Transgenerational Developmental Plasticity
An Inclusive Theory for Evolution of the Human Cognition
Literature Thesis Research Master Brain and Cognitive Sciences Author: Lynnet Frijling Student number: 10020012 Supervisor: Annemie Ploeger Date 20-08-2018
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Abstract
Directed by the modern synthesis, human cognitive evolution is often explained as the formation of cognitive modules adapted to stone age conditions. However, over the last thousands of years human cognition has evolved fast and adapted to the modern-day conditions. The modern synthesis does not provide an adequate explanation for this fast and flexible evolution. The extended
evolutionary synthesis includes multiple systems of inheritance, but a sufficient theoretical framework on how these systems could lead to accelerated evolution of the human mind is still missing. In this paper transgenerational developmental plasticity (TDP) is explored as a system that could account for a more flexible evolution of cognition. TDP leans on two exogenetic pillars, the epigenetic pillar and the behavioural/neuronal pillar. Both pillars need to be present and cooperate for the TDP system to function effectively. TDP has not proven to be a uniquely human system, but this paper will suggest that it could be most optimal in humans and could be especially applicable to brain and cognition.
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Index
Abstract ... 1
A Need for an Inclusive Theory for the Evolution of Human Cognition ... 3
A Stone Age Mind in a Modern Day ... 3
The Limitations of the Modern Synthesis ... 3
The Extended Evolutionary Synthesis ... 4
4E Cognition and Transgenerational Developmental Plasticity ... 6
Open Questions and Directions... 7
Transgenerational Developmental Plasticity: (How) Would it Work? ... 8
The Two Exogenetic Pillars of TDP ... 8
Epigenetic Inheritance; the Rise of Non-Coding RNAs ... 8
Behavioural/Neuronal Exogenetic Inheritance ... 9
Cooperation of Both Pillars as the Foundation of TDP ... 11
Discussing TDP and Evoked Questions ... 13
Transgenerational or Across Generational? ... 13
Is TDP a Uniquely Human System? ... 14
Does TDP Conflict with the Modern Synthesis? ... 14
Directions for the Future ... 15
Concluding Remarks on TDP... 17
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A Need for an Inclusive Theory for the Evolution of Human Cognition
A Stone Age Mind in a Modern Day
Over 150 years have passed since Charles Darwin first proposed his ideas on evolutionary biology and the origin of species. Over this time, our ideas and views on adaptation, natural selection and
evolution have evolved more rapidly and comprehensively than any species could evolve by natural selection over evolutionary time. Herein lies a hurdle the theory of evolution must overtake: the premise of evolution is that it is the product of adaptations caused by natural selection over generations. During several hundred millions of years, humans and their ancestors have adapted to Pleistocene conditions (Cosmides and Tooby, 1997). These adaptations include the formation of the human brain and its subsequent behavioural patterns. In the last few thousand years, human culture started to develop rapidly, from agriculture all the way up to our present-day society (Barkow, Cosmides and Tooby, 1992). On an evolutionary time scale a few thousand years is not nearly enough time for our brain and behaviour adapt by natural selection and be fit for the complexity of modern conditions. Therefor Barkow, Cosmides and Tooby (1992) argued our brain and behaviour has adapted to hunter-gatherer conditions, thereby shaping a ‘stone age mind’.
In evolutionary psychology, it is often claimed that the human mind is adapted to have universal psychological mechanisms that solve adaptive problems as encountered by
hunter-gatherers(Cosmides and Tooby, 1997). Barkow, Cosmides and Tooby (1992) compiled evidence for the existence of these mechanisms found in for instance social behaviour and mating behaviour. It is argued that these mechanisms are universal for all cultures, as their stone age origin is the same, but might be implemented differently within or between different cultures. For example, social exchange behaviour is observed in a wide variety of cultures over the world, but what goods are exchanged, and which set of decision rules are applied may vary.
It seems paradoxical that our behavioural patterns are not adapted for our modern-day conditions, yet it is our own brain and behaviour that gave rise to our culture and environment. Considering this, the theory of the stone age mind does not seem to cover the full extent of the human cognition, particularly not its fast evolution. In this paper we will recapitulate how the evolutionary theory as proposed by Darwin extended with the rise of new scientific insights. Consequently, the need for an inclusive theory for the evolution of human cognition will be explored. A comprehensive evolutionary theory could offer explanations for the exponential development of the human brain and cognition.
