The Archaeplastida Lineage:
The Archaeplastida lineage, which includes red and green algae as well as land plants, began with a eukaryotic cell engulfing a cyanobacterium—a process known as primary endosymbiosis. This event laid the foundation for plastids (organelles like chloroplasts) and the rise of photosynthetic eukaryotes. Over time, the Archaeplastida diversified, leading to the divergence of red algae (Rhodophyta) and green algae (Chlorophyta) approximately 1.5 billion years ago.

Evolutionary Context:
This split occurred roughly 200 million years after the broader divergence of eukaryotic supergroups, including animals, plants, and fungi (~1.7 billion years ago). While red algae adapted to marine environments with pigments like phycoerythrin for deep-water photosynthesis, green algae thrived in shallower waters with chlorophylls a and b, eventually giving rise to land plants. These evolutionary innovations played a crucial role in shaping Earth’s biosphere, paving the way for modern ecosystems.