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The Descendants
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Is a newfound prehistoric species our direct ancestor?
Matthew Berger wasn't looking to revise the story of human origins.
He was just chasing his dog Tau. But one day in August 2008, the 9-year-old
boy stumbled upon a 1.9 million-year-old collarbone at a South African dig
site. His discovery has sparked a new debate into the ancestry of human
beings.
Matthew's father is Lee Berger, a paleoanthropologist—someone
who studies the origins and the predecessors of the human species. Since
2008, Berger and his colleagues have been excavating the site where
Matthew made his discovery. They've dug up a big collection of wellpreserved bones, including a skull, a pelvis, a hand, and an ankle, that came
The Descendants
from two individuals. One was an adult female, the other a juvenile male
(around 10 to 13 years old). Likely those of a mother and her son, the fossil
remains are of hominids. Hominids are humans and the extinct humanlike
creatures from which we developed.
Berger believes the hominid fossils his team found are those of a
newfound species, one he calls Australopithecus sediba. The word sediba
means "wellspring" in Sotho, an African language. A.sediba, says Berger,
could be a direct ancestor of humans—the wellspring of our lineage.
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All In The Family
The term Australopithecus refers to a genus (group of
related species) that appeared in Africa between 4 million
and 5 million years ago. Also called australopithecines, they
had large apelike faces with big teeth. Their bodies were
light, roughly 32 kilograms (70 pounds), and relatively short,
about 1.2 meters (4 feet) tall.
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Current evidence indicates as many as six known
£=£-••" species of australopithecine lived before—and during—the
time of A.sediba. Like most primates, australopithecines were well adapted
t o climbing trees. Unlike most primates, however, they were also bipedal,
able t o walk upright on t w o feet.
Bipedal locomotion became more efficient later as the
australopithecines evolved into a new genus whose species had smaller
faces and larger brains—the genus Homo. The first known Homo species—
H. habilis—was named for the stone tools it apparently made. H. habilis
had a very precise handgrip and short finger length— features that support
the idea of toolmaking among the species.
As other species of the Homo genus evolved, their brains became
larger and more complex. Modern humans (H. sapiens) have the most
advanced and complex brains of all.
To Berger, A. sediba represents the species that connects the t w o
groups Australopithecus and Homo. The A. sediba fossils contain "mosaic"
features, he says, combining traits from both groups. A. sediba was adapted
in its upper body t o climbing trees, but its hands were hauntingly modern—
those of a toolmaker. And its brain, though small, was strikingly human in
structure.
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"The many advanced features found in the brain and body make it possibly
the best candidate ancestor for our genus Homo," he says.
The main problem with Berger's theory is timing. H. habilis appeared
2.5 million years ago. How could it have descended from an A.
sediba, whichlived half a million years in the future? That would
be like a daughter giving birth to her mother.
Berger isn't dissuaded by that criticism. He believes that
the fragmentary fossils that belong to the early members of the
Homo genus might have been dated incorrectly. They might
have come from individuals that lived after A. sediba and were actually its
descendants.
Dead End?
Critics do agree that A. sediba is a new species but believe it became
extinct without evolving into another species. That type of extinction is
common in evolution, says Ian Tattersall, a paleoanthropologist at the
American Museum of Natural History in New York City. "Nature's way of
experimenting is by throwing different species into the ecological arena,"
he explains. "The better-adapted species survive, while the others go
extinct."
Indeed, two or three early Homo species might have lived side by side 1.9
million years ago in Africa. It's possible that A. sediba lived among those
species.
"[Evolution is] not a linear process," says Tattersall, explaining that one
species doesn't necessarily give way to another and another. There are
many "evolutionary dead ends"—species that simply die out.
Even if A. sediba is one of those dead ends, Tattersall maintains, "it
gives us a better picture of what hominids looked like at the time."
Articulate Bones
The A. sediba fossils that Berger's team found are amazingly well
preserved and articulated—some of the bones were found still fused
together. Typically, with fossils that old, only bone fragments remain. With
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the A. sediba fossils, however, "the most astonishing thing is the
completeness," says Tattersall about the articulated remains.
Although Tattersall calls the mosaic features of A. sediba
"incredible," he is less sure than Berger that A. sediba is a linear ancestor of
H. sapiens. Nevertheless, the fossils tell us a lot about our evolutionary
past, he says. The "mix-and-match" mosaic features show how nature
experiments.
Whether or not humans are descended from A. sediba, our ancestors
underwent the same selection pressures—the conditions that force a
species to continue adapting to survive. "And out of this evolutionary
ferment," Tattersall says, a tinge of excitement in his voice, "our genus
Homo arose."
Family Tree
Prehuman evolution has wound its way through two genera (groups
of related species) during the past 5 million years. The first genus,
Australopithecus, existed roughly 5 million to 2 million years ago. The
second genus. Homo, arose 2.5 million to 2 million years ago. Species in the
second group had bigger brains and longer legs and used tools. Humans
(Homo sapiens) are the only surviving species in that group. Scientists are
trying to determine the exact connections between all the species in each
genus to find out which ones became extinct and which ones evolved into
later ones that finally became humans. Listed at right are many of the
known species in each genus.
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The Descendants
1. How were australopithecines different from primates?
A) They were bipedal.
B) They could climb trees.
C) They had big teeth.
D) They were shorter.
2. What was the effect of Matthew Berger finding the collarbone?
A) nothing of scientific significance
B) more information about australopithecines
C) a new species discovery
D) proof that Homo sapiens have the most complex brain of all
3. What does the author imply about Berger's assertions about A. sediba?
A) That they may be inaccurate.
B) That they are 100% true.
C) That there is no possible way they are true.
D) That we should not consult other experts.
4. Read the following sentences and answer the question below: "Matthew
Berger wasn't looking to revise the story of human origins. He was just
chasing his dog Tau. But one day in August 2008, the 9-year-old boy
stumbled upon a 1.9 million-year-old collarbone at a South African dig site."
What does the word revise mean?
A) change
B) bring back to life
C) confuse
D) deepen our understanding of
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5. The primary purpose of this passage is to describe
A) how much arguing happens within the scientific community when new
discoveries like this are made
B) the precise order of the different groups that evolved over time
C) what recent discoveries have added to our knowledge about human
ancestry
D how even kids and teenagers can make real, helpful contributions to
scientific research
6. How did the Australopithecus genus differ from the Homo genus?
7. What does the author imply about our knowledge about how all the
species in our past connect with one another?
8. The question below is an incomplete sentence. Choose the word that
best completes the sentence.
Scientists believe H. habilis were good toolmakers
precise hand grip and short fingers.
A) although
B) however
C) but
D) because
they had a
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9. Answer the following questions based on the sentence below.
Our ancestors faced selection pressures that forced them to adapt or die
out millions of years ago.
Who? our ancestors
(faced) What?
(that did) What?
When?
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Directions: Read the vocabulary word and definition below to complete
questions 10a, 10b, and 11.
Vocabulary Word: tinge (tinge): a slight amount or trace.
10a. Read the sentences below and underline all forms of the word tinge.
1. The dying leaves had a yellow tinge on the edge of them.
Add a tinge of pepper to the soup to add some flavor.
Her eyes were mostly brown but had a tinge of hazel to them.
4. I suggest you add a tinge of orange to the edge of your sunset painting.
5. She felt a tinge of sadness when she heard the news about the forest
fire.
10b. Circle the image shows something more likely to have a tinge of salt in
the taste?
s..'
x„.
11. If a recipe calls for 2 cups of flour, would you say that's a tinge of flour?