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Gene-culture co-evolutionary dynamics for language

3. Gene-culture co-evolutionary dynamics for language

Current studies in language evolution are putting more and more light on the relevance of a gene-culture co-evolution approach to language (Sterelny 2012; Jablonka et al. 2012; Dor and Jablonka 2014; Morgan et al. 2015). In gene-culture co-evolutionary models, the control of the phenotype is not exclusive matter of genetic determination; cultural transmission mechanisms of learning, phenotypic and behavioral plasticity are also involved (Suman, 2016).

Although biological and cultural evolution are processes operating at different time-scales, they interact with each other. The key point is trying to understand how far back in time this interaction could go and in which ways it is articulated. By doing so we might evaluate the proper causal role

of cultural transmission in language evolution, according to empirical evidence in paleoanthropology and archaeology.

Furthermore, understanding the causal role of cultural transmission might result pivotal to advance some predictions in order to bring clarity to the protracted debate on whether language evolved gradually at uniform rates or rather punctuationally, a topic we will discuss in the last paragraph before the conclusions.

According to these debates, we think that three main positions, concerning the processes at the basis of language evolution, can be identified today in literature.

1) a saltationist perspective, according to which a genetic mutation, occurred between 100 and 50 kya, triggered language appearance giving the alleged genetic endowment that makes humans unique. (Chomsky, 2010; Berwick et al., 2013; Bohluis et al. 2014). This stance, supporting a lucky mutation being responsible for letting the whole human language package emerging abruptly, without any role for natural selection, is believed to be anti-Darwinian and no longer tenable (Hillert, 2015; Lieberman, 2015; but see Bohluis et al. 2015), hence we won't further discuss it here.

2) a separationist perspective: language readiness (see below) was shaped by biological (genetic and epigenetic) evolution; then, approximately 50-100 kya, cultural conditions, often generically defined, allowed the emergence of modern (or true) language. In such a perspective, the causal role of culture enters pretty late the evolutionary scene and is exclusively associated to the emergence of modern “full-developed” language (Christiansen and Chater, 2010; Arbib, 2012; Pagel 2012).

3) an integrative perspective: cultural transmission played an evolutionary role not only in the evolution of modern language, but also in previous forms of communication (protolanguages) and it should be considered a causal evolutionary factor in shaping language readiness (Laland 2016, in press; Morgan et al. 2015 and discussion below).

Today, a term used to refer to the biologically evolved capacity to acquire language is language readiness or language-ready brain. This term stands for the neural features evolved during hominin evolution (not expressively for linguistic functions) and representing the scaffolds that allowed modern language to emerge (Arbib 2012; Boecks and Benitez Burraco 2014). Typically, in this perspective, language readiness is intended as a product of biological (genetic and epigenetic) evolution, while the emergence of modern language is intended as a cultural invention, as a consequence of specific cultural conditions (Kirby et al. 2008; Christiansen and Chater 2008;

Christiansen and Chater 2010; Scott-Phillips and Kirby 2010; Arbib 2012; Pagel 2012). In this sense, modern language appearance is usually dated approximately 100 kya, associating it to the

explosion of Behavioral Modernity (D'Errico and Banks 2013) observed in the archaeological record.

We label this stance the separationist perspective and we believe it presents some problems related to the alleged evolutionary processes that shaped language, in particular regarding the causal role assigned to cultural transmission in hominin evolution.

This separationist account is well depicted by some crucial passages in Arbib's (2012) argument, in the sixth chapter where the core hypotheses of the book are revealed: the hypotheses presented in Arbib (2012) distinguish

“biological evolution, which shaped the genome for a language-ready brain (and body), from cultural evolution, which took us from hominids with a language-ready brain and rudimentary manual-vocal communication (protolanguage) to humans with full language capability. I will argue in later chapters that the demands of protolanguage in hominids prior to Homo sapiens contributed to the evolution of the human brain, whereas those features that distinguish language from protolanguage did not” (Arbib 2012, p. 162)

And again:

“It has been the argument of this book that many different changes during biological evolution gave humans a language-ready brain but that it took cultural evolution to exploit the human brain’s capabilities to the point where the potential for language (in the singular) became realized in the development of diverse languages (in the plural) as Homo sapiens developed different groupings in Africa and spread from there around the world” (Arbib 2012, p. 251)

Arbib's argument can be summarized as follows: biological evolution built the scaffolds for a language-ready brain, with a central role played by the Mirror System; the scaffolds comprehend: a mirror system for grasping in the common ancestor between monkeys and humans; simple imitation in the common ancestor between humans and ape; complex imitation unique to genus Homo;

pantomime; multimodal protolanguage (protosign and protospeech). When cultural evolution entered the scene, true language (provided with compositional semantics) could emerge in all its diversity and complexity.

The aim of our work is not entering the linguistic, the cognitive and the neural details of Arbib's proposal, which certainly provides promising insights for language evolution studies, but just evaluating the causal role assigned to culture in its account of language evolution.

In the figure 6.2 (Arbib 2012, p. 159) a straight line goes from animal communication systems to protolanguages and biological evolution alone is the process responsible for such a transition. Then the same straight line starts from Homo sapiens, it goes towards Indo-European languages and cultural evolution is retained responsible of such a change.

