Read more: Instant Expert: The evolution of language
There are no fossils of words. So how can we find out about the evolution of language?
Humans have wondered about the origins of our unique capacity for language since the beginnings of history, proffering countless mythic explanations. Scientific study began in 1871 with Darwin’s writings on the topic in The Descent of Man. For nearly a century afterwards, however, most writing on the subject was highly speculative and the entire issue was viewed with distrust by reputable scholars.
Advertisement
Recently, we have moved towards specific, testable hypotheses. Because language does not fossilise, only indirect evidence about key past events is available. But the situation is no worse off in this respect than cosmology or many other mature empirical sciences and, as with these other disciplines, scientists studying the evolution of language now combine many sources of data to constrain their theories.
One of the most promising approaches compares the linguistic behaviour of humans with the communication and cognition of other animals, which highlights shared abilities and the characteristics that make human language unique. The comparisons allow us to build theories about how these individual traits might have evolved.
LINGUISTS define language as any system which allows the free and unfettered expression of thoughts into signals, and the complementary interpretation of such signals back into thoughts. This sets human language apart from all other animal communication systems, which can express just a limited set of signals. A dog’s barks, for example, may provide important information about the dog (how large or excited it is) or the outside world (that an intruder is present), but the dog cannot relate the story of its puppyhood, or express the route of its daily walk.
For all its uniqueness, human language does share certain traits with many animal communication systems. A vervet monkey, for example, produces different calls according to the predators it encounters. Other vervets understand and respond accordingly – running for cover when a call signals an “eagle”, for example, and scaling the trees when it makes a “leopard” call. This characteristic, known as functional referentiality, is an important feature of language. Unlike human languages, however, the vervets’ system is innate rather than learned. This makes their system inflexible, so they cannot create a new alarm call to represent a human with a gun, for example. What’s more, vervets do not seem to intentionally transmit novel information: they will continue producing leopard calls even when their whole group has moved to the safety of the trees. Thus, although the vervet communication system shares one important trait with human language, it still lacks many other important features.
Building time lines
Similarly, the honeybees’ complex dance routine offers some parallels with human language. By moving in certain ways, bees can communicate the location of distant flowers, water and additional hive sites to their hivemates – a system that is clearly functionally referential. More importantly, the bees are also communicating about things that aren’t present. Linguists call this characteristic “displacement”, and it is very unusual in animal communication – even vervets can’t do this. Nonetheless, since bees can’t communicate the full range of what they know, such as the colour of a flower, their system cannot be considered a language.
Looking at shared traits helps biologists to work out how those traits might have first evolved. Different animals might exhibit the same features simply because a common ancestor had the trait, which then persisted throughout the course of evolution. Such traits are called “homologies”. Obvious examples include hair in mammals or feathers in birds. Alternatively, similar traits can evolve independently without being present in a common ancestor, a process called convergent evolution. The emergence of wings in both birds and bats is an example of this kind of evolution, as is the displacement seen in the bee’s dance and human language.
Homologies allow us to build a time line of when different features first evolved. The fact that fish, mammals, birds, reptiles and amphibians all have skeletons, for example, suggests that bones evolved before lungs, which most fish lack but the other groups all share. Comparing creatures with convergent traits, by contrast, helps identify the common selection pressures that might have pushed the different species to evolve the trait independently.
This “comparative approach” has been instrumental in understanding where our abilities to learn, understand and produce new words came from. Together, these distinct traits allow free expression of new thoughts, so they are fundamental to human language, but they are not always present in other types of animal communication. Which other creatures share these abilities, and why?
The ability to learn to understand new signals is the most common. Typical dogs know a few words, and some unusual dogs like Rico, a border collie, can remember hundreds of names for different objects. The bonobo Kanzi, who was exposed from an early age to abundant human speech, can also understand hundreds of spoken words, and even notice differences in word order. This suggests that learning to understand new signals is widespread and broadly shared with most other mammals – a homology. But neither Kanzi nor Rico ever learned to produce even a single spoken word, as they lack the capacity for complex vocal learning.
“The ability to learn the meaning of new signals is widespread in the animal kingdom”
Many other species do have this ability, however. Almost everyone has seen a talking parrot, but there are more unusual examples. Hoover, an orphaned seal raised by fishermen, learned to produce whole English sentences with a Maine accent, and scientists have uncovered complex vocal learning in a wide variety of other species, including whales, elephants and bats. The fact that close relatives of these animals lack vocal learning indicates that this trait is an example of convergent evolution. Crucially, most animals who learn to speak do not understand the meaning of what they say. Hoover mostly directed his sentences at female seals during the mating period, for example, suggesting that vocal production and meaning recognition are two distinct traits that use different neural machinery. Only with specific training can animals learn to both produce and appropriately interpret words. Alex, the African gray parrot (pictured, left) of psychologist Irene Pepperberg, provides one example of a bird that used words for shapes, colours and numbers meaningfully.
In terms of creating a time line for human evolution, the evidence suggests our ability to recognise sounds – the homology – probably arose in a mammalian common ancestor, while our ability to produce complex sounds arose more recently in prehistory. Even more importantly, studying the various convergent examples of vocal learning have uncovered what was necessary for one important aspect of human language: speech.
Three kinds of evolution
Language develops through time at three different rates, all of which have sometimes been termed “language evolution”. The fastest process is ontogeny, in which an initially language-less baby becomes an adult native speaker. Then there’s glossogeny: the historical development of languages. This guide to language evolution, however, focuses on human phylogeny: the biological changes that occurred during the last 6 million years of our lineage through which our species Homo sapiens evolved from an initially language-less primate.