The Limitations of the Modern Synthesis
The segmentation of the mind into distinct psychological and behavioural mechanisms that tackle an adaptive behavioural issue, fits into the modern synthesis as an evolutionary theory. The modern synthesis, in which Darwinian and Mendelian insights are combined to form a modern evolutionary model, indicates the genome as the source of heritability, mutations as the source of variation and natural selection as the source of adaptation. According to the modern synthesis, mutations in the genome may have led to slight differences in brain and behaviour, which in turn could be naturally selected over millions of years, creating adaptations that were advantageous in the Pleistocene. It is a synthesis used at the base of the Santa Barbara school of Evolutionary Psychology, which explains the mind as a set of information processing modules, shaped by natural section to solve adaptive problems of our hunter-gatherer ancestors (Cosmides and Tooby, 1997).
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Even though the modern synthesis provides elegant explanations for the formation of simple and complex features of the (human) anatomy and physiology, it does not fully explain the depths of human cognition and behaviour, which is often deeply imbedded in culture and environment and differs more greatly around the world than that there are physical differences among the human population. Moreover, the modern synthesis seems to create features over millions of years of slight mutations and natural selection, while human cognition calls for a theory that explains rapid changes over only thousands of years.
It is an increasingly supported view that the modern synthesis is not so modern anymore. A more comprehensive model of evolution, including all knowledge of heritability and adaptiveness
elucidated since Darwin and Mendel, could perhaps give a more elegant explanation for the complex growth of our culture and the human mind in the last thousands of years.
The Extended Evolutionary Synthesis
An extended evolutionary synthesis was developed, in reaction of new data from various fields (figure 1): such as evolutionary developmental biology and developmental plasticity. Pigliucci proposed in 2009, that while our knowledge on evolutionary mechanisms extends, an extended theory should be considered. Danchin et al. (2011) integrated inclusive inheritance into this synthesis, knowing heritability does occur on more than one level. Jablonka and Lamb (2005) proposed four mechanisms of inheritance, as a replacement for the genome as the sole hereditary mechanism. They stated inheritance occurs genetically, epigenetically, behaviourally and symbolically (box 1).
Figure 1. The research fields recruited the extended evolutionary synthesis as proposed in the green circle, the modern synthesis (neo-Darwinism) in the red circle, and Darwinism in the blue circle. Figure adapted from Piglliuci et al. (2009).
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Stotz (2014) argues that exogenetic inheritance as a term can be wider than purely epigenetics, including all mechanisms under the last three systems in box 1. Moreover, Stotz (2014) suggest that non-genetically inherited resources should not be explained as competition to genetic resources. Different systems coexist and might even complement each other.
The extended evolutionary synthesis describes humans (and other organisms) as highly interconnected with their environment, engaging with and manipulating the surroundings and thereby contributing to their own evolution via multiple inheritance systems. Not only does the extended evolutionary synthesis give insight in what processes are and can be influencing adaptation, heritability and natural selection, but also does it provide us with new predictions on evolution. The extended evolutionary synthesis predicts that the relationship between genotype is phenotype is reciprocal, not causal (Laland et al, 2015). Laland et al. (2015) shows contrasting views on development and argues that environment and the genome are constructed, not programmed (figure 2). A wide range of exogenetic inheritance systems included under three inheritance systems organised by Jablonka and Lamb (2005), can describe how a phenotype is constructed by a reciprocal concurrence of the genome and the developmental niche, a system known as developmental niche construction (Stotz, 2017).
Box 1: Four Systems of Inheritance
Genetic inheritance: The Central Dogma as proposed by Crick (1958) states that hereditary
information is only transferred from nucleic acid to protein, and not the other way around.
Epigenetic inheritance: Gene regulation contributes to the phenotype by activating and
deactivating genes, by alternative splicing (selecting a subset of the coding sequence), or by creating new sequence information by insertion, deletion or exchange in the RNA (Davidson & Levine, 2005). These functions are triggered by environmental conditions, allowing specific control of the environment over genetics, and thereby over the phenotype. Epigenetic mechanisms can be passed down from parent germline to offspring germline, providing offspring not only with a blueprint (genome), but also with a manual on how to read it (exogenetic inheritance).
Behavioural inheritance: (including cultural and ecological) occurs when behaviour is
influenced, for instance via social learning, parental effects and habitat/niche construction. This too can work via exogenetic mechanisms, such as mRNAs, cytoplasmic inheritance and nutritional provisioning, but can also take place at a behavioural level, as in parental care or social status. The exact underlying mechanisms of inheritance of behaviour are not always elucidated.
