| 1. | It has a half-life of about 41/2 billion years .
它的半衰期大约为45亿年。 |
| 2. | At some time more than five billion years ago the universe started .
在50亿年前的某个时候，宇宙开始形成。 |
| 3. | The youngest rocks found on the moon are 3 billion years old .
在月球上发现的地质年代最晚的岩石已有30亿年历史了。 |
| 4. | Their ancestors have been eating according to flavor for upwards of a billion years .
他们的祖先已经根据滋味来吃东西吃了十亿年以上了。 |
| 5. | Yet oxygen gas did not begin to accumulate in the atmosphere until about 2 billion years ago .
但是大约在20亿年前，氧气才开始在大气层中积累。 |
| 6. | The best suggestion is that jupiter is still contracting gravitationally despite its age of several billion years .
最好的说明是：木星尽管已有几十亿年的历史，但由于引力作用，它仍在收缩。 |
| 7. | Practically every atom had its origin in the tiny violence of a radioactive disintegration some time within the past few billion years .
实际上每个原子都来源于过去几十亿年中某一时刻放射性分裂的微小激变。 |
| 8. | The organisms living between 3 and 1 billion years ago were not as versatile as modern species, probably due to energy limitation .
生活在过去二十亿年之间的有机体可能由于能量的限制，不如现代物种这样五花八门。 |
| 9. | A star can persist like this for 10 billion years
这样的恒星可以持续一百亿年以上。 |
| 10. | Our solar system was formed 4 . 6 billion years ago
我们的太阳系被形成了4 . 6十亿年前。 |
