阅读以下教材原句，仔细观察加颜色部分并补全空格。 1.What it was to become was uncertain until between 4.5 and 3.8 billion years ago when the dust settled into a solid globe. 2.The earth became so violent that it was not clear whether the shape would last or not. 3.What is even more important is that as the earth cooled down，water began to appear on its surface. 4.It was not immediately obvious that water was to be fundamental to the development of life. 5.Why they suddenly disappeared still remains a mystery. 6.So whether life will continue on the earth for millions of years to come will depend on whether this problem can be solved.