Do you want to live another 100 years or more? Some experts say that scientific advances will one day enable humans to last tens of years beyond what is now seen as the natural limit of the human life span.

    “I think we are knocking at the door of immortality(永生),” said Michael Zeya, a Montclair State University business professor and author of two books on the future. “I think by 2075 we will see it and that's a conservative estimate(保守的估计).”

    At the conference in San Francisco, Donald Luria, a professor at New Jersey Medical School in Newark said advances in using genes as well as nanotechnology(纳米技术) make it likely that humans will live in the future beyond what has been possible in the past. “There is a great push so that people can live from 120 to 180 years,” he said. “Some have suggested that there is no limit and that people could live to 200 or 300 or 500 years.”

    However, many scientists who study in ageing are doubtful about it and say the human body is just not designed to last past about 120 years. Even with healthier lifestyles and less disease, they say failure of the brain and organs will finally lead all humans to death.

    Scientists also differ on what kind of life the super aged might live. “It remains to be seen if you pass 120, you know; could you be healthy enough to have good quality of life?” said Leonard Pooh, director of the University of Georgia Gerontology Centre. “At present people who could get to that point are not in good health at all.”

    Not many years ago, some farmers were worried because hawks ( 鹰) were taking many of their chicken. The farmers didn't know what to do. Finally they went to the country officials and asked for help.

    "Kill the hawks," the officials said. "We will even pay for them." So the farmers began to think of ways to kill the hawks.

    The farmers killed many hawks. They no longer had to worry about their chickens. But they now had a new worry. Field mice were eating up a lot of the farmers' grain.

    How did this happen?

    Hawks cannot only chickens but also field mice. They eat more field mice than chickens. But the farmers didn't know this. When they killed a lot of hawks, they changed the balance.

    When people move into a new place, they often destroy many wild plants. Often these plants are food for the animals. If the animals can't find enough plants to eat, they will starve or have to leave the place.

    In one part of the USA, for example, the deer there like to eat a certain kind of wild roses. The mountain lions there eat the deer. The number of deer, mountain lions and wild roses doesn't change much if people leave things as they are. But people killed many mountain lions in order to protect the deer. Soon there were so many deer that they ate up all the wild roses. Then the deer began to eat the green leaves of young trees. These trees were important to the farmers. So the farmers thought of ways to protect their trees. Now the deer had nothing to eat, and many of them died. This was another lesson from nature.

    To keep the balance of nature is important for us to remember.

    People and dogs have been living together for hundreds of years. When did dogs and people become friends? Where did dogs come from?

    A group of scientists tried to answer these questions. They just finished a study about the history of dogs. In the study, they looked at the DNA of dogs all over the world and found that dogs, like humans, shared DNA with their family members. The scientists put together a big family tree for dogs. It went all the way back to the animal's beginning.

    In this way, scientists learned about the history of dogs. The first dogs lived in Asia. They were born about 33,000 years ago. At first, there were only about 6,400 of them. Their parents were wolves(狼).

    The new study answers many questions. It raises some too. For example, what caused wolves in Asia to become dogs all those years ago?

    One idea is that their DNA slowly changed and the wolves became dogs in the end. As time goes by, dogs became "man's best friends" Or it could happen the other way around. It was possible that some wolves started living with people before they became dogs. Living with people led to many changes to these wolves. It changed their DNA, so their offspring(后代) became dogs. In other words, it was possible that wolves were our friends before they became dogs.

    By studying DNA, scientists now have a much better understanding of the history of dogs. ________.

    How exactly dogs came to be our friends is still unclear.

    As we know, all living things have to eat food. For a man, a tiger, a fish, a bird, or even a worm, food is needed. We know all living things are sure to die without food. In every part of the world, different kinds of animals share living places and live in communities (群落) together. They are connected(联系) in a food chain.

    In a food chain, some living things are producers and some are consumers. Plants are producers because they use sunlight, soil and other things to make their own food. Animals are consumers because they have to eat other animals or plants.

    There are four different kinds of consumers in the animal world. A carnivore is an animal that only eats other animals. An herbivore is an animal that only eats plants. An omnivore is an animal that eats plants and animals. A scavenger is an animal that eats dead animal.

    Here is a picture of a food chain. The food chain shows how animal eat each other in a community. In the picture, you can see what animal or plant is food for another animal. See? The grass is food for a rabbit(兔子), which then becomes food for a hungry tiger. In this way, all of these animals are connected.