Human Teeth Likely Shrank Due to Tool Use

Wisdom teeth may have shrunk during human evolution says a new finding that could lead to a new way of figuring out how closely related fossil species are to modern humans.

Wisdom teeth may have shrunk during human evolution as part of changes that started with human tool use, according to a new study.

The research behind this finding could lead to a new way of figuring out how closely related fossil species are to modern humans, scientists added.

Although modern humans are the only surviving members of the human family tree, other species once lived on Earth. However, deducing the relationships between modern humans and these extinct hominins - humans and related species dating back to the split from the chimpanzee lineage - is difficult because fossils of ancient hominins are rare. [Image Gallery: Our Closest Human Ancestor]

Photos: Faces of Our Ancestors

Teeth are the hominin fossils most often found because they are the hardest parts of the human body. "Teeth are central to how a fossil ancestor lived, and can tell us about which species they belonged to, how they are related to other species, what they ate, and how quickly or slowly they developed during childhood," said lead study author Alistair Evans, an evolutionary biologist at Monash University in Melbourne, Australia.

Hominin teeth have shrunk in size throughout evolution, a trend perhaps most clearly seen with the wisdom teeth located at the back of the mouth, the researchers said. In modern humans, wisdom teeth are often very small or do not even develop, while in many other hominin species they were huge, with chewing surfaces two to four times larger than those of their modern human counterparts.

Previous research suggested this profound shrinking in modern human wisdom tooth size was due to the advent of cooking or other changes in diet unique to modern humans. However, Evans and his colleagues now suggest this shift may have begun much earlier in human evolution.

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The scientists analyzed tooth size in modern humans and fossil hominins. They found that hominin teeth fell into two major groups. One group was composed of the genus Homo, which includes both modern humans and extinct human relatives. The other group was made up of early hominins preceding Homo, such as the australopiths, the first primates to walk on two feet.

In australopiths and other early hominins, the scientists found that teeth tended to get bigger toward the back of the mouth, with proportions that stayed constant regardless of the overall size of the teeth. However, in the genus Homo, the smaller all the teeth were, the smaller the teeth were toward the back of the mouth.

"There seems to be a key difference between the two groups of hominins - perhaps one of the things that defines our genus Homo," Evans said in a statement.

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This change in how teeth developed between genus Homo and earlier hominins may have occurred due to the advent of advanced tool use in the genus Homo, Evans said.

"It's always been presumed that sometime in early Homo, we started using more advanced tools," Evans told Live Science. "Tool use meant we didn't need as big teeth and jaws as earlier hominins. This may then have increased evolutionary pressure to spend less energy developing teeth, making our teeth smaller."

In modern humans, tooth-size reduction has reached the point where wisdom teeth are increasingly failing to develop, Evans said. "The advent of cooking made food easier to eat, meaning we didn't need big teeth as much," Evans said.

Prior work suggested there was a lot of variation in how teeth evolved in hominins. "Now we're seeing some very simple, clear patterns in hominin tooth evolution instead," Evans said. [Infographic: Human Origins – How Hominids Evolved]

These patterns could help researchers decide whether ancient hominins were members of genus Homo or not, Evans said.

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"It's been suggested a number of times over the past 20 years that maybe Homo habilis, often considered the earliest member of Homo, should be considered an australopith instead," Evans said. "We found Homo habilis tooth proportions followed the australopith rule and not the Homo rule, which supports the argument that Homo habilis should be reclassified to something like Australopithecus habilis."

This new work builds on previous experiments with mice that suggested teeth could influence each other during development. In this "inhibitory cascade model," teeth that develop early can inhibit the size of teeth that develop later. These new findings suggest this mechanism underlying tooth size in mice and most mammals is seen in hominins as well, Evans said.

These findings suggest that by knowing the size of a single hominin tooth and the group to which it belongs, scientists could infer the size of the hominin's remaining teeth with considerable accuracy. "Sometimes we find only a few teeth in a fossil," Evans said. "With our new insight, we can reliably estimate how big the missing teeth were."

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Future research could analyze controversial hominin discoveries such as Homo naledi, recently unearthed in South Africa, Evans said. "It's got an interesting mix of traits, some that look like Homo, some that look australopith," Evans said. "It'd be interesting to examine its teeth and see which pattern it fits best."

The scientists detailed their findings in the Feb. 25 issue of the journal Nature.

Original article on Live Science.

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Back in the Beginning

To put a human face on our ancestors, scientists from the Senckenberg Research Institute used sophisticated methods to form 27 model heads based on tiny bone fragments, teeth and skulls collected from across the globe. The heads are on display for the first time together at the Senckenberg Natural History Museum in Frankfurt, Germany. This model is Sahelanthropus tchadensis, also nicknamed "Toumai," who lived 6.8 million years ago. Parts of its jaw bone and teeth were found nine years ago in the Djurab desert in Chad. It's one of the oldest hominid specimens ever found.