Symbolic inheritance: requires epistemic tools as mechanisms of transmission, such as
language and teaching techniques. In this way, humans inherit cultural traditions, institutions and technologies. This appeals to specific cognitive functions as learning, memory and plasticity, but it remains unclear if it has any epigenetic effects.
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4E Cognition and Transgenerational Developmental Plasticity
It is argued that the additions of extended evolutionary synthesis to the modern synthesis, as developmental niche construction and epigenetic inheritance, can only exist in an embodied, embedded, enacted and extended cognition (4E cognition) (Stotz, 2014). The 4E cognition describes organisms and especially humans as being highly interconnected with the environment and
embedded in their own niche. It states that human cognition cannot be considered a mind separated from body and outside world (Piaget, 1978). Stotz (2014) listed that organisms actively shape their evolutionary process in two ways: firstly, by influencing the environment, building a selective niche and thereby steering the selection pressure on the population. Secondly, by influencing the
developmental niche of offspring and others and thereby creating phenotypical variations. Stotz (2014) states that 4E cognition and niche construction are supported theories in human evolution, but that they are rooted in the crucial question on how and if these modifications are heritable. Only when traits, behaviour and adaptations can be passed on to future generations, can they influence the process of evolution, a term already set by Darwin.
Stotz asks for a broader view on exogenetic contributions of inheritance, such as epigenetics and transgenerationally transmitted resources as parental effects. It was proposed that any system that influences the dynamics of evolution should be considered as contributing to inheritance, no matter the dynamics. Transgenerational developmental plasticity (TDP) was named as a theme of study that could be elucidating in how we see the process of evolution in highly embedded and extended cognitive systems.
Figure 2. Contrasting views on development, the programmed view (a) and the constructed view (b). Figure adapted from Laland et al. (2015).
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Open Questions and Directions
The process of transgenerational developmental plasticity evokes questions, not least of them being:
This paper will explore exogenetic mechanisms and their ability to pass on developmental
phenotypical information to future generations. It will also argue how brain and behaviour makes special use of these forms of heritability. Confirming the influence of exogenetic inheritance systems and thereby presuming the extended evolutionary synthesis does bring about conflicting insights, which might interfere with ideas long proposed by Darwin. Questions arise as:
• How would transgenerational developmental plasticity work?
• Through what exogenetic processes can developmental effects be passed on to future
generations?
• When using exogenetic mechanisms, do we pass along developmental traits not only to
our own genetic offspring, but also to others around us?
• Does this conflict with for instance Darwins idea of ‘survival of the fittest’ or Dawkins
idea of ‘the selfish gene’?
• Is this system robust enough to accommodate stable inheritance? Or is the system
flexible and are phenotypes hereby adaptable to the environment?
• Is transgenerational developmental plasticity particularly applicable to human cognition? • Can it explain how human cognitive abilities evolved quickly and flexibly in the last
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Transgenerational Developmental Plasticity: (How) Would it Work?
The Two Exogenetic Pillars of TDP
To understand TDP as a system that could explain the evolution and adaptability of (human) cognitive abilities, first needs to be elucidated if and how TDP would work. Gomez-Robles et al. (2013) infers that human cognitive evolution is attributable to the pronounced developmental plasticity of human brains: they are genetically and morphologically not strongly constrained, making them highly evolvable and adaptable to selective pressures. In this paper it is argued that this TDP is largely ascribable to exogenetic inheritance systems. Stotz (2014) suggests that inheritance between cultural and genetic inheritance can be considered as TDP and are fruitful to investigate. Two exogenetic systems should be distinguished as most promising, especially in human cognitive evolution: Epigenetic inheritance (especially via RNAs) and behavioural inheritance. From here on out, these systems will be referred to as the two pillars on which TDP leans. As explained below, more information on the functioning of these systems has been elucidated in the past years and possibly reveals promising starting point for explaining why human cognitive evolution has expanded exponentially.
