A baby orangutan laughs while being tickled¹, in Sabah, Malaysian Borneo. Scientists say they've traced the origin of laughter into humankind's evolutionary² past.

When scientists set out to trace the roots of human laughter, some chimps³ and gorillas⁴ were just tickled to help.

That's how researchers made a variety of apes and some human babies laugh. After analyzing⁵ the sounds, they concluded that people and great apes inherited laughter from a shared ancestor that lived more than 10 million years ago.

Experts praised the work. It gives strong evidence that ape and human laughter are related through evolution, said Frans de Waal of the Yerkes National Primate⁶ Research Center at Emory University in Atlanta.

As far back as Charles Darwin, scientists have noted⁷ that apes make characteristic sounds during play or while being tickled, apparently⁸ to signal that they're interested in playing.

It's been suggested before that human laughter grew out of primate roots. But ape laughter doesn't sound like the human version. It may be rapid panting, or slower noisy breathing or a short series of grunts⁹.

So what does that have to do with the human ha-ha? To investigate that, Marina Davila Ross of the University of Portsmouth in England and colleagues carried out a detailed¹⁰ analysis of the sounds evoked¹¹ by tickling¹² three human babies and 21 orangutans, gorillas, chimps and bonobos.

After measuring 11 traits in the sound from each species, they mapped out how these sounds appeared to be related to each other. The result looked like a family tree. Significantly, that tree matched the way the species themselves are related, the scientists reported online yesterday in the journal Current Biology.

They also concluded that while human laughter sounds much different from the ape versions, its distinctive¹³ features could well have arisen from shared ancestral traits.

Jaak Panksepp of Washington State University, who studies laughter-like responses in animals but didn't participate in the new work, called the paper exciting.

Panksepp's own work concludes that even rats produce a version of laughter in response to play and tickling, with chirps¹⁴ too high-pitched for people to hear.

Robert Provine, a neuroscientist at the University of Maryland, Baltimore County, who wrote the book, Laughter: A Scientific Investigation¹⁵, said the new paper reveals some important insights, like ape sounds that hadn't been appreciated before.

当科学家准备探究人类笑声的起源时，被搔痒的大猩猩恰好帮了大忙。

科学家通过 “挠痒痒”这种方式来使不同种类的大猩猩和一些人类婴儿发出笑声。在分析了这些笑声之后，研究人员得出结论，人和大猩猩的笑声源自共同的祖先——他们生活在距今一千万年前。

专家称赞了这项研究成果。亚特兰大市埃默里大学耶基斯国家灵长动物研究中心的弗兰斯•德瓦尔称，这强有力地证明了在进化史上，猿和人的笑声密切相关。

早在查尔斯•达尔文时期，科学家就已经注意到，在玩耍或者被瘙痒后，猿类会发出特别的笑声，这似乎表明它们很喜欢玩耍。

以前曾有观点认为人类的笑声是从灵长类祖先进化而来的。但猿的笑声与人类的听起来不同，它有可能是急促的喘息声、较为缓慢且带有噪音的呼吸声或是短暂的呼噜声。

但这和人类的“哈哈”大笑有什么关系呢？为了研究这一点，英国朴茨茅斯大学的玛丽娜•达维拉•罗斯和同事仔细分析了三个人类婴儿和21个猩猩、大猩猩、黑猩猩、倭黑猩猩被搔痒后发出的声音。

在分析了每类笑声的11种特点后，研究人员绘制了笑声如何相互关联的图谱，图谱看起来像一个族谱。科学家在昨天发布于《当代生物学》网络版上的报告中称，这个族谱与各研究对象的内在关联不谋而合。

他们还总结称，尽管人类的笑声与猿类大不相同，但其特质却来源于共同的祖先。

华盛顿州立大学的雅克•潘克塞普专门研究动物的笑声反应，但没有直接参与这项新的研究。他称研究报告令人兴奋不已。

潘克塞普所做的研究成果也表明，即使老鼠在玩耍和被搔痒时也会笑，只是声调过高人听不到。

《笑的科学》一书的作者、马里兰大学巴尔的摩分校的神经学家罗伯特•普罗文称，新的研究报告揭示了一些重要线索，比如猿类的笑声，以前人们并没有意识到这些。

Vocabulary:

primate：灵长类动物

high-pitched：high in pitch, as a voice or musical tone.（声调高的，声音尖锐的）