes in environments characterized by differing amounts of light exposure. Ideally, the variable of interest should be tightly controlled.

For human subjects, the ability to perform manipulative experiments like reciprocal transplantation is limited by practical and ethical constraints. Researchers can’t pluck a baby from a Boston hospital nursery and swap it with a child from the highlands of Papua New Guinea to see how they react. Fortunately, the range of habitats successfully colonized by human populations makes for an excellent natural experiment, allowing researchers to apply comparative methods to examine the relationship between phenotypic expression and environmental context. That humans not only inhabit, but in fact thrive in an immense range of habitats despite sharing the majority of their genes is good evidence that humans are adaptively plastic animals. This still leaves open the question of what governs of plastic responses. Multiple mechanisms of heritability have been proposed to account for adaptive plasticity, including allelic sensitivity, regulatory loci, and specific plasticity genes14,18,  but the question of what genetic mechanisms ultimately underlie adaptive plasticity remain ultimately unresolved. For humans, it seems that the genetic architecture in question likely relates to our cognitive faculties – that is, the ability to search a landscape over the course of several generations and find adaptive solutions that are stored in an extra-genetic informational repository. That is, humans are species characterized by both genetic and cumulative cultural evolution.

Why Aren’t All Organisms Adaptively Plastic?

If organisms can evolve adaptively plastic traits in response to heterogeneous environments and nearly all environments exhibit heterogeneity on some spatial or temporal scale, this begs the question of why all organism do not exhibit plasticity in all traits18. Multiple answers can be found in the relative inflexibility of net fitness reaction norms (the most frequent response to a given environment)19, due in part to the frequently continuous nature of environmental variation, as well as the functional inter-dependence of phenotypic traits. First, the fact that selection ubiquitously acts to optimize fitness in a given environment constrains variation in fitness18. Since environments experienced by organisms tend to vary incrementally from one extreme to another, plastic responses can only be adaptive in the subset of environments where their performance is superior to alternative responses and their heritability is high. That is, a plastic response can only exhibit higher heritability and fitness than alternatives under certain conditions. In a transitional environment, the heritability of the response will be low, while in an environment distinctly different from than in which the response is beneficial, heritability for that trait will be non-existent, minimizing or completely eradicating its net fitness advantage19,20. Additionally, traits often exhibit some degree of inter-dependence. As a consequence, a plastic response that might otherwise be positive could be rendered deleterious as a result of associated changes in other important traits14. To some degree, adaptive plasticity may depend upon the functional independence of the trait in question, or at least on the relatively low impact correlated traits have on fitness outcomes.

Further limitations on the evolution of adaptive phenotypic plasticity can be found in the potential costs and limits imposed on plasticity by the environment in which the organism lives and the physical-chemical constraints levied by aspects of the organism’s physiology. Though their ultimate effect is the same (restricting the evolution and expression of adaptive plasticity) costs and limits differ in their functional mechanics21. Costs operate by decreasing the fitness of phenotypes, while limits prevent the development of a plastic phenotypic response in the first place. Maintenance costs, for example, detract from fitness in instances where the sensory and/or regulatory mechanisms upon which a plastic response depends are too energetically expensive. Lag time, on the other hand, imposes a limit on the expression of plasticity when the phenotype is unable to respond to environmental cues in a timely manner18,22. Since they have not been fully measured experimentally, the costs and limits that have been proposed as restrictions on phenotypic plasticity remain largely heuristic devices.

Conclusion

Given a prolonged association between the plasticity of response and improved fitness in consistently heterogeneous environments, phenotypic plasticity can be adaptive. Human evolutionary history seems to present just such a scenario, with frequent climatic oscillations selecting for a phenotypically plastic animal. Following our dispersal out of Africa, the challenges posed by migration between and colonization of differing habitats would have further selected for a malleable behavioral repertoire. However, the evolution of phenotypic plasticity is often inhibited by the inflexibility of mean fitness reaction norms, along with the range of possible costs and limits imposed by environmental and physiological constraints. Potentially fruitful avenues of research lie in identifying the specific heritable (genetic) mechanisms underlying adaptive phenotypic plasticity and measuring the relative importance of the various costs and limits that have been proposed as mechanisms preventing adaptive plasticity from being characteristic of all traits. With regard to humans, a complete explanation will be dependent on an understanding of the interaction between both genetic and cultural systems of informational inheritance23. Though the tractability (or lack thereof) of these questions can be discouraging, a full understanding of adaptive phenotypic plasticity depends on their resolution.

