It’shardtoimaginehowasingle-celledblob(单细胞斑点）intheoc...

问题详情：

    It’s hard to imagine how a single-celled blob (单细胞斑点）in the ocean evolved into a fish* a turtle, or eventually a human. 2.5 billion years ago, lives on Earth were all single-celled. Then, these tiny creatures started getting bigger, evolving into multi-celled  blobs that eventually became animals. Multicellularity is one of the most important steps in evolution, But it’s hard to study because tiny blobs in the ocean don’t really leave fossils behind.

    However, there're plenty of single-celled creatures still living today. And one of them, the choanoflagellate (领鞭毛虫)，seems to be our closest living smgle-celled cousin. This means that , long time ago, a common ancestor of both you and the choanoflagellate evolved in two different directions. By looking at what choanonagelates and animals have m common, scientists  can start to describe that single-celled common ancestor.“Choanoflagellates are really a unique Window back in time to the origin  of  ammals and humans.” said Dan Rokhsar of the University of California.

   Looking at a choanoflagellate,it's impossible to see any resemblance to an   seems by moving a long tail. The tail has tentacles that catch bacteria to eat. But inside that single cell in the genes, Dan Rokhsar and his team found something interesting. Choanoflagellates   genes produce cadherin proteins (钙粘蛋白)，which have a well-known role in animals: they slick cells together like glue. But a choanoflagellate only has one cell! Scientist Nicole King thought that the choanoflagellaes used this glue to hold themselves to the surlace of water. Fhe leap to multicellularity then makes more sense, because the first multi-ceiled creatures were able to put this glue to a new and different purpose—holding their cells together! Now that's a clever trick of evolution.

32. Which of the following statements is true according to the passage?

A. Multi-celled blobs didn^ exist on the earth until 1 billion years ago.

B. Choanoflagelates and animals share nothing in common.

C. Choanoflagellates and human beings share the same ancestors.

D. By watching the movement of a choanoflagellate, scientists have found that it resembles an aniinaL

33. What is the passage mainly about?

A. The function of a single-ce] led blob.

B. The features of multicellularity.

C. The result of scientists’ study on the multicelled blobs.

evolution of single-celled blobs.

34. Why is it difficult for scientists to study the process of multicellularity?

A. Because it happened billions of years ago.

B. Because they couldn't find any fossils in the ocean.

C. Because no singie-celiecl blobs have been left for them.

D. Because both humans and animals evolved from single-celled blobs.

35. Cadherin protein from choanoflagellates’ genes are probably used to        .

A. stick them to the surface of water

B. produce their own ceils

C. hold their cells together

D. swim with a long tail

【回答】

CDBA

知识点：阅读理解

题型：阅读理解