The available evidence suggests that the earliest organisms were primitive bacteria which were probably of extremely limited metabolic capability.
They were likely to have used as food abiotically synthesized molecules which they broke down without the use of oxygen. At some stage they developed the capacity for photosynthesis. They were also prokaryotic rather than eukaryotic, i.e. they possessed no nuclear membranes, nor did they have mitochondria or chloroplasts.
These are characteristics of present-day organisms such as the Cyanobacteria which include the blue-green algae.
Whilst the earliest organisms date back about 3.5 billion years, it is thought that it was not until about 1.4 billion years ago that the first eukaryotes evolved.
These organisms had all of their cell organelles enclosed in membranes, a situation which has persisted in the cells of all eukaryotic plants up to the present time.
The earliest single-celled plant fossils occur in rocks about 3.5 billion years old. Studies of these ancestral plants in successively younger rocks reveal more species in which the cells are arranged in long chains or irregularly in other groupings. These arrangements are examples of the process whereby plants consisting of many cells evolved from unicellular ancestors.
There are a number of famous localities in which these earliest of plants occur as fossils.
- The oldest known so far are the Black Cherts of Western Australia dating from about 3.5 billion years. Genera from this locality include Eosphaera, Kakabekia and Gunflintia.
- The Fig Tree Series in South Africa is only slightly younger, in geological terms, at 3.2 billion years. In this formation are seen Archaeosphaeroides and Eobacterium; the single cells of the former resemble in many of their characteristics those of modern unicellular blue-green algal species classified in the Chroococcales. Gunflintia was a filamentous form having a resemblance to extant chemosynthetic iron-forming bacteria.
Some attention must be paid to the establishment of the biological authenticity of the organisms described from these very early rock formations.
Admittedly some have such highly characteristic features also recognisable in extant organisms that they have every reason to be accepted for what they appear to be.
However there are those whose appearance is so similar to certain inorganic features in rocks that tests have to be applied.
One very useful one is the measurement of the 13C/12C ratio in the fossil material [radiocarbon dating; see Radioactive dating].
Recognition that the ratio of these two carbon isotopes in the fossil material under investigation is the same as would be obtained from a similar analysis of biological material at the present day, has gone a long way to establish the fossils as bona fide remains of biological organisms.