Chapter 16 – Early Brain Development

Wikis > Chapter 16 - Early Brain Development

1. Introduction

It can be said that many factors play a part in allowing language to continuously evolve into the complex and sophisticated tool of communication that it is today. According to researchers, there was a co-evolution of both brain and language to allow one to adapt to the other (Deacon, 1997; & Samado & Szathmary, 2012). But how did this happen? We shall attempt to explore the answers to this question through this chapter – how different parts of the hominid brain evolved to provide us with the basic capacities of language, the factors that facilitated hominid brain growth, and how language eventually emerged amongst us and why.

 

2. Structural brain evolution in early hominids

Since the appearance of hominids around 6 million years ago, the human brain has tripled in size in order to facilitate our now-advanced cognitive processing abilities, including the valuable ability to use language. As we have learnt from Chapter 2 of this Wikibook, taking into account the human brain-to-body ratio, the brain has increased in both absolute and relative terms across time.

2.1 Three major stages of brain evolution for language

Lieberman (1991) theorized three major stages in the evolution of the brain for language, the first being the lateralization of the brain – a function where some cognitive processes are more dominant in one hemisphere over another. For most humans, the left hemisphere provides the bulk of the neural circuitry needed for language production, and also controls the dominant hand used in tasks requiring fine motor control. Since tool-making occurred before language, brain lateralization could have evolved due to initial selection pressure for this fine motor control before subsequent evolutionary features led to the development and utilisation of language. This theory is supported by other studies concerning handedness, which is the tendency to use one hand more naturally than the other, and brain lateralization among non-human primates (Cziko, 1995).

With lateralization, the brain subsequently evolved to accommodate voluntary, intentional control of speech. Chimpanzees are unable to repress sound emission, as observed by Goodall (1986) in an instance where a chimpanzee pressed its hand across its mouth to muffle the sound he made upon locating a new food source as it did not want to alert the others and share it. Contrast this to humans who possess language productivity and communicate with displacement; these feats are made possible with the development of Broca’s area, as well as the augmentation of the prefrontal cortex and the rewiring of concentrations of neurons known as the basal ganglia.

Finally, the evolution for the precise control of the mouth cavity, tongue and vocal tract for speech itself facilitated the development of syntax. Words come about through intrinsic knowledge of the order of sounds and the well-timed sequential coordination of the movements of the tongue, lips and jaws. For example, to say cat in English, one knows exactly how and when to manipulate parts of their mouth to create the three different sounds corresponding to c, a, and t in that order to articulate the word. With this instinctive knowledge, humans thus developed syntax. Human communication such as expanding vocabulary grammar structure – which is now independent of speech motor control – is then naturally selected over time.

While it is widely known that language development in the human species was accompanied by an increase in brain size, there has been little attention paid to specific regions of the brain that contributed to this development. This section of the Wikichapter will focus on several key areas of the brain to understand how the development and growth of these specific parts facilitated our eventual manipulation of language.

   

Figure 1. Basic anatomy of the human brain. This diagram illustrates several main regions of the brain.

2.2 Broca’s and Wernicke’s areas

We cannot discuss language evolution in humans without examining Broca’s and Wernicke’s area. Broca’s area enables imitation, motor control and music cognition. At the cognitive level, language could be seen as the result of a coordinated combination of mechanisms – some language-specialised, some otherwise.

It must be noted that the existence of Broca’s and Wernicke’s areas is not unique to humans; they are found in non-linguistic species as well. However, their functions remain under-examined in these non-linguistic species (such as primates). By comparison, these areas of the brain do appear to be significantly larger in both absolute and relative terms in humans, thus suggesting that Broca’s and Wernicke’s areas may have developed specialized uses for language which other non-linguistic species have not (Schoenemann, 2009).

Figure 2. Arcuate fasciculus and mirror neurons. This diagram illustrates the neural connection between Broca’s and Wernicke’s areas via the arcuate fasciculus.

