A cross-species study of gesture and its role in symbolic development: implications for the gestural theory of language evolution

Abstract

Using a naturalistic video database, we examined whether gestures scaffold the symbolic development of a language-enculturated chimpanzee, a language-enculturated bonobo, and a human child during the second year of life. These three species constitute a complete clade: species possessing a common immediate ancestor. A basic finding was the functional and formal similarity of many gestures between chimpanzee, bonobo, and human child. The child's symbols were spoken words; the apes' symbols were lexigrams - non-iconic visual signifiers. A developmental pattern in which gestural representation of a referent preceded symbolic representation of the same referent appeared in all three species (but was statistically significant only for the child). Nonetheless, across species, the ratio of symbol to gesture increased significantly with age. But even though their symbol production increased, the apes continued to communicate more frequently by gesture than by symbol. In contrast, by 15-18 months of age, the child used symbols more frequently than gestures. This ontogenetic sequence from gesture to symbol, present across the clade but more pronounced in child than ape, provides support for the role of gesture in language evolution. In all three species, the overwhelming majority of gestures were communicative (i.e., paired with eye contact, vocalization, and/or persistence). However, vocalization was rare for the apes, but accompanied the majority of the child's communicative gestures. This species difference suggests the co-evolution of speech and gesture after the evolutionary divergence of the hominid line. Multimodal expressions of communicative intent (e.g., vocalization plus persistence) were normative for the child, but less common for the apes. This species difference suggests that multimodal expression of communicative intent was also strengthened after hominids diverged from apes.

Keywords: communication development; cross-species comparisons; gestural theory of language evolution; gesture; language development; language-enculturated apes; symbolic development.

Figure 1

An example of a lexigram use (top) and an image of lexigram board (bottom). Lexigram use was defined as touching a lexigram while the referent was glossed by caregiver or electronic voice on lexigram board.

Figure 2

Examples of a reach gesture by an ape and the human. Reaches involve actively extending a limb toward a referent without contacting it. Left: ape example – Sue, Panbanisha’s primary attachment figure, has been holding Panbanisha. Someone new (Linda) wants to hold her. Linda takes Panbanisha (1 year, 9 days) who reaches for Sue in this frame. Linda walks away with Panbanisha who vocalizes loudly in protest. Right: human example – Dad throws balloon; GN (15 months, 26 days) vocalizes and reaches toward it. She turns toward videographer, then dad. “You can go get it,” he says; and she does.

Figure 3

Examples of a reach-touch gesture sequence (reach on left and touch on right of each pair). Reach-touch is the same as reach except that contact is made with the referent, but only after a response from the caregiver. Top: ape example-Carrying Panbanisha and Panpanzee, Sue says, “Tell us where you wanna go.” Panpanzee guides Sue by taking her hand. Then as they near car, Panbanisha (17 months, 26 days) gestures to it and Sue walks toward it till they can touch it. “Oh you wanted to go in Steve’s car,” she says, and they peer inside. Bottom: human example – Mom is holding GN (11 months, 7 days) and washing something in the sink. GN reaches toward Cheerios. Mom walks closer so GN can reach into box and get Cheerios.

Figure 4

Examples of a point gesture. Point involves extending an arm with the index finger extended toward an object without touching it. Left: ape example – The caregiver asks Panpanzee (22 months, 20 days) where she wants to go and she points toward the car. They walk toward the car. Right: human example – GN (13 months, 9 days) points at picture on fridge while vocalizing. “What do you see?” Mom asks. “There’s GD in a picture,” Mom continues. GN points again (not shown) and then turns to point at actual GD, her older brother, who is out of view.

Figure 5

An example of a point-touch gesture by an ape and a human. Point-touch is a point wherein the participant ends the point by touching the referent without moving her finger along it or manipulating it. Left: ape example – Sue, the caregiver, and Panpanzee were walking around Sue’s house, and Panpanzee pointed toward the picture. They walked to it, and Panpanzee (22 months, 20 days) point-touched it. Sue then also point-touched it and commented on the picture. Right: human example – Mom is reading book to GN (12 months, 6 days). GN gestures toward it, then point-touches moon in it. “Look, there’s the moon,” says Mom.

Figure 6

Examples of a head-point gesture by an ape and a human. Head-point involves indicating an object by using one’s head. Left – Ape example: Sue shows Panbanisha (22 months, 27 days) a chain of keys and asks her to pick a key to open the door. Panbanisha touches the keys with her face/head. Sue opens the door. Right – human example: mom holds up finger puppet, saying “See it’s a baby.” She pretends to give it a bottle. GN (14 months, 1 day) laughs and head-points it. “Yeah it’s a baby,” Mom says.

Figure 7

Examples of an up gesture. Up involves raising the arm/arms above the head with the implied intention of being picked up. Left: ape example – Panbanisha climbing on car. Sue, holding Panpanzee on shoulders, says “Panban, don’t do that.” Panbanisha (17 months, 26 days) gets down and comes to Sue with arm raised for up. Sue puts Panpanzee down. Panpanzee briefly shoves Panbanisha and scampers off. Then Sue picks Panbanisha up. Right: human example – GN and Mom are playing with Lego blocks. GN (11 months, 7 days) raises arms up. Mom helps her up.

Figure 8

Examples of a go gesture by an ape and a human. Go involves reaching and pointing when no referent is visible (even when the camera pans to give clear view of scene). Left: ape example – Rose is standing near a fence holding Panpanzee (21 months, 2 days). Panpanzee gestures to go. Rose walks in the direction gestured. Right: human example – Dad asks GN (15 months, 26 days) if she wants pasta. She says no and points go. He stands up and says “Let’s go.”

Figure 9

A comparison of communicative (as defined by eye contact, vocalization, or persistence) relative to non-communicative gestures across species.

Figure 10

A comparison of the use of eye contact, vocalization, or persistence when gesturing relative to total communicative gestures across species.

Figure 11

A cross-species comparison of the frequency of communicative gestures relative to symbols during the first and the second half of the study. (The bonobo produced two lexigrams in the first 7 months of the study, but, because of the necessary scale of the graphs to capture high frequency categories, they are not visible in the right-hand panel.)

Figure 12

A comparison of types of communicative gestures/symbols across species. Head-point is classified as other due to its infrequency. (The chimpanzee and bonobo each produced one point-go and one other gesture, but because of the necessary scale of the graphs to capture high frequency categories, they are not visible.)