Plant Evolution and Diversity
Topics in Depth
The Theme of Plant Predecessors in Plant Evolution and Diversity
The Earth was once a dark and stormy place, where everything alive was found in the ocean, cooking away in a big pot of primordial soup. Over time, early ancestors of plants and animals moved to land and developed lots of new traits that allowed them to survive above water. Before we jump into the grimy details of plant prototypes kicking and fighting their way out of the primordial ooze onto land, take a few minutes to review the basic principles of evolution here.
The ancestors of land plants were algae. There are red, green and brown algae. But even though they can be green, algae are not plants. Scientists usually distinguish plants from algae because plants are embryophytes: they develop an enclosed embryo on the plant body, usually in archegonia, which produce eggs. Algae do not make embryos. They make spores, which they don't nurture in enclosed, protected spaces. Geez, it's like they don't love their babies. The ancestor of land plants was most likely shared with a group of green algae called the charophytes. Now, we don't want everyone to go picking favorites among the algae, because there are lots of awesome algae out there. But charophytes are special because they share a few important traits with land plants:
- Charophytes and land plants share certain enzymes that other green algae don't have. These enzymes help the cell hold onto organic products that it doesn't want to lose, like its cell phone and wallet. The enzymes in question are found in peroxisomes of charophytes and land plants, but not other algae. Coincidence? We think not.
- The proteins that make cellulose in both charophytes and land plants are arranged in circles. In other algae, they are arranged in lines.
- Sperm shape. Sperm of land plants looks much more like charophyte sperm than sperm of other algae.
- Cell division in land plants is more similar to charophyte cell division than other algal cell division.
In addition to the above traits, some charophytes also have a protein in their cell walls called sporopollenin, which was present in early land plants. The charophytes have DNA that is closer to land plants than other green algae. Together, these observations provide good evidence that land plants and charophytes shared a common ancestor.
Algae may not sound very exciting, but some people are betting that they will run our cars one day. Scientific research labs are working on extracting oils from algae for that very purpose.
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