We know plants evolved from algae, and over time they got more complex. But how is everything related? Why did plants evolve the way they did? And how do we know all this stuff about plant evolution, anyway?
The timeline of plant evolution looks something like this. Algae (on the left) are the oldest and angiosperms (on the right) are the youngest. BYA = billions of years ago; MYA = millions of years ago.
Please recreate a figure showing the evolutionary relationships of everything included here.
Each of the traits that appeared in plants somewhere along the evolutionary timeline probably arose because it was somehow advantageous in the environment. Remember that evolution has no direction—a plant can't evolve drought tolerance because it wants to, and just because a plant seed might end up in the desert doesn't mean that plant species will evolve drought tolerance.
However, if a plant lives in a dry place and is better at tolerating drought than some of its neighbors, drought tolerance will likely be selected for. That is, the plants that can't handle drought at all will be filtered out. This differential survival based on traits is how evolution works. Individuals don't evolve; populations evolve.
The natural selection driving evolution is often climate and other species. In the very beginning, geology was most important because before plants could move to land, there had to be a stable land surface and soil for them. When the lycophytes were growing tall and stately, the Earth's climate was very tropical. But later the climate cooled and dried out, and lycophytes were no longer favored. New plants did better in the natural selection battle (Darwin's "survival of the fittest"), at least temporarily.
Scientists know where plants lived in the past from the fossil record. Plant fossils are a valuable source of information, and it is possible to know how old they are thanks to radiometric dating. The fossil record is incomplete though, so there are not fossils of every single plant species that ever lived.
Another major tool for learning about plant evolution is DNA. By looking at similarities in DNA sequences, it is possible to see evolutionary relationships between plants. Scientists use plant DNA to learn about relationships on every level of organization, including populations, species, genera and families.
Plant structures are constrained by evolution. Even though evolution can produce new forms, it has to build on pre-existing structures. Let's look at the structures that evolved in plants over time.
Cuticles: The cuticle is a waxy layer that covers aboveground plant parts. Cuticles prevent drying out, which was an important adaptation for surviving on land where water isn't touching every surface of the plant body.
Transport systems: Xylem and phloem are the tissues that conduct water and nutrients around a plant body. When plant ancestors lived in water, they didn't need special transport systems because every cell was close to water and nutrients. But when plants moved to land, the lack of a transport system made it hard to get water to all the cells, and limited the height a plant could achieve. Early land plants developed tubular cells that water could pass through, and fossils have been found with tube-like cells that are similar to those in modern mosses. Eventually, stems got thicker and sturdier, which allowed transport of materials and also supported plant height.
Roots: Roots anchor plants to the ground and also absorb nutrients from the soil. The earliest roots from the fossil record are 408 million years old, showing that early vascular plants developed roots at least a few feet long. Scientists think that early root systems probably weren't very different from aboveground plant parts, but over time specialized for living underground.
Leaves: Early land plants did all their photosynthesis in their stems. After 40 million years of doing this, someone got the bright idea to grow leaves. No, not really, because remember evolution isn't directional. But leaf-like structures began developing in two different forms, and we can still see the different forms in modern plants.
One type of leaf that evolved was small and grew directly from the stem. These are called microphylls. Lycophytes had (and still have) microphylls, which look more like thin stems than leaves. Microphylls only have one strand of vascular tissue. The other leaves were larger and attached to the stem with a petiole. These are called megaphylls, and are more similar to the typical leaves we think of today. They also have branching vascular tissue, letting the leaf be broad. All flowering plants have megaphylls, as do ferns.