Here's why:
* Geological Time Scale: The Earth's history is divided into eons, eras, periods, and epochs, with the oldest layers at the bottom and the youngest at the top.
* Fossil Succession: Fossils found in lower strata represent life forms that existed earlier in Earth's history. Over time, life has evolved and become more complex.
* Evolutionary Trends: The fossil record generally shows a progression from simpler life forms to more complex ones. This is consistent with the theory of evolution.
Example:
* Trilobites are found in very old, lower strata. They are considered relatively primitive compared to mammals, which appear in much younger strata.
Important Note:
While this is a general trend, there are exceptions. Sometimes, fossils of more complex organisms can be found in lower strata due to:
* Local Geological Conditions: The precise order of strata can be disrupted by geological events like tectonic activity or erosion.
* Rapid Evolution: Sometimes, a new species can evolve rapidly and appear in a younger strata than expected based on general evolutionary trends.
However, as a general rule, the lower the geological strata, the more likely it is to contain fossils of simpler, more primitive organisms.
