Read more: Instant Expert: The evolution of language
Speech comes so easily to adult humans that it鈥檚 easy to forget the sheer amount of muscular coordination needed to produce even the most basic sounds
Speech comes so easily to adult humans that it鈥檚 easy to forget the sheer amount of muscular coordination needed to produce even the most basic sounds. How we came to have this ability, when most other animals find it so difficult, is one of the key questions in language evolution 鈥� and one of the few that has yielded to empirical studies.
Advertisement
SPEECH is just one aspect of human language, and is not even strictly necessary, since both sign language and written language are perfectly adequate for the unfettered expression of thought. However, since it is the normal medium of language in all cultures, it is reasonable to assume that its emergence must have represented a big step in the evolution of language.
Because no other apes apart from us can learn to speak, some change must have occurred after we diverged from chimpanzees, about 6 to 7 million years ago. The nature of the change has been somewhat unclear. Darwin suggested two possible explanations: either it was a change in our vocal apparatus, or there is a key difference in the brain. In each case, biologists have gained fundamental insights by examining other animals.
Let鈥檚 start with anatomy. Humans have an unusual vocal tract: the larynx (or voicebox) rests low in the throat. In most other mammals, including chimpanzees, the larynx lies at a higher point, and is often inserted into the nasal passage, creating a sealed nasal airway. In fact, humans begin life this way: a newborn infant can breathe through its nose while swallowing milk through its mouth. But as the infant grows, the larynx descends, and by the age of 3 or 4 this feat is no longer possible.
The reconfigured human vocal tract allows the free movement of the tongue that is crucial to make the many distinct sounds heard in human languages. For a long time, the descended larynx was considered unique to our species, and the key to our possession of speech. Researchers had even tried to place a date on the emergence of language by studying the position of the larynx in ancient fossils.
鈥淎 lower larynx allows the free movement of the tongue that is crucial to make complex sounds鈥�
Evidence from two different sources of comparative data casts doubt on this hypothesis. The first was the discovery of animal species with permanently descended larynges like our own. We now know that lions, tigers, koalas and Mongolian gazelles all have a descended larynx 鈥� making it a convergent trait. Since none of these species produce anything vaguely speech-like, such changes in anatomy cannot be enough for speech to have emerged.
The second line of evidence is even more damning. X-ray observations of vocalising mammals show that dogs, monkeys, goats and pigs all lower the larynx during vocalisation. This ability to reconfigure the vocal tract appears to be a widespread, and probably homologous, feature of mammals. With its larynx retracted, a dog or a monkey has all the freedom of movement needed to produce many different vocalisations (see diagram). The key changes must therefore have occurred in the brain instead.
Direct connections
The human brain is enormously complex, and differs in many ways from that of other animals. We expect different neural changes to underlie each of the different components of language, like syntax, semantics and speech. Others presumably underlie abilities like improved tool use or increased intelligence. Determining the specific neural changes that correspond to particular capabilities is often very difficult, and in many cases we don鈥檛 even have good guesses about what changes were needed.
Biologists have been more fortunate when studying the neural machinery of speech, however. Motor neurons that control the muscles involved in vocalisation 鈥� in the lips, the tongue and the larynx 鈥� are located in the brainstem, and after decades of painstaking research we now know that humans have direct neural connections between the motor cortex and these brainstem neurons which nonhuman primates lack. Could these direct neural connections explain our enhanced ability to control and coordinate the movements necessary for speech? The explanation seemed plausible. Fortunately, we can test the hypothesis with the help of other species that exhibit complex vocal learning.
If direct neural connections are necessary for vocal learning, we predict they should appear in other vocal learning species. For birds at least, this prediction appears to hold true: parrots or songbirds have the connection while chickens or pigeons, which are not vocal learners, lack them. For many vocal learning species, including whales, seals, elephants and bats, we don鈥檛 know, because their neuroanatomy has yet to be fully investigated, providing untapped sources to test the 鈥渄irect connections鈥� hypothesis.
An ability to produce the correct sounds for speech is one thing, but complex vocal control in humans also relies on our ability to control the different articulators in the correct, often complicated, sequences. The discovery of the FOXP2 gene has recently provided insights into the origins of this ability (New 杏吧原创, 16 August 2008, p 38). Modern humans all share a novel variant of this gene which differs from the one most primates have, and disruptions of this gene in people create severe speech difficulties. But what does it do? Various studies have found that the gene seems to be crucial for memory formation in the basal ganglia and cerebellum, which are involved in coordinating the patterns of movements that are crucial for our complex vocalisations. Recently, fossil DNA recovered from Neanderthals has shown that they shared the modern variant, suggesting that they already possessed complex speech.
Speech is just one component of language, though, and similar questions must be asked about syntax and semantics before we can hope to understand the evolution of language as a whole.