You might not realise it, but, according to researchers at the Université de Genève, Switzerland, a region of the your brain - the auditory cortex - just 'lit up'. This region is responsible for voice recognition and it responds not only to human voices but also to the calls of common chimpanzees (Pan troglodytes). Notably, the same response is not seen with the calls of bonobos (Pan paniscus) or rhesus macaques (Macaca mulatta). Their findings have been published open access in eLife.
This discovery presents creationists with yet another problem to be ignored, misrepresented or lied about.
Using William A. Dembski’s so-called “proof of intelligent design” — complex specified genetic information, widely cited by creationists as evidence for design and against evolution — we are entitled to ask an obvious question. Why would an intelligent designer create genetic information for a supposedly “too complex to have evolved by random chance” region of the human brain that responds selectively to chimpanzee calls?
What, precisely, was this ability designed for?
By contrast, the evolutionary explanation is straightforward. If humans and chimpanzees share a relatively recent common ancestor, we would expect some neural processing traits to be retained, particularly where there has been no strong selection pressure to eliminate them.
The finding does, however, raise an interesting secondary question: why do we not respond in the same way to bonobo calls?
The answer is likely to come from evolutionary biology. Chimpanzees and bonobos diverged fairly recently, and there may have been a selective advantage for bonobo calls not to be recognised by chimpanzees. Chimpanzees are known to kill and eat bonobos if given the opportunity, so selection may have favoured divergence in vocal signals — with the consequence that humans also lost sensitivity to bonobo calls.
Once again, we encounter a feature of nature that is difficult to reconcile with the notion of an intelligent designer, yet entirely consistent with evolutionary processes acting on shared ancestry, divergence, and selection pressures.
Scientifically, the work is also of considerable interest, as it may shed light on how human speech recognition and language development arise in children. For the creationist, however, it is merely one more inconvenient piece of evidence — to be filed under “not wanted — reject” or “evidence of a Satanic conspiracy — ignore”.
Humans, Chimpanzees, and Bonobos — At a Glance.
Shared ancestry
Human–Pan last common ancestor: ~6–7 million years ago
Chimpanzee–bonobo split: ~1–2 million years ago
Genetic similarity
Human ↔ chimpanzee: ~98.8%
Chimpanzee ↔ bonobo: ~99.6%
Behavioural differences
Chimpanzees (Pan troglodytes)
Aggressive, territorial, documented infanticide; will kill bonobos if encountered
Bonobos (Pan paniscus)
Generally less violent, different social structure and vocal signalling
What Do Competing Explanations Predict?
Question
Intelligent Design
Evolution
Should humans recognise chimpanzee calls?
No clear reason
Expected
Should the response be selective rather than general?
Unexplained
Expected
Should unused traits persist without a function?
No
Yes
Does this align with common ancestry?
No
Yes
Why this matters
Evolution predicts inherited neural traits that persist in the absence of strong selection to eliminate them. Intelligent Design must instead explain why a supposedly purpose-built brain region recognises chimpanzee voices but not bonobo voices — a distinction for which it offers no functional rationale.
For those creationists brave enough to look, the research by the Université de Genève group is summarised in a press release.
Our brains recognise the voices of our primate cousinsA UNIGE team shows that certain vocal processing skills are shared between humans and great apes.The brain doesn’t just recognise the human voice. A study by the University of Geneva (UNIGE) shows that certain areas of our auditory cortex respond specifically to the vocalisations of chimpanzees, our closest cousins both phylogenetically and acoustically. This finding, published in the journal eLife, suggests the existence of subregions in the human brain that are particularly sensitive to the vocalisations of certain primates. It opens a new window on the origin of voice recognition, which could have implications for language development.
Our voice is a fundamental signal of social communication. In humans, a large part of the auditory cortex is dedicated to its analysis. But do these skills have older roots? To find out, scientists from the UNIGE’s Faculty of Psychology and Educational Sciences adopted an approach based on the evolution of species. By comparing the neural processing of vocalisations emitted by species close to humans, such as chimpanzees, bonobos and macaques, it is possible to observe what our brain shares, or does not share, with that of other primates and thus to investigate the emergence of the neural bases of vocal communication, long before the appearance of language.
Visualising vocalisations
In this study, researchers at UNIGE presented 23 human participants with vocalisations from four species: humans, as a control; chimpanzees, which are close to us both genetically and acoustically; bonobos, also genetically close but whose vocalisations are more reminiscent of birdsong; and finally macaques, more distant from humans in both respects. Using functional magnetic resonance imaging (fMRI), they analysed the activity of the auditory cortex.
Our intention was to verify whether a subregion sensitive specifically to primate vocalisations existed.