Epigenetic Inheritance; the Rise of Non-Coding RNAs
Hu et al. (2011) state that humans and apes have largely comparative genomes, but very diverging cognitive abilities. Also, that cognition of humans has evolved over a relatively short evolutionary time. They show that micro-RNAs (miRNAs), a kind of non-coding RNAs (ncRNA), have a large role in gene expression divergence between species. Complementary to this idea, Qureshi and Mehler (2012) propose that ncRNAs mediate the accelerated human evolution of especially the central nervous system (CNS) and are responsible for many unique functional features of the human brain. This proposal is based on the observation of Taft et al. (2007) that non-protein-coding sequences scale consistently with developmental complexity. Taft et al. (2007) stated that non-protein-coding sequences contain regulatory information transacted by RNAs, and that the regulatory architecture is momentous in the evolution of developmentally complex species. Moreover, Qureshi and Mehler (2012) state that many ncRNAs are related with the developmental brain. These findings suggest that ncRNAs might have mediated an accelerated evolution of the human brain, especially via the
developmental period.
Heimberg et al. (2007) show that micro-RNA (miRNA) families can be traced back millions of years and are fixed in lineages throughout evolution. The acquisition rate of miRNAs correlates with morphological complexity in organisms. This suggests a role for miRNAs in evolution as an exogenetic inheritance system. Moreover, Heimberg et al (2007) state that the increase in miRNAs is no result of gene duplication, indicating that it might be a result of environment. Salient environmental
information is indeed encoded into the genome, according to Qureshi and Mehler (2012), delineating a mechanism that uses environmental cues to guide evolution.
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When ncRNAs encode information into the genome during development, a way to transfer this code to a future generation is a missing piece of the puzzle, which would make this exogenic inheritance system complete. Sharma (2017) states that the exact mechanisms that underlie epigenetic inheritance are yet unclear but assembles
increasing evidence for parental gamete-borne epigenetic factors, especially RNAs. She proposes a communication of RNAs from soma to the
germline, as a basis to integrate epigenetic inheritance (see figure 3).
It seems so that the functional features, by evolutionary psychologists often referred to as cognitive modules, might not have been entirely shaped by genetic evolution in the Pleistocene, but also more quickly by the rise of ncRNAs. A plausible reconciliation would be that millions of years of genetic selection has shaped robust general cognitive modules present in humans, but a faster few
thousands of years of exogenetic evolution has given our brain the plasticity to adapt and use these modules for flexible behaviour.
Behavioural/Neuronal Exogenetic Inheritance
As the mechanisms of epigenetics and their influence on the process of evolution are becoming clearer, a complementary factor of epigenetics still needs to be highlighted. As discussed before, organisms are embedded in their (developmental) environment and need environmental cues to develop certain traits. Purely epigenetic transmission to the offspring is not sufficient, parental effects and environmental stimulation is needed for an organism to develop its own phenotype. For this, a developmental niche is created providing one with developmental stimulation and in time, one could provide similar stimulation to its own offspring. Here it is stated that this behaviour and the underlying neuronal networks work as a second pillar to TDP.
Pallas (2017) states that neural circuits are build dependent on environmental cues and circuits are usually more sensitive to these influences during a critical developmental period. As an example, the development of the visual sensory pathways in different species are studied, stressing the
importance of the ecological niche. Pallas (2017) argues that synaptic connections can vary in strength and maintenance depending on experience, according to the ‘fire together, wire together’ rules of Hebbian learning (Hebb, 1949).
Figure 3. Model of transgenerational epigenetic inherintance, based on soma to germline communication. Figure adapted from Sharma (2017).
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In this sense, it could be argued that our brain, and its learning and memory capacities, acts as an exogenetic mechanism itself. We learn and experience during development and throughout life. From memory, we create the best
circumstances for development for own offspring. The information is stored in the network of our brain and passed on to the next generation by niche construction and active teaching. It could be argued that what ncRNAs are to epigenetics, neuronal plasticity and Hebbian learning are to behavioural exogenetics. Moreover, the cognitive science points towards unique human capacities that may contribute to the TDP system (box 2). There is little evidence that shows Hebbian learning or long-term potentiation (LTP) to work significantly different in humans than in other mammalian species. Still, Han et al. (2013) show a role for human glial cells, as mice engrafted with human glial cells express enhanced learning and LTP. These findings reveal a neurobiological basis for unique human plasticity and learning.
Herculano-Houzler (2014) concludes that the neuron-glial ratio varies uniformly across species that have diverged over 90 million years ago, ascribing an important role for neuron-glial interaction to memory capacity across species. Obviously, adaptations resulting in better learning and memory in humans could very well contribute to the neuronal exogenetic mechanisms that form the second pillar of TDP. It would confirm the amplified role of TDP in humans compared to other species.