 

Works Cited:

 

  1. Wells, Jonathan C. K. 2012. Ecological Volatility and Human Evolution: A Novel Perspective on Life History and Reproductive Strategy. Evolutionary Anthropology 21:277
  2. Potts, Richard. 1998. Variability Selection in Hominid Evolution. Evolutionary Anthropology 7:81-96.
  3. Wells, Jonathan C. K. 2012. The Capital Economy in Hominin Evolution: How Adipose Tissue and Social Relationships Confer Phenotypic Flexibility in Stochastic Environments. Current Anthropology. 53(6): 466-478
  4. Sterelny, Kim2003. Thought in a Hostile World: The Evolution of Human Cognition. Malden, MA: Blackwell Publishing
  5. Odling-Smee, John. 2010. Niche Inheritance. In Evolution: The Extended Synthesis. Massimo Pigliucci and Gerd B. Muller eds. Pp. 195-207. Cambridge, Massachusetts: MIT Press.
  6. Laland, Kevin N. and Gillian R. Brown. 2006. Niche Construction, Human Behavior, and the Adaptive-Lag Hypothesis. Evolutionary Anthropology. 15:95-104
  7. Gerbault, Pascale, AnkeLiebert, Yuval Itan, Adam Powell, Mathias Currat, Joachim Burger, Dallas M. Shallow, and Mark G. Thomas. 2011. Evolution of lactase persistence: an example of human niche construction. Philosophical Transactions of the Royal Society B. 366:863-877.
  8. Pigliucci, M. 2010. Phenotypic Plasticity. Pp. 355-378. inEvolution: The Extended Synthesis. [M. Pigliucci and G.B. Muller, eds]. MIT Press, Cambridge, MA
  9. Agrawal, A. A. 2001. Phenotypic plasticity in the Interaction and Evolution of Species. Science. 294:321-326
  10. Maes, H.H.M., M. C. Neale, & L.J. Eaves. 1997. Genetic and Environmental Factors in Relative Body Weight and Human Adiposity. Behavior Genetic. 27(4): 325-351
  11. Barsh, G.S. 2003. What Controls Variation in Human Skin Color? PLoS Biol 1(1): e27. doi: 10.1371/journal.pbio.0000027
  12. Frazer, K. A., S. S. Murray, N.J. Schork, & E.J. Topol. 2009. Human genetic variation and its contribution to complex traits. Nature. 10: 241-251
  13. http://www.aaanet.org/stmts/racepp.htm
  14. Via, S. 1994. The evolution of phenotypic plasticity: what do we really know? Pp. 35-57 in L.A. Real (ed.), Ecological Genetics. Princeton University Press, Princeton, NJ.
  15. Trauth, M. H., J. C. Larrosoana, & M. Mudelsee. 2009. Trends, rhythms, and events in Plio-Pleistocene African climate. Quaternary Science Reviews. 28:399-411
  16. Dudley, S.A., and J. Schmitt. 1996. Testing the adaptive plasticity hypothesis: density-dependent selection on manipulated stem length in Impatiens capensis. American Naturalist 147(3):445-465.
  17. Torres-Dowdall, J., C. A. Handelsman, D. N. Reznick, and C. K. Ghalambor. 2012. Local adaptation and the evolution of phenotypic plasticity in Trinidadian guppies (Poecilia reticulate). Evolution 66(11):3432-3443
  18. Pigliucci, M. 2001. Phenotypic plasticity. Pp. 58-69 (chapter 5) inEvolutionary ecology: concepts and case studies [C.W. Fox, D.A. Roff, and D.J. Fairbairn, eds.]. Oxford University Press, Oxford, UK.
  19. Davidson, A.M., M. Jennions, & A. B. Nicotra. 2011. Do invasive species show higher phenotypic plasticity than native species and, if so, is it adaptive? A meta-analysis. Ecology Letters 14:419-431
  20. Ghalambor, C.K., J.K. McKay, S.P. Carroll, & D.N. Reznick. 2007. Adaptive versus non-adaptive phenotypic plasticity and the potential for contemporary adaptation in new environments. Functional Ecology 21:394-407
  21. Pigliucci, M. 2005. Evolution of phenotypic plasticity: where are we going now? Trends in Ecology and Evolution 20(9):481-486
  22. Auld, J.R., A. A. Agrawal, and R. A. Relyea. 2010 Re-evaluating the costs and limits of adaptive phenotypic plasticity. Proceedings of the Royal Society B. 277:503-511
  23. Richerson, P. J. & R. Boyd. 1999. Built for Speed: Pleistocene Climate Variation and the Origin of Human Culture. Perspectives in Ethology 13:1-45
  24. Jonathan F. Donges, Reik V. Donner, Martin H. Trauth, Norbert Marwan, Hans-Joachim Schellnhuber, and Jürgen Kurths. 2011. Nonlinear detection of paleoclimate-variability transitions possibly related to human evolution. Proceedings of the National Academy of Science. Early edition http://www.pnas.org/lookup/suppl/
    doi:10.1073/pnas.1117052108/-/DCSupplemental