Final answer:
Fossils in the lowest geological layers are generally the most primitive. Paleontologists may conclude a mass extinction event based on a sharp decrease in fossil variety, such as findings from pre-Permian rock layers. Dating methods are essential in constructing the historical timeline of life on Earth.
Step-by-step explanation:
Fossils found in the lowest geological layers are generally the most primitive. When paleontologists reason that a particular layer correlates to a mass extinction, it likely involves evidence such as a sudden decrease in the variety and number of fossils post that layer, indicating a significant loss of species. For instance, collecting fossils from rock layers deposited prior to the Permian period that contain early vertebrate bones might suggest a previous abundance of life forms that were decimated by a mass extinction event. Paleontologists use various dating methods to determine the age of fossils, and the earliest known fossil evidence for life dates back to microbial mats from 3.5 billion years ago. Through the fossil record, scientists can track the abundance and diversity of life through different geological time periods, noting how certain types of fossils are found more frequently in certain strata.