By Christine Daniloff, phys.com, February 21, 2013
"The sounds
uttered by birds offer in several respects the nearest analogy to
language," Charles Darwin wrote in "The Descent of Man" (1871),
while contemplating how humans learned to speak. Language, he speculated, might
have had its origins in singing, which "might have given rise to words
expressive of various complex emotions."
Now researchers
from MIT, along with a scholar from the University of Tokyo, say that Darwin
was on the right path. The balance of evidence, they believe, suggests that human language is a grafting of two communication forms found
elsewhere in the animal kingdom:
first, the elaborate songs of birds, and second, the more utilitarian,
information-bearing types of expression seen in a diversity of other animals.
"It's this
adventitious combination that triggered human language," says Shigeru
Miyagawa, a professor of linguistics in MIT's Department of Linguistics and
Philosophy, and co-author of a new paper published in the journal Frontiers in
Psychology.
The idea builds
upon Miyagawa's conclusion, detailed in his previous work, that there are two
"layers" in all human languages: an "expression" layer,
which involves the changeable organization of sentences, and a
"lexical" layer, which relates to the core content of a sentence. His
conclusion is based on earlier work by linguists including Noam Chomsky,
Kenneth Hale and Samuel Jay Keyser.
Based on an
analysis of animal communication, and using Miyagawa's framework, the authors
say that birdsong closely resembles the expression layer of human
sentences—whereas the communicative waggles of bees, or the short, audible
messages of primates, are more like the lexical layer. At some point, between
50,000 and 80,000 years ago, humans may have merged these two types of
expression into a uniquely sophisticated form of language.
"There were
these two pre-existing systems," Miyagawa says, "like apples and
oranges that just happened to be put together."
These kinds of
adaptations of existing structures are common in natural history, notes Robert
Berwick, a co-author of the paper, who is a professor of computational
linguistics in MIT's Laboratory for Information and Decision Systems, in the
Department of Electrical Engineering and Computer Science.
"When
something new evolves, it is often built out of old parts," Berwick says.
"We see this over and over again in evolution. Old structures can change
just a little bit, and acquire radically new functions."
A new chapter in
the songbook
The new paper,
"The Emergence of Hierarchical Structure in Human Language," was
co-written by Miyagawa, Berwick and Kazuo Okanoya, a biopsychologist at the
University of Tokyo who is an expert on animal communication.
To consider the
difference between the expression layer and the lexical layer, take a simple
sentence: "Todd saw a condor." We can easily create variations of
this, such as, "When did Todd see a condor?" This rearranging of
elements takes place in the expression layer and allows us to add complexity
and ask questions. But the lexical layer remains the same, since it involves
the same core elements: the subject, "Todd," the verb, "to
see," and the object, "condor."
Birdsong lacks a
lexical structure. Instead, birds sing learned melodies with what Berwick calls
a "holistic" structure; the entire song has one meaning, whether
about mating, territory or other things. The Bengalese finch, as the authors
note, can loop back to parts of previous melodies, allowing for greater
variation and communication of more things; a nightingale may be able to recite
from 100 to 200 different melodies.
By contrast, other
types of animals have bare-bones modes of expression without the same melodic
capacity. Bees communicate visually, using precise waggles to indicate sources
of foods to their peers; other primates can make a range of sounds, comprising
warnings about predators and other messages.
Humans, according
to Miyagawa, Berwick and Okanoya, fruitfully combined these systems. We can
communicate essential information, like bees or primates—but like birds, we
also have a melodic capacity and an ability to recombine parts of our uttered
language. For this reason, our finite vocabularies can generate a seemingly
infinite string of words. Indeed, the researchers suggest that humans first had
the ability to sing, as Darwin conjectured, and then managed to integrate
specific lexical elements into those songs.
"It's not a
very long step to say that what got joined together was the ability to
construct these complex patterns, like a song, but with words," Berwick
says.
As they note in the
paper, some of the "striking parallels" between language acquisition
in birds and humans include the phase of life when each is best at picking up
languages, and the part of the brain used for language. Another similarity,
Berwick notes, relates to an insight of celebrated MIT professor emeritus of
linguistics Morris Halle, who, as Berwick puts it, observed that "all
human languages have a finite number of stress patterns, a certain number of
beat patterns. Well, in birdsong, there is also this limited number of beat
patterns."
Birds, bees—and
dolphins?
Norbert Hornstein,
a professor of linguistics at the University of Maryland, says the paper has
been "very well received" among linguists,
and "perhaps will be the standard go-to paper for language-birdsong
comparison for the next five years."
Hornstein adds that
he would like to see further comparison of birdsong
and sound production in human language, as well as more neuroscientific
research, pertaining to both birds and humans, to see how brains are structured
for making sounds.
The researchers
acknowledge that further empirical studies on the subject would be desirable.
"It's just a
hypothesis," Berwick says. "But it's a way to make explicit what
Darwin was talking about very vaguely, because we know more about language
now."
Miyagawa, for his
part, asserts it is a viable idea in part because it could be subject to more
scrutiny, as the communication patterns of other species are examined in
further detail. "If this is right, then human language has a precursor in
nature, in evolution, that we can actually test today," he says, adding
that bees, birds and other primates
could all be sources of further research insight.
MIT-based research
in linguistics has largely been characterized by the search for universal
aspects of all human languages. With this paper, Miyagawa, Berwick and Okanoya
hope to spur others to think of the universality of language in evolutionary
terms. It is not just a random cultural construct, they say, but based in part
on capacities humans share with other species. At the same time, Miyagawa
notes, human language is unique, in that two independent systems in nature
merged, in our species, to allow us to generate unbounded linguistic possibilities,
albeit within a constrained system.
"Human
language is not just freeform, but it is rule-based," Miyagawa says.
"If we are right, human language has a very heavy constraint on what it
can and cannot do, based on its antecedents in nature."
More information: www.frontiersin.org/Language_Sciences/10.3389/fpsyg.2013.00071/abstract
Provided by
Massachusetts Institute of Technology
This story is
republished courtesy of MIT News (web.mit.edu/newsoffice/), a popular site that covers news about MIT research, innovation and
teaching.
"How
human language could have evolved from birdsong." February 21st, 2013. http://phys.org/news/2013-02-human-language-evolved-birdsong.html
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