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Skull shocker: a 7-million-year-old skull has scientists asking "who is it?" (Earth/life science: fossils/hominids).

When it lived, the creature was probably just another small flat-faced animal dwelling on the wooded shore of a giant lake. But now--7 million years after death--its remains are shaking the human family tree clear to the roots. A skull, discovered last year in the Djurab Desert of Chad, Africa, is arguably the most momentous fossil (remains preserved as rock) find of the past 75 years.

Michel Brunet and his international team of fossil hunters scoured vast tracts of Chad's hostile desert for 10 years before unearthing the find of a lifetime: a cracked brown skull, two lower-jaw fragments, and three teeth. After scrutinizing the specimens for months, Brunet--a French paleoanthropologist (human-origins expert)--boldly concluded his fossil wasn't an ancient ape but an early human. "It's a hominid--the oldest hominid," he says. A hominid is a member of the family that includes modern humans (Homo sapiens) and their extinct ancestors and cousins, but not apes.

Scientists classify humans in the primate order, along with 200 species of monkeys and apes (see chart, p. 17). Modern humans' unique features include their upright walk, spoken language, advanced use of tools and fire, small canine teeth, and level of intelligence. "Human ancestors are recognizable because they have a few of these features, in particular upright posture and small canines," says Harvard University paleontologist David Pilbeam.

To determine if the skull belonged to a hominid, Brunet's team obsessed over its every detail (see diagram, p. 16). Because the skull resembled no known species, the discoverers classified it in a new genus and species, naming it Sahelanthropus tchadensis. But the fossil is nicknamed Toumai, meaning "hope of life" in a Chadian language.

A few scientists say Toumai might be an ancient ape. But Brunet holds his ground: "I'm completely confident this is a hominid." And many who have examined the skull support Brunet's hypothesis (testable scientific explanation).

"Most scientists agree that Toumai is a hominid," says Pilbeam. But one factor could silence skeptics: Proof of bipedalism--the ability to walk upright, a key hominid trait. A pelvis or thigh bone would be the best proof. But for now the skull offers a tantalizing piece of evidence: The position of the spinal-cord opening (foramen magnum) in the skull's base resembles that of skulls belonging to known bipedal species.

At 7 million years old, Toumai is 3.5 million years older than Australopithecus afarensis, a family of small, long-armed upright walkers discovered in 1972 in the eastern African nation of Tanzania. A. afarensis was the earliest known human ancestor until 1992. Since then, a series of early hominid fossil discoveries have thrust back the fossil record beyond 4 million years ago. A handful of 6-million-year-old fossils vied for the title of oldest hominid--until Toumai was found.

WAKE-UP CALL

Fossil hunters discovered Toumai thousands of miles from the Rift Valley in eastern Africa, where previous hominid fossil troves led scientists to assume they might ultimately discover the oldest human ancestor (see map, above left). "In hindsight, we should have expected this," Harvard paleontologist Daniel Lieberman told The New York Times. "We weren't looking at all of Africa. This fossil is a wake-up call."

In the Djurab Desert, ceaseless wind moves the dunes and exposes new fossils, says Brunet. After sifting sand there for a decade, his team amassed a hoard of 10,000 fossils, dating from 3 to 7 million years ago and representing 42 animal species. They would have preferred to find the bones sandwiched between layers of volcanic ash, which naturally contain the elements potassium and argon. These minerals can be radiometrically dated because they decompose at a measurable rate and serve as an accurate historical clock.

Luckily, scientists could tell a lot about Toumai's age by studying nearby fossils: The specimens included extinct relatives of elephants and pigs. French paleontologist Patrick Vignaud at the University of Poitiers compared them with nearly identical specimens found and radiometrically dated in eastern Africa, where volcanic ash is plentiful. This comparison helped him estimate Toumai's age at nearly 7 million years old.

FAMILY TREE

Toumai now reigns as the oldest known hominid, but that doesn't mean it was the first. "It will never be possible to know precisely where or when the first hominid species originated," Brunet says. The hominid fossil record now includes thousands of specimens, representing as many as 18 species over 7 million years of history, says Pilbeam. "If you pack all the fossils nearly in boxes and stack them, they'd fill quite a large building."

How do those species relate to each other? Scientists are busy connecting the dots. Anthropologists think modern humans first appeared around 100,000 years ago in Africa and spread throughout the world. Today, Homo sapiens is the only living hominid species. "In some parts of Europe and Asia, they lived alongside the humanlike species Homo neanderthalensis, or Neanderthal," Pilbeam says. Neanderthals went extinct 30,000 years ago. "But they were a population that appears to have no genetic link to living humans."

Could the same be true of Toumai? It's too soon to tell. Says Pilbeam: "Prior to 4 million years ago, the hominid fossil record is thin. Toumai is exciting because it offers a glimpse at one of the earliest chapters."
IT'S YOUR CHOICE

Choose the correct answer to these questions:

1 What is the genus and species of the new skull?

A Sahelanthropus tchadensis C Homo sapiens
B Homo neanderthalensis D Australapithecus afarensis

2 Which is a defining feature of bipedal animals?

A use of fire C use of tools
B use of language D two-legged upright walking

3 Which statement best explains how scientists
classified Toumai?

A They compared the skull with other fossilized mammals they
found near it.

B They compared Toumai's anatomical features with the skulls
of other primate species.

C They compared the skull's teeth to other primates' teeth.

D They examined the fossil with a radiometric-dating machine.

4 Which of the following would help scientists
accurately identify Toumai as bipedal?

A a pelvis C a nearly complete skeleton
B a thigh bone D all of the above

ANSWERS IN TEACHER'S EDITION


APE OR HOMINID: SIX POINTS OF COMPARISON

How scientists classified the new fossil

1 SKULL

The skull's size suggests Sahelanthropus was no larger than a chimpanzee.

2 BRAIN

The creature's brain was about the size of a modern chimp's.

3 BROW RIDGE

This 18-millimeter-thick brow ridge is thicker than any modern male ape's brow ridge. Modern humans don't have a prominent brow ridge, but early members of the genus Homo did.

4 FACE

The face is short and flat. By comparison, the lower part of a chimp's face juts forward to hold its large canines, The lower part of a modern human's face is even smaller and flatter.

5 TEETH

Canines are smaller and shorter than a chimp's. Tooth enamel is thicker than a chimp's, which suggests a diet with less fruit.

6 FORAMEN MAGNUM

The opening at the base of the skull where the spinal cord connects to the brain appears to be oval. This shape hints that Toumai might have walked upright like a hominid. Chimps have a round opening.

[ILLUSTRATIONS OMITTED]
HOW SCIENTISTS CLASSIFY PRIMATES

Scientists place humans, gorillas, and chimpanzees (along
with 200 other ape and monkey species) in the primate order.

 TOUMAI MODERN MODERN MODERN
 HUMAN CHIMPANZEE GORILLA

KINGDOM ANIMALIA ANIMALIA ANIMALIA ANIMALIA
PHYLUM CHORDATA CHORDATA CHORDATA CHORDATA
CLASS MAMMALIA MAMMALIA MAMMALIA MAMMALIA
ORDER PRIMATES PRIMATES PRIMATES PRIMATES
FAMILY HOMINIDAE HOMINIDAE PONGIDAE PONGIDAE
GENUS SAHELANTHROPUS HOMO PAN GORILLA
SPECIES TCHADENSIS SAPIENS TROGLODYTES GORILLA
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Article Details
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Author:Masibay, Kim Y.
Publication:Science World
Geographic Code:60AFR
Date:Nov 8, 2002
Words:1255
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