Australopithecus afarensis

With each new discovery, paleoanthropologists have to rewrite the origins of man's ancestors, adding on new branches and tracking when species split. This model was fashioned from pieces of a skull and jaw found among the remains of 17 pre-humans (nine adults, three adolescents and five children) which were discovered in the Afar Region of Ethiopia in 1975. The ape-man species, Australopithecus afarensis, is believed to have lived 3.2 million years ago. Several more bones from this species have been found in Ethiopia, including the famed "Lucy," a nearly complete A. afarensis skeleton found in Hadar.

Australopithecus africanus

Meet "Mrs. Ples," the popular nickname for the most complete skull of an Australopithecus africanus, unearthed in Sterkfontein, South Africa in 1947. It is believed she lived 2.5 million years ago (although the sex of the fossil is not entirely certain). Crystals found on her skull suggest that she died after falling into a chalk pit, which was later filled with sediment. A. africanus has long puzzled scientists because of its massive jaws and teeth, but they now believe the species' skull design was optimal for cracking nuts and seeds.

Paranthropus aethiopicus

The skull of this male adult was found on the western shore of Lake Turkana in Kenya in 1985. The shape of the mouth indicates that he had a strong bite and could chew plants. He is believed to have lived in 2.5 million years ago and is classified as Paranthropus aethiopicus. Much is still unknown about this species because so few reamins of P. aethiopicus have been found.

Paranthropus boisei

Researchers shaped this skull of "Zinj," found in 1959. The adult male lived 1.8 million years ago in the Olduvai Gorge of Tanzania. His scientific name is Paranthropus boisei, though he was originally called Zinjanthropus boisei -- hence the nickname. First discovered by anthropologist Mary Leakey, the well-preserved cranium has a small brain cavity. He would have eaten seeds, plants and roots which he probably dug with sticks or bones.

Homo rudolfensis

This model of a sub-human species -- Homo rudolfensis -- was made from bone fragments found in Koobi Fora, Kenya, in 1972. The adult male is believed to have lived about 1.8 million years ago. He used stone tools and ate meat and plants. H. Rudolfensis' distinctive features include a flatter, broader face and broader postcanine teeth, with more complex crowns and roots. He is also recognized as having a larger cranium than his contemporaries.

Homo ergaster

The almost perfectly preserved skeleton of the "Turkana Boy" is one of the most spectacular discoveries in paleoanthropology. Judging from his anatomy, scientists believe this Homo ergaster was a tall youth about 13 to 15 years old. According to research, the boy died beside a shallow river delta, where he was covered by alluvial sediments. Comparing the shape of the skull and teeth, H. ergaster had a similiar head structure to the Asian Homo erectus.

Homo heidelbergensis

This adult male, Homo heidelbergensis, was discovered in in Sima de los Huesos, Spain in 1993. Judging by the skull and cranium, scientists believe he probably died from a massive infection that caused a facial deformation. The model, shown here, does not include the deformity. This species is believed to be an ancestor of Neanderthals, as seen in the shape of his face. "Miquelon," the nickname of "Atapuerca 5", lived about 500,000 to 350,000 years ago and fossils of this species have been found in Italy, France and Greece.

Homo neanderthalensis

The "Old Man of La Chapelle" was recreated from the skull and jaw of a Homo neanderthalensis male found near La Chapelle-aux-Saints, in France in 1908. He lived 56,000 years ago. His relatively old age, thought to be between 40 to 50 years old, indicates he was well looked after by a clan. The old man's skeleton indicates he suffered from a number of afflictions, including arthritis, and had numerous broken bones. Scientists at first did not realize the age and afflicted state of this specimen when he was first discovered. This led them to incorrectly theorize that male Neanderthals were hunched over when they walked.

Homo floresiensis

The skull and jaw of this female "hobbit" was found in Liang Bua, Flores, Indonesia, in 2003. She was about 1 meter tall (about 3'3") and lived about 18,000 years ago. The discovery of her species, Homo floresiensis, brought into question the belief that Homo sapiens was the only form of mankind for the past 30,000 years. Scientists are still debating whether Homo floresiensis was its own species, or merely a group of diseased modern humans. Evidence is mounting that these small beings were, in fact, a distinct human species.

Homo sapiens

Bones can only tell us so much. Experts often assume or make educated guesses to fill in the gaps in mankind's family tree, and to develop a sense what our ancestors may have looked like. Judging from skull and mandible fragments found in a cave in Israel in 1969, this young female Homo sapien lived between 100,000 and 90,000 years ago. Her bones indicate she was about 20 years old. Her shattered skull was found among the remains of 20 others in a shallow grave.