September 13, 2008
By Bower, Bruce
Recent changes in hearing-related genes may have influenced language development Imagine, for a moment, that you are smaller than a speck of dust and in the mood for some teeny-tiny sightseeing. It's a perfect opportunity to take a scenic trip to the inner ear.
First, stroll up the ear canal. This is a fantasy, so no waxy buildup blocks the way. At the end of the fleshy tunnel, squeeze around the huge, circular membrane better known as the eardrum. Gingerly sidestep the precariously balanced, oddly shaped middle ear bones and proceed into the inner ear. Up ahead, rising like skyscrapers from a flat landscape, looms a cluster of stereocilia. These slender, interconnected projections sit atop the basic sensory elements of hearing- the inner ear hair cells. Bundles of gently waving stereocilia serve as receptacles for sound waves delivered from hair cells, transforming those waves into electrical signals that travel to the brain to be interpreted.
But the inner ear is more than just the mediator of hearing. As a core player in the human system for receiving and creating spoken language, it's a hotbed of recent evolutionary change as well.
In a new study, anthropologist John Hawks of the University of WisconsinMadison finds that eight hearing-related genes show signs of having evolved systematically in human populations over the past 40,000 years. Some alterations on these genes took root as recently as 2,000 to 3,000 years ago.
"Hawks makes a compelling case that not only is human evolution ongoing in the past 10,000 years, but it has sped up," says anthropologist Clark Larsen of Ohio State University in Columbus.
Seven genes identified by Hawks produce proteins that make stereocilia and the membrane that coats them. The eighth gene assists in building middle ear structures that transmit sound frequencies to the inner ear.
It all points to the evolutionary sensitivity of at least one part of the human language system in the post-Stone Age world, Hawks reported in April in Columbus at the annual meeting of the American Association of Physical Anthropologists. Language depends not just on a vocal tract capable of making certain speech sounds but on ears designed to hear particular sound frequencies, as well as on a variety of other brain and body features. Relatively recently in evolutionary history, genetic revisions within populations have upgraded ear structures needed for discerning what other people say, he proposes.
"It takes a long time for a biologically complex system like language to evolve," Hawks says. "We're still genetically adapting to language."
His findings challenge the influential idea that the way humans now talk emerged full-blown about 50,000 years ago thanks to a single genetic mutation that improved vocal articulation. Hawks' results instead play into a growing appreciation that rapid population growth toward the end of the Stone Age, followed by the rise of agriculture and village life around 10,000 years ago, triggered cultural changes that prompted genetic accommodations.
Speak up speedup
Speech-related genes must have succumbed to evolutionary pressures for improved communication in the expanding populations of the late Stone Age and at the dawn of farming, Hawks reasons.
He wanted to test whether certain genes that foster the ability to hear what others say might have become more common as more and more people lived year-round in one place.
To do that, Hawks analyzed a database of 3.9 million single nucleotide polymorphisms - regularly occurring variations of individual DNA letters within genetic sequences - that researchers had earlier identified in 90 Europeans, 90 Africans, 45 Chinese and 45 Japanese.
These single letter mutations are passed down as part of a larger chunk of DNA, a section of chromosome with a characteristic pattern of other nearby DNA alterations. These sections, along with their sets of mutations, break down over time due to the remixing of DNA that occurs each time a sperm and egg fuse during conception. It's thus possible to estimate how long ago a specific mutation arose based on the pattern of accompanying mutations.
Consider a gene necessary for forming filaments that join stereocilia into sound-transmitting bundles. A particular mutation of this gene appears frequently in Chinese and Japanese people and probably originated in the past 10,000 to 15,000 years, Hawks says.
Other common variants of genes required for making stereocilia occur either in Europeans or Africans. These DNA changes emerged as early as 40,000 years ago and as late as 2,000 years ago.
"I have no idea why certain variants show up in some populations and not in others," Hawks remarks.
