To answer this question, we need to look at the history of Earth's life forms as recorded by fossils. The timeline provided to us is divided into segments labeled A through E, with the oldest times starting at A and the present at E. The labels correspond to major eons and eras in Earth's geological history.
Now, let's look at each label and their possible corresponding eons or eras:
- Label A most likely represents the earliest part of Earth's history, which would include both the Hadean and Archean eons. During the Hadean eon, Earth was forming and was mostly inhospitable to life as we know it. The Archean eon followed and spans from about 4.0 to 2.5 billion years ago, during which the earliest life, simple single-celled organisms, appeared.
- Label B corresponds to the Proterozoic eon, which extended from about 2.5 billion to 541 million years ago. During this time, the planet saw significant changes, including an increase in oxygen levels and the appearance of the first multicellular life forms.
- Label C is linked to the Paleozoic era, ranging from about 541 to 252 million years ago. This era is known for the diversification of marine life in its early stages and later the colonization of land by plants and animals.
- Label D represents the Mesozoic era, known as the age of reptiles, running from approximately 252 million to 66 million years ago. This era saw the rise and fall of dinosaurs.
- Finally, label E indicates the Cenozoic era, which is the current era, starting around 66 million years ago to the present. Mammals dominate this era, and it is in this era that humans appeared.
We are specifically looking at the appearance of "increasingly complex multicellular organisms." The first well-known complex multicellular life forms, such as the Ediacaran biota, are believed to have appeared in the late Proterozoic eon. Therefore, we infer that increasingly complex multicellular life began to appear during the transition from the Proterozoic eon to the following era—the Paleozoic. This corresponds to the time interval from label B to label C on the given timeline.
Given this interpretation, the answer is B to C. This is the interval during which increasingly complex multicellular organisms are widely acknowledged to have first appeared on Earth according to the fossil record.
