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LDT “told people, basically, don’t bother to go look” for speech abilities in anything other than modern humans, says Thomas Sawallis, one of the authors of the new paper. Those speech abilities could include distinct vowels and consonants, syllables, or even syntax—all of which, according to LDT, should be impossible for any animal without a human vocal tract. “There was always this idea,” says Greg Hickok, a cognitive-science professor at the University of California at Irvine who was not involved in the study, “that there was one thing that had to happen and that released the linguistic abilities.” For Noam Chomsky and his followers, that thing was the invention of syntax. For proponents of LDT, it was the reshaping of the human throat.
Part of the reason LDT caught on to begin with is that language evolution, as a field, lacks concrete data. As John Locke, a linguistics professor at Lehman College, put it, “Motor control rots when you die.” Soft tissues like tongues and nerves and brains generally don’t fossilize; DNA sequencing is impossible past a few hundred thousand years; no one has yet found a diary or rap track recorded by a teenage Australopithecus. So the anatomical argument presented by LDT gave researchers something to latch on to. Until the ’60s, people who studied language evolution “were considered crackpots because they didn’t have any data,” Locke says. “When you’ve got nothing on the table, a little something goes a long ways.”
The researcher generally credited with developing laryngeal descent theory is Philip Lieberman, now a professor at Brown University. He called the new paper “just a complete misrepresentation of the entire field,” among other things. One of the quantitative models the new study relies on, he says, doesn’t properly represent the shape of the larynx, tongue, and other parts we use to talk: “It would convert a mailing tube into a human vocal tract.” And according to Lieberman, laryngeal descent theory “never claimed language was not possible” prior to the critical changes in our ancestors’ throat anatomy. “They’re trying to set up a straw man,” he said.
Yet other experts I spoke with told me that setting an upper bound on when speech, and therefore language, could have possibly evolved was exactly the effect that LDT had on anyone studying language evolution. Hickok said that when he was being trained in linguistics, “this was an established, almost dogmatic idea.” The new study is a dramatic reversal of the status quo, he said: “The phrase that came to mind when I finished it was mic drop.”
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Still, he doesn’t agree entirely with Sawallis and his co-authors’ conclusions. Rather than 27 million years, Hickok proposes that the earliest bound on any sort of speech ability would be nearer to human ancestors’ split with the Pan genus, which includes chimpanzees and bonobos, our closest living relatives. That split happened about 5 million to 7 million years ago—certainly longer than 200,000 years, but a far cry from 27 million. Lieberman argues that the precursors of speech might have emerged about a little more than 3 million years ago, when artifacts like jewelry appear in the archaeological record. The idea is that both language and jewelry are intimately related to the evolution of symbolic thinking.