Becoming human: could newly discovered fossils belong to a long-lost human ancestor?
As they sorted through rocks blasted out of a nearby cave--called Malapa--decades earlier by limestone miners, Berger and a team of scientists turned up more fossils. The bones belonged to a 12- to 13-year-old male and a female in her 20s that lived about 2 million years ago. The specimens, dubbed Australopithecus sediba, have an unusual mix of features. In part, they resemble primitive, apelike animals. But they also have some of the advanced characteristics of the genus Homo, the category of classification to which modern humans belong (see Family Reunion, p. 10).
Most scientists believe that millions of years ago, a species that was descended from apes evolved, or changed over a long period, into something more humanlike. Berger's team thinks A. sediba could be that transitional species, which led to the earliest Homo--and eventually us. But not everyone agrees with this interpretation.
Before the first humans appeared, many apelike hominids existed. Hominids make up the family that contains primates and their ancestors. "Trying to fend the root of our species out of the extremely diverse group of ancient relatives is not easy," says Ian Tattersall, an anthropologist at the American Museum of Natural History in New York City.
ANCIENT DEATH TRAP
When scientists uncover the remains of ancient hominids, they often don't find much more than single bones or teeth. A. sediba's remains, though, are amazingly intact. "It's extraordinarily rare to find such complete fossils," says Darryl de Ruiter, a professor at Texas A&M University and member of Berger's team, who is studying A. sediba's skull and teeth.
The remains are in such good condition because of the geology of the area where the fossils were found. Long ago, an underground water source called an aquifer carved out a series of caves (see Excavating a Death Trap, left). The researchers think that while looking for water, the male and female A. sediba may have fallen down a hidden cave shaft to their deaths. Inside, sediment quickly buried the bodies, preserving their bones. Since the skeletons were found near each other, the pair probably died around the same time and may have even known each other or been related. Berger's team has since found traces of at least four more skeletons that may have been part of this same group.
Once Berger's team uncovered the fossils, they compared the bones with those of other members of A. sediba's genus Australopithecus, with early Homo, with modern humans, and with apes. This led to the discovery of A. sediba's mix of old and new traits that scientists believe makes A. sediba a good candidate as the species that became our earliest Homo ancestor (see Missing Link?, p. 9). "They show more characteristics that align them with Homo than [has] any other Australopith we've ever encountered," says de Ruiter.
Like apes, A. sediba had a small stature and long, powerful arms good for climbing trees. But like more advanced hominids, the female's hand had a precision grip necessary for grasping tools. Its foot also had both a primitive heel and a modern anklebone. So although A. sediba walked upright, it probably did so differently than humans do.
A scan of the cavity inside the male's skull shows that he had a small, chimp-size brain, quite different from future, big-brained humans. The images also revealed how his brain was shaped. One side of the frontal lobe is larger than the other--similar to what's seen in humans. Our brain's two sides have evolved to become specialized in things like language.
One challenge to the claims of Berger's team is that some Homo fossils---the most important being a jawbone---are older than their supposed ancestor, A. sediba. But de Ruiter argues it's difficult to figure out whether such scrappy fossil fragments really belong to the genus Homo. He points to A. sediba's mixed-up nature. "If we had found a skull and jawbone separately, we wouldn't have thought they were from the same species--they are that different."
Although scientists may disagree on whether A. sediba is our direct ancestor, they do agree that the fossils are extraordinary. They show that a lot of evolutionary changes were going on 2 million years ago, resulting in a variety of hominids, out of which humans sprang, says Tattersall. He thinks our lineage may be hard to trace because the leap to humans may have occurred suddenly instead of gradually, as Berger's team suggests.
"We're challenging our colleagues' interpretations, and they're challenging ours back, which is the nature of science," says de Ruiter. Berger's team has provided casts of their fossils to museums around the world. Now the scientific community can examine the new fossil evidence and draw its own conclusions.
AUSTRALOPITHECUS AFARENSIS 3.2 million years ago
AUSTRALOPITHECUS SEDIBA 1.98 million years ago
HOMO ERECTUS 1.6 million years ago
A. sediba has a mix of primitive and modern physical characteristics. Here's how it compares with an older Australopithecus species and a more recent Homo species.