The arcuate fasciculus, which is a white-matter fiber tract that is involved in human language (Behrens et al., 2008), directly connects these two areas together and has apparently grown during human evolution. In primates, their equivalent of both areas project neural information to prefrontal regions that are close to them, but not directly to them (Aboitiz & Garcia, 1997). This direct neural connection between Broca’s and Wernicke’s areas could have thus led to more expansive information interaction, where these brain areas would work together and allow humans to develop the capacity to use language. Connections between Broca’s area and the middle temporal regions (crucial for semantic processing) are only obvious in chimps and humans, and appear to be most advanced and accurately projected in humans.

2.3 – Cortical folding

Figure 3. A comparison between mammalian brains. This diagram illustrates that humans have more advanced cortical folding compared to other mammals.

Cortical folding is a term used to describe the phenomenon in which the surface of the cerebral cortex of the brain is folded, thus creating grooves (sulci) and bumps (gyri) on the surface of the brain (Serendip, n.d.). More cortical folding is found in larger, thicker brains, like those of humans as observed in the illustration above. An important evolutionary advantage of this design principle is that it enables human brains to become more compact and become capable of faster processing due to the minimal distal lag in cortical connections with the increasing brain size, and thus a huge advantage for our manipulation and comprehension of language.

2.4 – Cerebral cortex

Figure 4. A sagittal view of the brain. This figure indicates the location of the cerebral cortexes.

The cerebral cortex is the outer layer of grey matter neural tissue of the brain in humans and other mammals. A larger proportion of the human cortical surface is committed to the higher-order association cortex rather than primary sensor and motor areas (Swenson, 2006). This suggests that more of the human cerebral cortex is devoted to conceptual processing, such as memory, language, abstraction and judgement. The evolution of the cerebral cortex has thus played a key role in the advancement of information processing in the mammalian brain, advancing tremendously from a primate cortex equipped with structures that once constrained its size and the amount of information it could previously store and process, to one capable of processing complex and sophisticated language structures.

2.5 – Prefrontal cortex

Figure 5. The human brain evolution is not as special as we thought. This figure depicts the similarities between mammalian brains.

The prefrontal cortex area has increased approximately twofold compared to what would be predicted for a primate brain the size of our own. Presently, it is involved in a variety of linguistic tasks, such as various semantic aspects of language (Maguire & Frith, 2004), syntax (Novoa & Ardila, 1987), and higher level linguistic processing, such as understanding the reasoning behind conversations (Caplan & Dapretto, 2001). However, how much language itself is responsible for these changes in the prefrontal cortex remains unclear, because the area also controls other crucial nonlinguistic behavioural processes that could also have contributed in human behavioural evolution, such as planning, social information processing, memory and attention (Schoenemann, 2009).

2.6  – Temporal lobes

Figure 6. The brain’s lobes. The different lobes and their locations are depicted in this figure.

In primates, the temporal lobes help to distinguish between different sounds and calls of their own species. They thus play an important part in auditory communication. The significant increase (about 23% larger than predicted based on the size of the human brain) in the temporal lobe dimensions (volume, surface area and white matter volume) could be related to the intricacies of human auditory communication systems (Schoenemann, 2009).

Figure 7. What and where. This figure depicts the mangocellular and pavocellular pathways in the brain.

There are also two major pathways in the temporal lobe where information in humans are processed – the dorsal (‘where’ pathway) and the ventral stream (‘what’ pathway). The comprehension of proper nouns and cognizance of common nouns are dependent on the temporal lobe. Since the primary auditory cortex of the temporal lobe and its adjacent areas are three times larger than in apes, it is plausible that crucial developments occurred with relation to auditory processing for language (Rauschecker & Scott, 2009) This also highlights the elaboration of circuits involved in conceptual and semantic processing.