It nonetheless appears that evolution has increasingly promoted genes that mediate the ability to hear speech sounds. Hawks suggests that as social life became more demanding in the late Stone Age, these particular gene variants must have aided survival and reproduction. People who inherited them may have developed special proficiency at detecting subtle emotions conveyed by a speaker's vocal tone or recognizing familiar voices in a chattering crowd.
Hawks initially suspected that the need to counteract hearing loss in aging populations spurred much recent evolution in genes involved in hearing. But to his surprise, genes that have been implicated in aging-related hearing loss showed no evidence of systematic change in the past 40,000 years.
In contrast, the systematic changes of the hearing- and speech- related genes echo those of a 2007 paper in which Hawks and his colleagues - using the same genetic database from Europeans, Africans and Asians - concluded that late- Stone Age population surges spurred abevy of rapid evolutionary changes. The pace of such changes has sped up ever since, the researchers propose.
About 1,800 genes, roughly 7 percent of the human total, show signs of survival-enhancing change in the past 80,000 years, Hawks' team estimates. Around 80,000 years ago, human groups left Africa and adapted to a series of new habitats and climates. In a second wave of change, the agricultural revolution reshaped physical and cultural environments with particular vigor.
That may explain the researchers' finding that gene variants related to several forms of disease resistance have spread through populations in different parts of the world. Agricultural groups witnessed sharp increases in mortality from contagious epidemic diseases, including smallpox, malaria, yellow fever and tuberculosis, Hawks notes.
Similarly, a switch to milk drinking by farmers fostered the spread of a gene variant involved in efficiently metabolizing lactose, a sugar found in milk. This lactose-tolerance gene flourished independently in Europe and in Africa, allowing its inheritors to drink milk without unpleasant side effects.
Other recent evolutionary changes defy explanation. For instance, Chinese, Japanese and Europeans display systematic alterations to a couple of serotonin transporter genes. These genes produce a protein necessary for regulating serotonin, a mood-related chemical messenger in the brain. Any emotional or behavioral consequences of tweaks to these genes remain unknown.
Although apparently adaptive mutations such as these arose in the past 10,000 years, some geneticists doubt that the agricultural revolution jump-started the pace of genetic evolution. Accurate techniques for identifying and dating the single DNA letter variants characteristic of certain populations are still being developed. Hawks and his associates' analysis may have missed many ancient instances of genetic evolution, leading them to overestimate the pace of recent evolution, remarks geneticist Sar ah Tishkoff of the University of Pennsylvania in Philadelphia.
"We have to be cautious in making inferences about the rate of selective change in human populations," she says.
Hawks sees a greater need for caution in evaluating the recent suggestion, based on fossil discoveries, that humanlike speech emerged long before modern Homo sapiens did about 200,000 years ago.
Throat and ear bones of a Neandertal ancestor that lived at least 530,000 years ago in northern Spain point to a remarkably advanced speech capacity, say paleontologist Ignacio Martinez of the University of Alcala near Madrid, Spain, and his colleagues.
In the January Journal of Human Evolution, the scientists describe two hyoid bones from this ancient Homo species, a possible common ancestor of later European Neandertals and modern humans. A cave in Spain's Atapuerca mountains yielded the fossils. The hyoid, a horseshoe-shaped bone in the neck, supports the tongue and larynx and is the only bone in the skeleton not connected to any other bone.
Some researchers regard hyoid shape in humans as an indicator of a vocal tract designed for speaking. Others argue that the hyoid contains no reliable clues to vocal tract functions.
What's beyond debate is that the Atapuerca hyoids look like those of people today, the researchers say. But hyoids from chimpanzees, gorillas and 3- to 4-millionyear-old human ancestors differ substantially from the Spanish fossils.
Human-looking hyoids alone don't prove that Atapuerca folk gabbed with one another a half million years ago. But add earlier finds of humanlike outer and middle ear structures in five skulls from the same site and the possibility of ancient speech at Atapuerca gains some traction, according to Martinez's group. Neandertal ancestors in northern Spain had ears specialized for transmitting midrange sound frequencies used in speech today, the Spanish scientists reported in 2004.
