Final answer:
Land plants evolved from freshwater green algae over 400 million years ago, acquiring adaptations such as protective cuticles, stomata, and vascular tissues, which facilitated their transition to and diversification on land.
Step-by-step explanation:
Land plants evolved from forms of freshwater green algae, and this transition is estimated to have occurred a little over 400 million years ago. The evidence for this evolution includes common cellular structures like cellulose in the cell walls and shared biochemical compounds. Green algae such as those from the Charophyta division are seen as the closest relatives to early land plants. Importantly, over time, land plants developed several adaptations for life on land, which included the retention of the embryo within the plant, the evolution of a waxy protective cuticle, the development of gas-exchanging stomata, and the innovation of vascular tissue for water and nutrient transport.
The progression from green algae to more complex plant forms was driven by adaptations that allowed these organisms to overcome challenges posed by terrestrial habitats. Vascular tissues allowed for the transport of water and nutrients from the soil, which was critical for survival on land, a major difference from algae which could directly absorb water from their surrounding water environment. As adaptations like seeds, and later flowers, evolved, plants became even more successful in terrestrial ecosystems, eventually leading to the vast diversity of plant life we see today.
The fossils of the earliest land plants show distinct stems and rudimentary leaves, and though they required moist environments for reproduction, they laid the groundwork for future plant evolution. The move onto land offered vast new ecological niches for plants, and as a result, they became a fundamental part of terrestrial ecosystems, eventually facilitating the colonization of land by other organisms, including animals.