Cyanobacteria have a close evolutionary relationship with eukaryotes. They have the same photosynthetic pigments as the chloroplasts of algae and land plants. Chloroplasts are the right size to be descended from bacteria, reproduce in the same manner, by binary fission, and have their own genome in the form of a single circular DNA molecule. The enzymes and transport systems found on the folded inner membranes of chloroplasts are similar to those found on the cell membranes of modern cyanobacteria, as are their ribosomes. These similarities between cyanobacteria and chloroplasts suggest an evolutionary link between the two, and can be explained by the theory of .
DNA evidence suggests that the first eukaryotes (green plants) evolved from prokaryotes (through events) between 2500 and 1000 million years ago. Fossils of eukaryotes that resemble living brown algae have been found in sedimentary rocks from China that are 1700 million years old, while possibly the oldest photosynthetic eukaryote, , comes from rocks 2100 million years old. Note that the diversity of modern algal groups, and particularly of their chloroplasts, suggests that these endosymbiotic events were not unusual. Modern algae comprise a range of organisms with very different structures but identical photosynthetic pigments. This suggests that very different host organisms have formed a symbiosis with the same photosynthetic cells. That is, the algal groups must have evolved through separate endosymbiotic events, and the group as a whole is identified on the basis of a similar level of structure, rather than on its evolutionary origins. Such groups, where the members have several different evolutionary origins, are described as .
It is another non-leafy thallous liverwort anchored by
unicellular rhizoids and differentiated into a photosynthetic epidermis
and cortex and a starch-storing medulla.
Flowering plants first appeared about 180 million years ago. Today, 90% of the plant kingdom are flowering species, while algae, conifers, ferns, horsetails, mosses, liverworts, and their relatives, make up the remaining 10%.
While the diploid generation in mosses is larger and longer-lived than in liverworts, it is still reliant upon the photosynthetic, haploid gametophyte out of which it grew for its nutritional needs.
Gametophores are haploid tissue, just like the primary photosynthetic tissue of the liverwort, and produce sperm and egg cells with mitotic haploid nuclei identical to the cells in the plant body and gametophore.
With th exception of algae that can developon land, liverworts are currently regarded as the oldest terrestrialplants on Earth (fossil evidence indicates that thalloid liverworts areprobably older than leafy liverworts).
The earliest photosynthetic organisms on land would have resembled modern algae, cyanobacteria, and lichens, followed by bryophytes (liverworts & mosses, which evolved from the group of green algae). Bryophytes are described as seedless, nonvascular plants. Their lack of tissue for transport of water and nutrients limits their size (most are between 2 and 20 cm high). Bryophytes don't have typical stems, leaves, or roots, but are anchored to the ground by rhizoids. They can grow in a wide range of environments and are : when the environment dries so does the plant, remaining dormant while dry but recovering rapidly when wetted. These features make them important pioneer species.
These first land plants evolved from the green algae, with which they share a number of traits. All store energy reserves, as starch, inside plastids. Their cell wall is built of cellulose microfibrils and the photosynthetic pigments are chlorophylls a and b, plus b-carotene.