Pallas (2017) summarizes that the use-dependent plasticity of the neural network diminishes with age. This provides flexibility in early life during the developmental period, and stability in later life. Neural circuits are shaped under sensory input, ensuring the network is shaped to the
environment. The network incorporates environmental change by developmental processes. This supports the theory that the developmental period is crucial to TDP.
Box 2: Uniqueness of the
Human Cognitive
Capacities – Mental Time
Travel
All mammals seem to display learning and memory properties, yet human memory stands out as more advanced. Suddendorf & Corballis (2007) suggest that flexible memory systems that help predict future situations offer evolutionary advantage. They declare episodic memory as the most flexible system, a system embedded in autonoetic consciousness. This is the ability of ‘mental time travel’, wherein humans can place themselves into the past and future and therefore accurately plan the future. It is demonstrated that autonoetic consciousness is a cognitive capacity unique to humans (Suddendorf & Corballis, 2007). This ability would indeed contributory to TDP. While growing up in a developmental environment, or while learning from other environments, humans can actively plan the environment and teachings they want to construct for their own offspring. Suddendorf and Corballis (2007) ascribe the human capacity of autonoesis to the larger human prefrontal cortex and its reorganisation.
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Cooperation of Both Pillars as the Foundation of TDP
Both epigenetic systems and behavioural systems being highly interdependent is an undeniable dynamic. Danchin et al. (2011) argued that the epigenetic profile is in part experience-dependent, while the epigenetic profile in turn affects the parental behaviour, generating a similar experience and epigenetic profile for the offspring. This interdependency fits in the vision of organisms being embedded, embodied, extended and enacted in their environment. Both systems cannot thrive without one another. They balance and influence each other: epigenetics encodes environmental information and pass it on to the next generation, and the adaptable brain stores and remembers environmental information (especially form a critical developmental period) and then uses its functions to create a niche for the offspring. They form two interactive routes through which heritability can be influenced, while the genetic code is fixed (see figure 4).
These two routes and the way they keep each other balanced, supports the broader view of organisms being embedded in their environment. It amplifies the call for an extended evolutionary synthesis, that incorporates exogenetic inheritance systems in a broader overview together with aspects as genetics, plasticity and natural selection.
It can be argued that the balance of these two systems has a significant influence on brain
development and thereby on cognition and behaviour. More so than other human features shaped by evolution, the brain might be specially under control of these systems. As Qureshi and Mehler (2012) implied, many ncRNAs are related to the developmental brain. Moreover, the neural circuits of the brain can implement behavioural information and thereby flexibly adapt. It suggests that the interaction of these systems especially evolves the brain faster and more flexibly than other features of mammals can evolve; features that might be more fixed in genetic evolution.
Earlier glial cells have been proposed to be connected to the unique human learning skills that shape TDP. Elaboration of glial cell lineage has in turn been suggested to have a complex interrelationship with ncRNAs as a regulatory mechanism (Qureshi and Mehler, 2018). This suggests that epigenetics as the first pillar of TDP might have had a significant role in the development of the neuronal second pillar of TDP.
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The way transgenerational developmental plasticity is specifically important to brain and behaviour, it also has a special role in humans as a species, compared to other mammalian species. Again, both pillars of TDP support this. There is evidence for a large role of ncRNAs and epigenetics in humans. Moreover, humans have obtained specific learning and memory abilities with complex capacities, which allows for more flexible and complex niche construction. Transgenerational developmental plasticity might therefore be a crucial system for understanding the evolution of the brain and specifically the exponential evolution of the human brain and its cognitive capacities.
Figure 4. Proposed model of TDP. While genetic profile is fixed, the epigenetic and behavioural pillar flexibly influence the phenotype and each other.
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Discussing TDP and Evoked Questions
Transgenerational or Across Generational?
One of the more striking characteristics of TDP lies just in the 4E view of humans being embedded in their environment. In their natural habitat, humans influence and are influenced by many
environmental features, including each other. It is not just that we build a developmental niche for our offspring, or influence our offspring epigenetically, we also contribute to the developmental niches of other people’s offspring and influence other people’s offspring epigenetically. And not only this, we also influence each other as adults, and other generations above us, in one large network of exogenetic influence. For instance, we could influence peers behaviourally on many subjects (to like music, to be more patient, to speak another language, to be more anxious), and people can
incorporate these experiences when creating a developmental niche for their offspring. Also, we could affect each other’s epigenetic profile by interacting. In this way, transgenerational
developmental plasticity does not only work transgenerationally. It works interactively across generations.
