Green algae contain diverse structures and live in a variety of habitats, including inside other organisms. Biologists believe green algae most closely relate to the plants living on land. Because of their place in phylogeny, green algae have been proposed as precursors to the evolution of Earth's green plants. Biologists have considered the evolutionary role of algae in terms of a structure called a holdfast that is present in some aquatic green algae. Holdfasts anchor algae to surfaces for part of their life cycle yet do not draw in any nutrients as do root systems in terrestrial habitats. Some green algae growing underwater and held in place by holdfasts resemble lawn grass. The grass- green pond scum that grows into long filaments and covers the water's surface also depends on holdfasts to keep the algae attached to the earth.

Chlamydomonas is a genus of flagellated green algae prevalent in freshwater and moist soils. Because of its ability to use sexual or asexual reproduction, Chlamydomonas resides at a key point in the evolution of different types of algae. The first evolutionary path contains Chlorella, a nonmotile alga that reproduces asexually. Chlorella's reproduction involves the formation of a protoplast (a cell lacking a cell wall), which then divides to form from two to 16 daughter cells that are identical to the parent cell. In Chlorella metabolism, the cells ferment the sugar glucose to lactate when growing in anaerobic conditions and use aerobic photosynthesis to make energy when oxygen is available (Chlorella's preferred metabolism).

A second important path in evolution contains Volvox. This organism forms a hollow sphere of a single layer of from 500 to 20,000 individual cells. Flagella on each cell beat in a coordinated way to rotate the entire colony and propel it through its watery habitat. When using sexual reproduction, Volvox employs male and female gametes each containing half of the chromosome. Volvox represents a critical step in evolution in which single cells combined and worked together in coordination as a multicellular organism. Volvox thus offers a glimpse at two important phases in the evolution of higher organisms: sexual reproduction in which male and female gametes combine their genes, and independent motility created by cells working together in a coordinated manner.