Speech is a human hallmark, but its evolutionary origins continue to defy scientific explanation. Recently, the open-close mouth rhythm of 2-7 Hz (cycles/second) characteristic of all spoken languages has been identified in the orofacial signals of several nonhuman primate genera, including orangutans, but evidence from any of the African apes remained missing. Evolutionary continuity for the emergence of speech is, thus, still inconclusive. To address this empirical gap, we investigated the rhythm of chimpanzee lip-smacks across four populations (two captive and two wild). We found that lip-smacks exhibit a speech-like rhythm at approximately 4 Hz, closing a gap in the evidence for the evolution of speech-rhythm within the primate order. We observed sizeable rhythmic variation within and between chimpanzee populations, with differences of over 2 Hz at each level. This variation did not result, however, in systematic group differences within our sample. To further explore the phylogenetic and evolutionary perspective on this variability, inter-individual and inter-population analyses will be necessary across primate species producing mouth signals at speech-like rhythm. Our findings support the hypothesis that speech recruited ancient primate rhythmic signals and suggest that multi-site studies may still reveal new windows of understanding about these signals' use and production along the evolutionary timeline of speech.

中文翻译：

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黑猩猩的口罩证实了灵长类动物在语音节奏进化中的连续性。

语音是人类的标志，但其进化起源仍无视科学解释。最近，在几种非人类灵长类属（包括猩猩）的口齿信号中，已经确定了所有口语特征性的2-7 Hz（周期/秒）的张口节奏，但仍缺少任何非洲猿类的证据。因此，语音出现的进化连续性仍然没有定论。为了解决这一经验差距，我们调查了四个种群（两个圈养和两个野生种群）中黑猩猩唇ze的节律。我们发现，口罩在大约4 Hz处表现出类似语音的节奏，从而弥补了灵长类动物内语音节奏演变证据的空白。我们观察到黑猩猩种群内部和种群之间的节奏变化很大，每个级别的差异都超过2 Hz。但是，这种差异并未导致我们样本中系统的群体差异。为了进一步探讨这种变异的系统发育和进化观点，有必要对灵长类物种产生类似语音节奏的口信号的个体间和种群间分析。我们的发现支持了语音招募了古老的灵长类动物有节奏的信号这一假设，并表明，多点研究仍可能沿着语音的进化时间轴揭示对这些信号的使用和产生的新认识。跨灵长类物种需要进行个体间和种群间分析，以语音般的节奏产生口信号。我们的发现支持了语音招募了古老的灵长类动物节律信号的假设，并表明，多地点研究仍可能沿着语音的进化时间轴揭示对这些信号的使用和产生的新认识。跨灵长类物种需要进行个体间和种群间分析，以语音般的节奏产生口信号。我们的发现支持了语音招募了古老的灵长类动物有节奏的信号这一假设，并表明，多点研究仍可能沿着语音的进化时间轴揭示对这些信号的使用和产生的新认识。