Our planet was born out of the Milky Way some 4,700 million years ago. At first, it was probably a swirling cloud of gases and stardust where life could not have existed. At an early stage, it must have been tremendously hot, due to its own radioactivity. Not until it cooled enough to form a hard crust could life begin to appear.
To know about past life, we must search for its remains in the rocks. Not all rocks contain fossils. One kind of rock, called igneous (meaning fire), is made from the molten lava out of volcanoes. Granite is an example, and it contains no fossils. Other rocks are slowly made from tiny particles of land and mountains worn away by weather. Depending on the size of these particles, they form into clay, limestone or sandstone. In them, there can be found the fossils of creatures that died long ago.
Other fossils come from lake-bed rocks, or from gravel and sand washed down by rivers. In rare cases, almost perfect fossils are found in caves or, like the mammoths from Siberia, in frozen ground. Tiny animals such as insects may be found in amber, which is the hardened resin from prehistoric trees. Other fossils turn up in tar pits where they drowned long ago.
Usually, only the hard pieces of an animals are preserved, such as bones, teeth and shells. Paleontologists try to piece them together and tell us what the animals looked like, also how they lived. With the help of models and drawings, we can learn something about the prehistoric animals, which form part of the evolution story of life.
During 4,700 million years ago, our planet was _________.
gases and a cloud
gases and stardust
stardust and a cloud
a swirling cloud