Leonardo Ceravolo, lead author
Faculty of Psychology and Educational Sciences
University of Geneva, Geneva, Switzerland.
And that is precisely what the research team observed. A region of the auditory cortex known as the superior temporal gyrus, which is involved in processing sounds, including language, music and emotions, is activated in response to the vocalisations of certain primates.
When participants heard chimpanzee vocalisations, this response was clearly distinct from that triggered by bonobos or macaques.
Leonardo Ceravolo.
This specificity is all the more remarkable given that bonobos, although genetically as close to us as chimpanzees, produce vocalisations that are very different acoustically. It is therefore the dual proximity, both evolutionary and sonic, that seems to determine the human brain’s response.
Implications for understanding the evolution of language?
This discovery opens up interesting avenues for studying the evolution of the neural basis of communication. It suggests that certain regions of the human brain may have retained, over the course of evolution, a sensitivity to the vocalisations of close cousins.
We already knew that certain areas of the animal brain reacted specifically to the voices of their fellow creatures. But here, we show that a region of the adult human brain, the anterior superior temporal gyrus, is also sensitive to non-human vocalisations.
Leonardo Ceravolo.
These findings reinforce the hypothesis that certain vocal processing skills are shared between humans and great apes, and therefore predate the emergence of articulate language. They could also contribute to a better understanding of the development of voice recognition, and even language in children, for example by helping to explain how babies manage to recognise the voices of their loved ones while still in utero.
eLife Assessment
This valuable study shows that regions of the human auditory cortex that respond strongly to voices are also sensitive to vocalizations from closely related primate species. The study is methodologically solid, though additional analyses - particularly those isolating the acoustic features that differentiate chimpanzee from bonobo calls - would further strengthen the conclusions. With additional analyses and discussions, the work has the potential to offer key insights into the evolutionary continuity of voice processing and would be of interest to researchers studying auditory processing and evolutionary neuroscience in general.
Abstract
In recent years, research on voice processing in the human brain—particularly the study of temporal voice areas (TVA)—was dedicated almost exclusively to conspecific vocalizations. To characterize commonalities and differences regarding primate vocalization representations in the human brain, the inclusion of closely related nonhuman primates—namely chimpanzees and bonobos—is needed. We hypothesized that neural commonalities would depend on both phylogenetic and acoustic proximities, with chimpanzees ranking closest to Homo. Presenting human participants (N=23) with the vocalizations of four primate species (rhesus macaques, chimpanzees, bonobos and humans) and regressing-out relevant acoustic parameters using three distinct analyses, we observed within-TVA, sample-specific, bilateral anterior superior temporal gyrus activity for chimpanzee vocalizations compared to: all other species; nonhuman primates; human vocalizations. Within-TVA activity was also observed for macaque vocalizations. Our results provide evidence for subregions of the TVA that respond principally—but not exclusively—to phylogenetically and acoustically close nonhuman primate vocalizations, namely those of chimpanzees.
Introduction
The study of cerebral mechanisms underlying speech and voice processing has gained importance since the early 2000s with the advent of functional magnetic resonance imaging (fMRI) [1]. Voice-sensitive areas, commonly referred to as “temporal voice areas” (TVA) or simply “voice areas”, have been highlighted along the upper, superior portion of the temporal cortex [2]. Since then, great efforts have been made to better characterize these TVA, with particular attention to their spatial division into functional subregions [3–5]. A fairly large body of literature points to the critical role of the TVA in voice perception and processing in healthy participants [4, 6–8] as well as in lesioned patients [9]. Subregions of the TVA have also been directly linked to social perception [10], vocal emotion processing [11, 12], voice identity [13, 14], and gender perception [15]. The developmental axis of voice processing has also been studied in infants, demonstrating the existence of the TVA in the human brain as early as 7— but not 4—months of age [16], while the ability to respond specifically to the voice of their parents has been observed in fetuses in utero [17]. With the ongoing development of brain imaging and analysis techniques [18], it is realistic to expect successful, albeit noninvasive, fMRI results on task-related voice perception in utero in the near future. Along the evolutionary axis, evidence for TVA or, more generally, conspecific vocalization-sensitive brain areas has emerged primarily in dogs [19] and monkeys [20, 21] (Macaca mulatta), raising the question of whether such specialized brain areas are species-specific [22] and to what extent human and nonhuman primates share neural mechanisms that enable them to preferentially process conspecific vocalizations [23]. However, less attention has been paid to paradigms in which animal vocalizations are presented to humans, and to the best of our knowledge no study to date has reported selective human TVA activations for processing such