Answer:
Fossils are important features that are present in the sedimentary rocks. This provides evidence of how life has evolved over a large geological time scale.
Initially, the first life forms were unicellular, microorganisms, whose remnants are found to be present in the chert rocks of Australia, that are about 3.46 billion years old belonging to the Archean era.
Next to this, there occurred a new species, known as the trilobites which were first appeared in the Cambrian (540 million years back) and got extinct by the end of Permian (250 million years). These are found to be present in the Paleozoic rocks of Utah, the United States, and Oklahoma.
The next well-developed species existed were the dinosaurs, which first appeared in the Triassic, dominant in the Jurassic and got extinct by the end of Cretaceous. This involved the time from about 250 million years back to 65 million years back.
Thus, this is how life has evolved from simpler to complex organisms, including the present-day humans. This study of changes that occurs in the fossils found in the rocks from the bottom (older) to the top (younger) is considered as faunal succession, and the geologist evaluates the age of the rocks that comprises the distinct fossils.
An image has been attached below, showing the occurrence of different life forms namely microorganisms, trilobites, and dinosaurs in the geological history at different places on earth.
