The accepted theory of evolution suggests that. species change over time
living species are related to extinct species.
different species can interbreede
animals that look alike are the most closely related
traits acquired in ones life time can be passed on
inherited variation allows some members of a population to be more fit than others.

unicellular organisms, like bacteria, are able to perform all life functions within one single cell. multicellular organisms need many different types of cells to carry out the same life processes. each of these special types of cells has a different structure that it perform a specific function.

cell/ tissue/ organ/ organ system/ body

for a different answer set the answer would be because the oxygen in the water had depleted (apex)