2.7 – Basal ganglia

The basal ganglia (refer to Figure 1)  is made up of a group of subcortical nuclei mainly accountable for executive functions and behaviours, emotions and motor learning (Lanciego, Luquin & Obeso, 2012). It participates in a vital circuit loop that functions in the selection and voluntary execution of movements (Bear et al., 2007). These circuits connecting the cortex and the basal ganglia have been found to aid in both language production and comprehension, since it has been found that afflicted basal ganglia results in motor problems, and issues with the comprehension of syntax and the processing of semantic information. Since mankind’s motor skills do not seem to significantly surpass primates, the increase in twice the absolute size of the basal ganglia could be attributed to contribute in aiding higher cognitive functions, such as language (Schoenemann, 2009).

2.8 – Cerebellum

In addition to monitoring and regulating motor signals from the cortex (Carpenter & Sutin, 1983), the cerebellum (see figure 1) also participates in speech perception and production, as well as semantic and grammatical processing (Baillieux et al., 2007). Since the cerebellum also contributes to the control of timing mechanisms, its growth could thus be pinpointed to reasons of language development, as it works alongside temporal information in language production and perception (see section 2.1, where the precise control of the mouth and vocal tract is vital for language production). The cerebellum’s increase in size is hence the result of language evolution, or from natural selection – specifically behavioral capabilities that depend on it.

 

3. Factors influencing early brain development

Now that we have examined various brain regions which have contributed towards the emergence of language in humans, this section will thus explore several distinct forces that powered the evolution of the human brain.

3.1 – Molecular changes in biology of Hominids

It is believed that the encephalization (the increase in complexity and size of the brain as a result of the shift in function from non-cortical parts of the brain to the cortex – see previous section on cerebral and prefrontal cortexes) of hominid brains did not derive from external environments because modest brain growth was only seen in very specific genus such as the hominids from the Homo ergaster, Homo habilis and Paranthropus boisel branches. These subspecies did not see any significant growth in brain size and activity despite sharing the same environment and diet 2.5 million years ago. Hence, researchers are inclined to believe that the other factors of early brain development played secondary but cumulative roles to the primary factor of cranial molecular change in the Homo species. (Bradshaw Foundation, n.d.) Hence, researchers suspect that changes to the anatomy of the hominids at a molecular level could have caused an increase in metabolic demand (that helped power brain usage) and the synaptic plasticity of the hominids’ brains, which helped make brains more functional by increasing the complexity and amount of brain activities that could be carried out simultaneously (Sherwood, 2008).

3.2 – Selective mating

On top of the molecular changes, the relatively brainier subspecies probably favored mates that showed abilities that required brains of comparable size, or in other words, partake in selective mating. As a result, their progeny were very likely to inherit the large-brain traits of their parents (Bradshaw Foundation, n.d.). Now that we have established the hominid brain, we can dive into the factors that may have accelerated and greatly increased the potential of the relatively large Homo brain.

3.3 – Changes in hunting behaviors

First, the food-gathering behavior of hominids probably had an impact on brain development in terms of immersing them in neuron-building mental and physical exercise. Interestingly, the fact that our ancestors were more scavengers than hunters meant that they probably had to devise ways to ensure that larger predators did not eat away at their food source. The plasticity of their brains would have allowed them to eventually think of ways to make tools and also simple systems that could make their scavenge trips and subsequent storage more effective (Bradshaw Foundation, n.d.). From there, researchers deduced that our ancestors probably managed to hone their tool-making abilities and systems of collaboration such that they were able to hunt together.

3.4.1 – Nutrition: Increase in caloric count

Some paleontologists find that in changing their diet such that it included meat, the hominids’ brain may have evolved as well since their caloric count would have been multi-fold of that of mere plant and insect-based scavengers. The increase in caloric count was said to have allowed the growth in brain size and also help maintain the relatively high levels of cranial activity (Cornelio, 2016).

