Final answer:
The rareness of fossils in Precambrian rocks compared to the Phanerozoic eon's abundance of fossils suggests a significant increase in the diversity of life, especially following the Cambrian explosion. This period of evolution brought about an expansion of biodiversity with the emergence of new animal phyla.
Step-by-step explanation:
The fossils found in Precambrian rocks are rare compared to those in rocks from the Phanerozoic eon, which suggests a major change in the diversity of life occurred during the Phanerozoic eon. This change was likely due to the Cambrian explosion, which is a term used to describe the rapid increase in the diversity of multicellular life forms on Earth. This era is marked by significant evolutionary developments and exponential growth in the complexity and variety of life. Following the Precambrian mass extinction, the Cambrian period witnessed the emergence of a multitude of new animal phyla, representing a vast expansion of biodiversity.
Before the Cambrian period, the Ediacaran Period of the late Precambrian era saw the rise of early multicellular life, including the Ediacaran biota. Despite these early developments, it was not until the Cambrian explosion that life on Earth truly diversified. The rareness of Precambrian fossils can be attributed to either the simplicity and small size of Precambrian life forms, which made their fossilization less likely, or to a lesser extent of biological activity during that time. Whereas, the abundance of fossils in the Phanerozoic rocks underscores a remarkable increase in biological diversity and complexity.
Mass extinctions, like the Permian extinction at the end of the Paleozoic Era, can drastically reduce biodiversity, but also open up ecological niches for the evolution of new species. In contrast, environmental changes can create new niches, leading to rapid speciation and increased diversity. This evolutionary record, marked by mass extinctions and subsequent recoveries, shapes the biological diversity of Earth through time.