auditory material, namely the vocalizations of other animals. Human processing of animal vocalizations has been studied with both monkey and cat material, but no specific cross-species activations have been observed within the TVA with respect to either species [24]. Other studies have focused more specifically on phylogenetic distance and have included nonhuman ape (chimpanzee, Pan troglodytes) and ‘Old World’ monkey (rhesus macaque, Macaca mulatta) vocalizations as stimuli. Such studies failed to identify species-specific brain activations—despite correctly discriminating chimpanzee affective vocalizations [25]—and observed ambivalent results for below [25] vs. above [26] chance discrimination of macaque affective vocalizations by human participants. A recent exception is a study in which functionally homologous anterior TVA activity was observed in both humans and macaques: this region was indeed specific to macaque calls in the macaque’s anterior TVA, and specific to human voices in the anterior TVA of humans, but no macaque-specific activity was observed in the human TVA [27]. This sparse literature motivated the present study, which aims to investigate cross-species TVA activations in humans when asked to categorize vocalizations from phylogenetically—and acoustically-close and -distant—species while undergoing fMRI scanning. The importance of acoustic differences between species and more specifically acoustic distance, particularly through fundamental frequency variations [28, 29] was indeed of great interest. Acoustic distance—calculated using Mahalanobis distance with 16 acoustic parameters extracted from the stimuli—was in fact a determining parameter in assessing affective cues recognition in nonhuman primate calls by human participants [30]. In this study, affiliative chimpanzee—but not bonobo—calls were acoustically the closest to positive human voice stimuli, suggesting a distinct evolution of bonobo calls [30]. Bonobo vocalizations are of particular interest because this species is thought to have undergone evolutionary changes in their communication, in part due to a neoteny process involving acoustic modifications, and although they are as phylogenetically close to humans as chimpanzees—with an estimated separation with the Homo lineage only 6-8 million years ago [31]. Previous research has shown that bonobos have a shorter larynx—a valid predictor of a species’ mean fundamental frequency [32]—compared to chimpanzees, resulting in a higher fundamental frequency in their calls [28]. Such a difference has been demonstrated in juvenile bonobo calls compared to chimpanzee and human baby calls [33], arguing for a greater acoustic distance between bonobo calls and human or chimpanzee vocalizations. For these reasons, we included vocalizations from both Pan species (chimpanzees, Pan troglodytes; bonobos, Pan paniscus), as well as a phylogenetically more distant species (Cercopithecidae: rhesus monkeys), with an estimated separation with the Homo lineage dating back to 25 million years ago. Indeed, any claim of human ‘uniqueness’ for TVA recruitment remains on hold and should be tested in light of these closely related species. Using the same stimuli, we previously investigated the specific frontal mechanisms involved in the categorization of nonhuman primate vocalizations independently of a selection of low-level acoustic parameters [34], but the possibility that acoustic differences would affect, at the auditory level, the ability of human participants to recognize nonhuman primate calls should be thoroughly examined, as we did in the present study. As suggested by research mentioned above, monkey vocalizations are overall less likely to be identified compared to ape vocalizations due to both phylogenetic and acoustic differences. Therefore, our mechanistic hypothesis of the difficulty for humans to recognize bonobo calls is that frequencies of the human tonotopic map in the auditory cortex—adapted and adjusted to the frequencies of the human voice during evolution—would not be tailored to process the frequencies generated by bonobo calls. It would also be the case for macaque calls, while frequencies of chimpanzee calls—being closer to the range of human voice fundamental frequency [28, 30]—would be better represented in the human auditory cortex and therefore more easily processed and better identified by humans.
According to the literature mentioned so far and to the mechanistic hypothesis underlying the processing of chimpanzee as opposed to bonobo and/or macaque calls by human participants, we therefore predicted: (i) more acoustic proximity between human and chimpanzee vocalizations, whereas more distance would separate those of bonobos and macaques from the human voice; (ii) a recruitment of temporal brain areas—within the TVA—for the processing of vocalizations from the Pan taxon (chimpanzee, bonobo) but not Cercopithecidae (rhesus monkey) vocalizations, taking into account acoustic features of interest through a discriminant analysis of the parameters that best underlie our stimuli.
Timecourse of the species categorization task with stimuli example and acoustic distance data.
(A) Detail of the timecourse of four trials of the species categorization task in non-representative order, including waveform and spectrogram graphs for one example stimulus of each species. (B) Scatter plot and histogram of the acoustic Mahalanobis distance data of each stimulus for each species including mean (numbers represent exact mean value) and violin plots of the standard error of the mean in addition to distribution fit. ITI: inter trial interval; Hum: human; Chimp: chimpanzee; Bon: bonobo; Mac: macaque.
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