3.4.2 – Nutrition: Increase in consumption of “brainy” foods

Additionally, meat from marine animals usually contained nutrients that helped with brain growth such as docosahexaenoic acid (DHA) and omega-3 fatty acids. This theory was supported by a comparison study of the brain development of African hominins who lived near large water bodies and those who did not due to regional aridity (deficiency in moisture) and climate changes (Schultz & Maslin, n.d.). The hominins throughout the period experienced varying growth rates, and scientists believe that whenever regional aridity took place, there would be a slow growth rate until the hominins migrated and settled at a new large water body. Those that settled near freshwater lakes were the ones who were more likely to consume foods that were naturally high in DHA and poly-unsaturated fatty acids (found in food like salmon, which was available in these bodies of water) and hence grow larger brains (Coqueuegniot, Hublin, Houe’t, Jacob & Veillon, 2016).

 

4. Gestures and language

Research has found evidence to support the idea that verbal language and sign language depends on similar neural structures. Patients who used sign language and suffered from a left-hemisphere lesion showed the same disorders as vocal patients did with their oral language. Other researchers found that the same left-hemisphere brain regions that were active during sign language were also active during the use of vocal or written language (Pollick & Waal, 2007).

To illustrate the correlation between gestures and speech and consequently on brain development, let us take a look at primates. Gesturing for primates are an intrinsic characteristic and is crucial in primate communication. For example, apes of different species would perform gestures specific to their species although they have never seen its execution by another member. To exemplify, the beating of the chest by gorillas is an indication of aggression and is specific only to their species (Pollick & Waal, 2007). On the other hand, chimpanzees would organize coordinated assaults to portray aggression (Geggel, 2014). This supports the postulation that gestures laid the foundations for the development of language.

Further evidence suggests the correlation between gestures, language, and brain development. For humans, gestures affect concurrent vocalizations, creating certain natural vocal associations of manual efforts. Similarly, chimpanzees would move their mouths when performing fine motor tasks. These mechanisms might have contributed to the development of intentional vocal communication as a supplement to gestural communication. Voice modulation could have been prompted by pre-existing manual actions, and consequently the brain developed naturally to accommodate for such environmental demands.

However, critics of the gestural theory highlighted the difficulty in determining reasons to explain why primates have abandoned their initial ability to communicate vocally for a less effective non-vocal, gestural mode of communication. Furthermore, primates have complete control only over hand movements and not over vocal communication. Thus, it can be demonstrated that gestural communication is a precursor to human language development albeit that primate vocalization is homologous and involuntary. Additionally, gestures are not necessarily less effective if placed in certain contexts such as a hunt where it is advantageous to be silent (Bradshaw Foundation, n.d.).

These observations greatly support the gestural hypothesis of human language origins, which are further supported by the differential growth of the brain and vocal instruments. On top of that, the existence of gestural communication in human infants prior to the development of speech, and the right-hand (hence left-brain development) bias of both ape and human gestures (Pollick & Waal, 2007) both provide support for the concurrent development of the brain structures, enhanced gestural communication and subsequently language capabilities.

 

5. Language as a tool: Why did we start using language?

Figure 9. Neanderthals were collecting unusual rocks. Primates demonstrate their capability in constructing tools.

In our everyday lives, we constantly engage in activities and interact with others. To ensure successful interactions and completion of activities, we are required to use language. Just as a spear allows us to hunt for animals, language use allows us to complete certain activities.  It is commonly thought that language appeared alongside physical tools such as spears when gesturing could not be used due to our hands becoming preoccupied. Hence we started using a series of sounds to communicate with each other. Not only that, for infants to learn about successful in-group living they would have to learn about the norms that govern that group. This set of sounds developed into a properly structured language when it underwent social and cultural developments. In this section, we will be discussing how language was shaped through different developmental milestones as our ancestors started in-group communications.

