Final answer:
All plants, including green algae and land plants, are monophyletic and share a single common ancestor, believed to be an ancestral green alga. The earliest plants evolved in the ocean and moved to land, developing various adaptations for terrestrial life, such as vascular tissues. The oldest known fossils of land plants date back about 470 million years.
Step-by-step explanation:
According to current evolutionary thought, all plants, including green algae and land plants, are considered monophyletic, meaning they descend from a single common ancestor. This ancestor is thought to have been an ancestral green alga which gave rise to the first vascular plants. The transition from an aquatic to a terrestrial environment required plants to evolve various adaptations such as mechanisms to avoid desiccation, ways to disperse reproductive cells in air, structural support, and efficient capture and filtration of sunlight.
While the evolution of vascular tissues has allowed plants to grow larger and become more independent from water, not all plants have achieved full independence. Many seedless plants, resembling early land plants like liverworts, still require moist environments for their reproductive processes. Plants may have colonized land about 700 million years ago, but the oldest known fossils of land plants date back to approximately 470 million years.
Thus, while flowering plants like the dense blazing star and the purple coneflower can vary greatly in appearance, they still maintain the fundamental characteristics inherited from their common ancestor. This example demonstrates that despite the vast diversity of plants, they all share a similar basic morphology and evolutionary history.