It opens possibilities to pass along hereditary material epigenetically and neuronally to not only the next generation, but also in parallel to peers and non-genetic offspring. This notion makes one wonder whether this conflicts with Dawkins’ ideas on the selfish gene. Dawkins states that a gene can only be stable (or thriving) in a gene pool when its phenotypic output is ‘selfishly’ benefitting the replication of the gene. Passing on epigenetic and behavioural information to non-relatives, could enhance the fitness of others and thereby promote the replication of other genes at the expanse of its own replication.
However, it is also Dawkins who states:
‘Replicators do not have to be made of DNA in order for the logic of Darwinism to work’ – Dawkins (2004)
In other words, ncRNAs and even neuronal networks might be considered as replicators undergoing natural selection. A hypothesis that cannot be seen separately from TDP. It can be stated that when a certain ncRNA is advantageous to the survival or reproduction of a specie, it will be passed along more frequently than ncRNAs that are disadvantageous. They will be passed along through the gametes (transgenerationally) and through behavioural influence (transgenerationally and across generationally).
The same logic can be applied to neural networks that encode cognitive/behavioural patterns in our brain. When a certain behavioural pattern is advantageous, its prevalence in the population will be high. The behavioural pattern will be passed along behaviourally (for instance via imitation or active teaching) transgenerationally and across generationally. In this way, the neural network of the successor is constituted in the same or a similar way as the network of the predecessor. Note how the term successor is used instead of descendant: the successor does not typically need to be offspring.
Even though the logic of Darwinism seems to apply to both pillars of TDP, Dawkins would argue that any extended phenotype should directly benefit the replicators (Dawkins, 2004). He would state that, underlying the variance in ncRNAs and in neuronal patterns, should be variance in genes exerting a causal influence.
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It is undeniable that both systems of exogenetic inheritance as plasticity of the human neural networks have an underlying genetic origin that evolved over evolutionary time. This does however not conflict with the notion that these systems, once established by genetic evolution, do now work as replicators their selves. Benefits of neuronal patterns can directly benefit the prevalence of such neuronal patterns, as would it work with other exogenetic inheritance systems.
Is TDP a Uniquely Human System?
As both pillars of TDP provide evidence for features being particularly present in humans, it is quick to conclude that TDP is a uniquely human system. After all, TDP as a system would contribute to an explanation of why human cognitive capacities evolved so flexibly with the quickly changing human culture. Still, there are indications that TDP also works in other animals.
By example, Brusini et al. (2018) show that brains of domestic rabbits have brains with altered brain structure in grey and white matter, compared to their wild counterparts, leading to less fearful behaviour. Previously, Carneiro et al. (2014) already showed a polygenetic base for this change in phenotype. As suggested by Brusini et al. (2018), genetic changes underly the structural
reorganisation of the brain of the domestic rabbit. However, it was also noted that genetic alterations took place at noncoding sequences in the vicinity of genes with a role in brain
development. This suggests that ncRNAs might have a part in the reshape of the domestic rabbit brain. It could be proposed that not only genetic evolution, but moreover the influence of these two exogenetic inheritance systems accelerates and facilitates such fast evolvement of the brain to a changing environment.
It could well be that TDP is a system at work in all animals, but sometimes to a greater or lesser extent. The stability and strength of TDP in species can be assigned to the presence of both pillars of TDP. When species hardly make use of any epigenetic mechanisms they rely on a much more stable genetic inheritance system, making the evolution of the species more robust but also slower and less flexible. Besides, when species have no or limited neuronal systems of memory, the flexibility of creating a developmental environment for the offspring is also limited. As it is proposed here that both pillars cooperate and enhance each other, it can be predicted that the strength of the TDP system grows exponentially when both pillars are stably implemented in a species.
TDP could well be at work in other, especially mammalian species. Humans are not alone in employing epigenetic systems of inheritance and are not exclusive having neuronal systems that control behaviour. Still, as there is evidence for humans having advanced cognitive capacities compared to other mammals (see box 2), as well as evidence for enhanced epigenetic and neuronal properties, it can be suggested that TDP stands most firmly on both pillars in humans.
Does TDP Conflict with the Modern Synthesis?
The short answer is yes. The Modern synthesis indicates the genome as sole source of heredity, and TDP extends towards an epigenetic and neuronal system that together establish a plastic system of heredity. TDP can be regarded as an elaboration on the extended evolutionary synthesis.