 

5.1 – Migration due to overpopulation

Given the amount of time our ancestors had to roam the earth and reproduce, there would definitely be a biological reason contributing to their impetus to use language. There are many theories of migration of the Homo genus, but the most widely-accepted human evolution theory would be the “Single Origin, Out of Africa Theory” (Hays, 2016). It is said that earliest hominids (from the Australopithecus species to the Homo genus) evolved in Africa. Only after 2 million years later did they migrate to Asia and the now-known Americas (Hays, 2016). While climate change and extreme aridity were postulated to be critical factors in forcing migration, such events did not occur often enough for hominins to consider migration. Instead, it was the fact that there were not enough resources in the community that probably forced them to split in search of new territory. However, in order to achieve the migration of sizable populations, our ancestors had to formulate a means of communication for teamwork and collaboration. Not only did they have to coordinate the movement of people and possessions, an intricate planning process would have to be established to ensure that they picked a prime location for resettlement, and developing the necessary technology to travel across water to further territories. These needs thus provided a strong impetus for the development of the most primitive forms of language. If the community had not split, those who were not able to enjoy the resources would have naturally died out (Bailey & Greary, 2009), causing a roadblock in the evolution of the human species and the lack of need for language in the first place.

5.2 – Cultural development

Culture and language are interdependent factors with each influencing the development of the other. In other words, the development of language depends on the elaboration of culture, where only group endorsement of particular cultural practices are deemed suitable for transmission to future generations via language (Durkheim & Nisbet, 1976). In the initial stages of cultural development, there was no form of communication in place to transmit these symbolic cultural facts. Language only started developing when there was a common understanding that there were a substantial amount of cultural practices that were ritualised and deemed necessary for cultural transmission.

In the earliest days of human civilisation, it was unlikely that there were cultural practices pertaining to tradition and social festivities when meeting their needs of survival took up the bulk of their existence. However, as humans migrated and eventually began to settle, there was more time freed up for developing such activities other than hunting and building shelters.

Since infancy, children begin learning about both culturally accepted and taboo practices in their group of membership (Shneidman & Woodward, 2016). In order to learn and effectively engage with their environment, the children’s primary educators (their mothers) would need to engage in interactions with them to teach them important practices and  rules that govern their specific group (Csibra & Gergely, 2006). As traditions and practices grew more diverse and elaborate, a need for communication thus arises for educational cultural transmission purposes, now known as child-directed interactions. Child-directed learning is often characterised by eye contact and a series of gestures and sounds made by the mother to her child in order to communicate specific information about a ‘referent object’ (Shneidman, Todd & Woodward, 2014). The development of language for the communication of increasingly complex cultural practices thus ensures that practices that are deemed important by the ingroup would be passed down to the next generation, and that subsequent generations can in turn build upon existing cultural knowledge to further human civilisation. Subsequently, a child would learn how to turn-take and participate in a back and forth engagement that may have been the driving force behind the advancement of primitive language forms.

5.3 – Technological and social advancement via tool-making

We have already looked at the ‘cradle of language’ borne out of mother-child interactions, and we will now take a look at what caused the further development of language. From 5.2, we understand that the volatility and continued development of language depends on the cultural factors surrounding it. While the development of language stemmed from the need to communicate basic in-group norms (which subsequently became increasingly advanced and complex), tool-making incited more reasons for language to be elaborated and used in order to ensure mankind’s survival. It has been found that language developed hand in hand with tool-making. (Aldo, Dietrich, Jan & Bruce 2010) With the development of basic tools, such as sharpened rocks and primitive spears, the hands of our ancestors were now occupied in a bid to develop better and more efficient hunting methods. Where manual gestures might have taken precedence as an effective means of communication, the tools humans held in their hands soon made such forms of communication an arduous process. There was thus a need to utilize a better form of communication that would ease their daily lifestyle. Furthermore, as tool-making increased in sophistication over time, there rose a need to verbally explain the tool-making process to each other such that they could be replicated and have their designs preserved and improved upon for future generations, driving the appearance of the beginnings of language – Morgan et al. (2015) found that active verbal instruction from teachers yielded the best results when participants of their particular experiment attempted to recreate Oldowan tools, as opposed to methods like reverse engineering or pure imitation. Hence, tool-making became a valuable reason behind the eventual production of language to ensure survival and continuity of our species.

 

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