SIMILARITIES WITH A. AFARENSIS
(1) Small brain size
(2) Long, high cheekbones
(3) Primitive molar cusps
(4) Small body size
(5) Long upper limbs
(6) Primitive heel bone
SIMILARITIES WITH H. ERECTUS
(7) Front of brain reorganized
(8) Projecting nose
(9) Smaller teeth and chewing muscles
(10) Hips less flared, similar to that of humans
(11) Longer legs
(12) Hands with precision grip
The team of anthropologists studying A. sediba think that it could be the closest link to the branch of the human family tree that contains the genus Homo and modern-day humans. Take a look at when each hominid subgroup arose and its members.
EXCAVATING A DEATH TRAP
Two million years ago, the Malapa cave where A. sediba's fossils were found contained an underground aquifer. A group of A. sediba, looking for water, may have fallen into the cave. Sediment then quickly buried their bodies.
NATIONAL SCIENCE EDUCATION STANDARDS:
Grades 5-8: diversity and adaptations of organisms
Grades 9-12: Biological evolution
COMMON CORE STATE STANDARD: LITERACY IN SCIENCE: 5. Analyze structure the author uses to organize text.
Learn about the discovery of a new species that may be a direct ancestor to humans.
* What are fossils? (hardened traces of ancient organisms)
* What can scientists learn from fossils? (what species lived in the past, where they lived, and what species they share characteristics with)
* What genus do modern-day humans belong to? (Modern-day humans belong to the genus Homo. Students may know that the scientific name for humans is Homo sapiens.)
[I] 1. Go to www.scholastic.com/scienceworld. Open the digital edition to pp. 10-11, and use the masking tool to block out everything except the diagram "Family Reunion." Ask students what the diagram shows (species in the human family tree). Explain that although scientists have found fossils from a variety of species sharing characteristics with humans, they still don't know which species is a direct ancestor to humans' branch of the family tree--the genus Homo. Explain to students that a new fossil find may change that.
[E] 2. Have students open their magazines to page 9. Open the digital edition to the same page. Highlight the phrase "transitional species" in the second column and ask students what a transitional species is. (A species that represents a middle step in the evolution from an older species to a newer one. It usually has some features from both.)
[E] 3. Ask a volunteer to read the first two paragraphs. When the student gets to the second column, highlight the term Australopithecus sediba. Have students practice pronouncing this word: AW-stray-low-PITH-ick-us said-EE-bah. Explain that this is the scientific name of the new species. Its name consists of its genus and species.
4. Have volunteers take turns reading the rest of the article. Ask students to identify one of the challenges to the claims made by Berger's team.
[H] Expand the text box labeled "what Do You Think?" on page 11 Will scientists ever be able to say for sure which hominid species led to present-day humans?" Hold a class discussion. This is a good time to remind students of the relationship between evidence and claims in science.
Use the "Two Ancient Hominids" skill sheet from the online database at www.scholastic .com/scienceworld. In this activity, students will use a Venn diagram to demonstrate what they have learned from the article.
DON'T TEACH LIFE SCIENCE?
Go to www.scholastic.com/scienceworld to download these assessment skills sheets instead:
EARTH SCIENCE: THE GREAT COLLAPSE
Anthropologists think that the cave where the bones of A. sediba were found played an important role in preserving their fossils. Read this passage to learn about another type of underground formation, called a sinkhole.
PHYSICS: DATING FOSSILS
Scientists can determine the age of a fossil by measuring the amount of radioactive elements found in surrounding rocks. Use this graphing activity to learn about the half-lives of various elements used for dating.
* VIDEO EXTRA: Watch a video about A. sediba's discovery at: www, scholastic.com/scienceworld.
* Learn more about A, sediba from this collection of resources from the American Association for the Advancement of Science: www.sclencemag.org/site/extra/sediba.
* Check out the National Museum of Natural History's human-evolution Web site: http://humanorigins,si.edu.
DIGITAL ISSUE KEY:
[A] SHOW ALL PAGES
[C] MASK TOOL
[D] DIGITAL STICKY NOTES
[E] TEXT HIGHLIGHTER
[F] DRAWING TOOL
[I] VIDEO PLAYER
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|Title Annotation:||BIOLOGY: EVOLUTION|
|Article Type:||Cover story|
|Date:||Jan 23, 2012|
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