Reality → Life → Evolution → Origin of life
Precambrian stromatolites are the oldest fossil record of unicellular life [1] . Conserved relicts of any earlier life form do not exist. Lab experiments have shown that the likely conditions of the early Earth's atmosphere can produce organic molecules from inorganic gases [2] . How the first organic molecules could arrange themselves to form such complex structures as RNA/DNA, functional proteins, and finally a living cell, is a fascinating subject of hypotheses [3] . The first appearances of self-replicating biomolecules [4] , photosynthetic reactions [5] , and eukaryotic cells [6] are key early evolutionary events whose details are shrouded in mystery.
Cyanobacteria have been identified in 1.5 - 2.0 billion years old stromatolites (rocks with thin microbial layers) found in Australia, South Africa, and North America. Cyanobacteria are prokaryotes with photosynthesizing capability, a relatively advanced feature. Some stromatolite-like rocks may be up to 3.5 billion years old, but it cannot be ascertained whether their thin layers are of biological or mineralogical origin.
Guided by Oparin's theory of the ' primordial soup', the 1952 Miller–Urey experiment simulated Earth's Archean atmosphere and produced organic molecules, including amino acids, from inorganic matter (for the role of amino acids in biology see Proteins and related Note 3).
The slow and erratic progression to complex macro-biomolecules and the first life form or forms (e.g., LUA ?) happened during the long fossil-deprived 'dark' (Precambrian) era. Modern genomic research points to thermophilic microorganisms in the sulphur and iron-rich environment of deep sea hydrothermal vents as a possible scenario for life's origin (for more details and other hypotheses see Abiogenesis, From soup to cells, Schritte zum Leben, and Entropy and life).
In all modern organisms DNA, RNA, and proteins are necessary for replication of genetic information and biosynthesis. It is now generally believed that the highly complex processes could not have evolved without a long extinct RNA world that used simpler forms of replication and was a precursor of today's DNA, RNA, and protein world.
Based on geological evidence and related studies of Earth's early atmosphere, it is believed that photosynthetic activity first appeared with anaerobic bacteria (possibly more than 3.5 billion years ago), before oxygenic photosynthesis emerged with cyanobacteria, and increased to present levels after the advent of land plants (see slides for the early evolution of photosynthesis).
The first eukaryotes appeared possibly more than 2 billion years ago. According to the endosymbiotic theory, proto-eukaryotic cells engulfed prokaryotes to form some of the most important organelles (mitochondria for animal cells and chloroplasts for plant cells). The oldest eukaryotic fossils found are the Ediacaran biota, which lived 600 million years ago, before the Cambrian explosion. It is unclear where in the early 'tree of life' these organisms can be accommodated.