A rock with different fossils is shown below: A diagram labeled Fossil Layers is shown. The topmost layer is labeled Fish and shows the skeleton of a fish, the second layer from the top is labeled Shell and shows a shell, the third layer from the top is labeled Fern and shows a fern leaf, and the lowermost layer is labeled oak and shows an oak leaf

Based on the fossil, which organism evolved first?

Fern
Fish
Oak
Shell