But skeletal similarities such as these tell a limited story about language evolution, Hawks argues. Recent genetic modifications to nonfossilizing stereocilia in the inner ear suggest that modern humans produce and hear speech differently than ancient Atapuercans did, in Hawks' view.
Anthropologist Robert McCarthy of Florida Atlantic University in Boca Raton agrees. McCarthy studies head and neck fossils in order to reconstruct the vocal tracts of ancient members of the human evolutionary family.
Neandertals and other Stone Age species must have spoken, only not as clearly as people now do, he says. At the physical anthropology meeting, McCarthy played synthesized re-creations of what a Neandertal speaker would have sounded like making vowel sounds, based on earlier reconstructions of vocal tracts.
The combination of a long face, short neck, unequally proportioned vocal tract and large nose would have decreased the intelligibility, at least to modern human ears, of Neandertals' vowel sounds, according to his analysis.
A transition to facial and neck traits needed for modern speech occurred in H. sapiens between 100,000 and 40,000 years ago, McCarthy estimates.
Even then, languages continued to change and assume different structures. For instance, Asian languages came to rely on musical- sounding tonal shifts for meaning and some African tribes adopted clicking sounds in place of vowels.
In a volatile linguistic world, it makes sense to McCarthy that language-related hearing genes still attract evolutionary adjustments. "Hawks has done a nice job of showing that all kinds of genes have been changing at a faster rate in the last 50,000 years, so it's not surprising that genes related to hearing are on that bandwagon," he says.
The biggest bandwagon in human evolutionary history started rolling around 10,000 years ago, when sedentary farming rapidly replaced nomadic hunting and foraging, the anthropologists agree. On the plus side, the spread of agriculture and animal domestication provided enough food for growing populations. On the minus side, farming's rise caused many people to suffer marked declines in health and well-being, says Ohio State's Larsen.
Larsen's unpleasant picture of the agricultural lifestyle stems from analyses of human skeletons excavated at a variety of prehistoric farming villages. Crowds of villagers acted as petri dishes for infectious diseases. Water sources near villages became contaminated by parasites, which also infected people. Infants and young children died more frequently than they had at the height of the Stone Age.
A newfound emphasis on eating domesticated plants prompted deficiencies in nutrients formerly obtained from meat, such as iron, zinc, vitamin A and vitamin B12. As a result, people grew increasingly smaller and shorter. Size reductions of the face and jaw outpaced those of the teeth, often resulting in dental misalignments. To top it off, competition for fertile land encouraged the rise of organized warfare and mass slaughters.
A brave new agricultural world of deteriorating health must have provoked unprecedented adaptive changes in human genes, Larsen asserts.
Recent genetic evolution bolstered immune defenses in response to epidemic diseases that ravaged farming populations, hypothesizes anthropologist George Armelagos of Emory University in Atlanta.
Hawks agrees but adds that postStone Age evolution has not been solely a race against death and disease. The need to communicate in groups of expanding size and complexity sparked genetic changes in humans' ability to hear what others say, he maintains.
As genetic data from people around the world continue to accumulate, Hawks will keep his eye on the ear for further signs of language evolution. If you know how to listen, even DNA can talk.
This scanning electron micrograph shows sensory hair cells in the cochlea of the inner ear. As sound enters the ear, waves cause the stereocilia (orange) on the hair cells to move. This movement is converted to an electrical signal, which passes to the brain.
The modern hyoid bone, found in the human neck, supports the root of the tongue. The bone In the skeleton (left) Is held up by wires since It Is not connected to any other bones. The fossilized hyoid bones (below) belong to an early Homo species. Found In Atapuerca, Spain, they date back to the Stone Age.
* John Hawks et al. "Recent acceleration of human adaptive evolution."
Proceedings of the National Academy of Sciences. December 26, 2007.
Copyright Science Service, Incorporated Aug 30, 2008
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