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Still, TDP should not be considered conflicting. In the same fashion the extended evolutionary synthesis should not be considered to disprove the modern synthesis. It extends the modern synthesis. The modern synthesis has provided a very robust, stable and verifiable explanation for heredity and evolution. It very well explains the evolution of species over millions of years, and more so the stability of geno- and phenotypes. However, it does provide a satisfying explanation of rapid phenotypical changes in a quickly evolving environment. TDP does provide such an explanation, and ads a more plastic and flexible system to the robust genetic system. These systems can complement each other seamlessly:
It can be argued that TDP is merely an adaptation arising from the genetic system. Species or individuals with a functioning TDP system have an evolutionary advantage, as they can adapt more quickly to the environment. Does this mean that genetic heredity is still the only true form of heredity? It is undeniable that TDP originally came forth from genetic evolution. Genetic changes gave rise to ncRNAs or glial cell changes in humans, which set up the pillars of TDP. However, now that the system is in place it co-operates with the genetic system and contributes meaningfully to the human mind and behaviour. Both systems can coexist without disarming one another.
Directions for the Future
Provided that TDP is a meaningful system to the evolution of human cognition, more research should be conducted to supply empirical evidence for this theoretical framework. A closer look can be taken at both pillars of TDP. As for the epigenetic pillar, according to Burrgren (2016) epigenetics have taken an important position in the understanding of inheritance and evolution, but less than 7% of the papers that discuss epigenetics mention evolution or inheritance. As seen in figure 5, epigenetics are mostly researched in the biological fields of physiology, genetics and cellular and molecular biology. Just a small percentage of publications studied epigenetics in the field of evolution (~1%), evo-devo (~0.5%) or behaviour (~2%).
• The genetic hereditary system provides species with a robust and stable system that causes a slow and steady evolution, resistant to the whimsicality of the environment. • Transgenerational Developmental Plasticity provides species with a flexible and plastic
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To understand the exact influence of epigenetics, transgenerational research is required. For instance, it would be interesting to see if and which epigenetic profiles are passed along to offspring. Also, it would be elucidative to study if these profiles are inherited through the gametes or recreated by the developmental
environment.
As for the behavioural/neuronal pillar of TDP, various research has been done on the evolution of the (human) brain and cognitive capacities. Still, it remains unclear to what extent the human cognitive capacities improve flexible developmental stimulation for the offspring. It would for instance be interesting to enhance or diminish certain learning qualities within species, like Han et al. (2013) executed with engrafting human glial cells in mice and explore if this accelerates developmental plasticity and eventually evolution compared to a control group.
Furthermore, it would be valuable to study the collaboration of the epigenetic and the behavioural pillar and see how they balance each other. It can be seen to what degree the behaviour of the parents sets the epigenetic profile in the developmental niche and to what extend it is inherited through the gametes. Epigenetic profiles of siblings growing up in the same environment could be compared to each other and to profiles of adopted children with no genetic link to their parents. This way, it could be determined to what degree the epigenetic profile is inherited or set by the
developmental environment.
Another interesting route to take is to study how the epigenetic profile is influenced across generations. Is it set after a certain developmental period, or can adults influence each other’s profiles in later life? Epigenetic profiles of experimental animals can be studied during development and in later life. Moreover, multiple variables can be explored, such as the influence of a social or solitary environment in later life. Results of such studies could provide insight in if TDP works not only transgenerationally but also across generationally.
Figure 5. Radar diagram of the relative distribution of epigenetic publications in the 12 biological fields. Figure adapted from Burrgrend (2016).
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Concluding Remarks on TDP
Transgenerational Developmental Plasticity could for a large part explain the fast and flexible
cognitive evolution of humans in the last thousands of years. Data from both pillars suggest that TDP can play a crucial role in brain development and evolution. However, more research is still required to elucidate the exact dynamics of both pillars and to prove TDP had a role in human brain evolution. Even though it has not yet proven to be an exclusively human system, TDP shows promise to have a particular influence on the human brain. TDP does not conflict with the modern synthesis or the notion of the stone age mind. Cognitive modules designed for the stone age can still build the human brain and form a genetically driven basis, but TDP can have flexibly adapted cognition to a quickly changing environment. Within the confinements of genetic basis, a lot of flexible variation can be created through TDP. It can be said that the stone age mind is complemented by modern day adaptations.
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