Evidence of a worm-like creature about the size of a grain of rice has been uncovered in South Australia,and researchers believe it is the oldest ancestor on the family tree that includes humans and most animals.The creature lived about 535 million years ago. It is considered to be an evolutionary step forward for early life on Earth.
Researchers have been trying to find evidence of the common ancestor of most animals.Developing its body structure and organization successfully allowed life to move in specific,purposeful directions.This includes everything from worms and dinosaurs to amphibians（两栖动物）and humans.But for our common ancestor,they knew that fossils（化石）of the tiny,simple creatures they imagined would be nearly impossible to find because of its size and soft body.Then,they turned to fossilized burrows,dated to the Ediacaran Period,found in South Australia.
For 15 years,scientists knew the burrows were created by the creature.There was no evidence of what made the burrows and lived in them until researchers decided to take a closer look at the burrows.Geology professor Mary Droser spotted impressions shaped like ovals near the burrows.
A 3-D laser scan revealed the impressions containing evidence of a body shaped and sized like a rice grain,with a noticeable head and tail which suggested it had muscles.The muscles would have enabled the creature to move and create the burrows,like the way a worm moves.And the size of the creature matched with the size of the burrows they found.
The researchers involved in the study named the creature Ikaria wariootia.The first name translates as "meeting place" in the Adnyamathanha language.Adnyamathanha is the name of Australian people that live in the area where the fossil was found.



What does the underlined word "It" mean in the first paragraph？ ______

A. A worm-like creature.
B. A grain of rice.
C. A family tree.
D. An evolutionary step.

What did the researchers do during the study？ ______

A. They developed the creature in the expected way.
B. They studied a large number of worms and dinosaurs.
C. They observed the burrows close up.
D. They went back to the Ediacaran Period some years ago.

What inspired the scientists to explore the origin of the burrows？ ______

A. A 3-D laser scan.
B. The impressions.
C. Some good ovals.
D. More burrows.

What does the text mainly tell us？ ______

A. The researchers worked very hard in the burrow.
B. The worm-like creature may be animals' ancestor.
C. It is very difficult to name a new kind of creature.
D. Australian people like fossils of many creatures.