Depicting the evolutionary processes responsible for language evolution as a straight line that shifts from biological to cultural evolution represents exactly what we labeled a separationist account.

We argue that there are at least two critical points in Arbib's account:

1) the absence of overlapping between the biological and cultural evolution lines;

2) cultural evolution starting only with Homo sapiens appearance.

The same problematic perspective is present in Pagel (2012):

“language evolved to solve the crisis that began when our species acquired social learning – probably some time around 160 to 200 kya” (Pagel 2012, p. 280)

and in Christiansen and Chater (2010):

“Instead of viewing the brain as having a genetically specified, domain-specific system for language, which must somehow have arisen over the course of biological evolution, we see the key to language evolution to be evolutionary processes over language itself. Specifically, we view language as an evolving system, and the features of languages as having been shaped by repeated processes of acquisition and transmission across successive generations of language users”

(Christiansen and Chater 2010, p. 5)

“Cultural evolution will work against biological (co)evolution in the case of malleable aspects of culture— rapid cultural change leads to a fast-changing cultural environment, which serves as a

‘‘moving target’’ to which biological adaptation cannot occur” (Christiansen and Chater 2010, p.

12)

Such an account of language evolution poses a sharp separation between biological and cultural evolution, not supported by the updated empirical evidence about hominin phylogeny. We believe that the quoted accounts underestimate the role played by cultural transmission and social learning, as cultural evolution and social learning simply did not enter the evolutionary scene with Homo

sapiens, but instead played a significant role during all the evolution of genus Homo.

Each hypothesis on language evolution should be primarily in accordance with the most updated data coming from the archeological and paleontological records. Today, the archeological and the paleoanthropological records coming from hominin branching phylogeny suggest that these two processes have worked side by side (inclusive inheritance and explanatory pluralism perspective, see Suman 2016) at least since the appearance of genus Homo (or even before) and so they should be integrated in a gene-culture co-evolutionary interaction in order to understand how uniquely human and interdependent features of cumulative culture and language have emerged.

Here we briefly list some of the main compelling evidence. The first evidence of material culture are today set with the Lomwekian tools 3,3 mya, and tentatively associated to Kenyanthropus platyops (Harmand et al 2015). The presence of such a lithic industry indicates that those hominins were already relying on rather advanced social learning practices, hence cultural transmission.

Oldowan technology (Semaw et al., 1997), usually associated to Homo habilis, appears in the archeological record around 2,4 – 2,6 mya and shows stasis for 700 ky. Acheulean technology (Lepre et al. 2011), usually associated to Homo erectus, the first hominin species coming out of Africa, appears 1,7 mya and shows significantly different and novel characteristics, such as bifacial processing. Erectines showed morphological, behavioral and developmental features suggesting that they were benefitting from cumulative cultural knowledge (Antón et al., 2014) and probably they already exhibited a pre-syntactic protolanguage (Tallerman and Gibson 2012).

Moreover, theoretical analysis showed that “individual learning, social learning (from the parental generation), and innate determination of behavior are favored by natural selection when environmental changes occur at short, intermediate, and long generation intervals, respectively”

(Aoki et al. 2005, p. 335). Climatic fluctuations indeed played a pivotal role in the evolution of human behavioral flexibility and adaptations (Parravicini and Pievani 2016, in press). Consequently, we could expect that hominin species evolved individual learning as a fundamental adaptive strategy. However, evidence coming from the fossil and archeological record suggest that hominins have relied on complex social interactions at least since the appearance of the first lithic industries, hence Lomekwian, 3,3 mya (Harmand et al. 2015). Climatic fluctuations effects should have been attenuated in order to let hominins evolve social learning adaptive strategies. This consideration support the idea that (cultural) niche construction activities have long been in place in hominin evolution, allowing hominins to build environments in which to fit. Counteractive niche construction (Odling Smee et al. 2003; Laland et al. 2010) is an example of such an evolutionary mechanism favouring hominins fit to their constructed environment. As culture may have been the main factor allowing this environmental modification and eventually human adaptation to a wide

range of environments (out of Africa), selection for more and more efficient cultural capacities should have been strong in hominin evolution, especially on those hominin species coming out of Africa, coping with several different environments, from at least two million years ago. Constant selective pressures for cultural variants, allowed human cultures to grow in diversity and complexity, in several human species.

However, culture, in order to be cumulative, requires high-fidelity transmission (Lewis and Laland 2012), and verbal language is such a high-fidelity transimission mechanism. In order to support the transmission of cumulative culture, hominins evolved more and more complex forms of communication along with the increasing complexity of cultural variants in a co-evolutionary dynamic.

Summing up, the archeological and the paleontological records provide strong evidence today in favor of the third stance we labeled the “integrative” perspective, that is a gene-culture co-evolutionary scenario in genus Homo evolution. This scenario supports a co-co-evolutionary dynamic between higher and higher-fidelity transmission mechanisms and culture, and supplementary experimental data seems to support such a prediction (Morgan et al. 2015), as argued below.

4. Causality in co-evolutionary feedback processes: cultural transmission shaped the