A rough timetable of some key events in the history of the biosphere
125 ka early modern Homo sapiens
4 Ma early Australopithecus
150 Ma early flowering plants
440 Ma earliest higher land plants
500 Ma earliest vertebrates
600 Ma earliest animals with skeletons
2 Ga early eukaryotes
3.5 Ga oldest evidence for life 23223v2112x on Earth
4.0 Ga oldest rocks on Earth - early hydrosphere
4.0 Ga major meteorite bombardment - degassing?
4.6 Ga origin of Solar System and Earth
13 Ga 'Big Bang' - origin of the Universe
Serious scientific explanations focus on a chemical origin for life on Earth.
Oparin (1938) and Bernal (1951) published major books on the chemical evolution of life.
Miller (1953) carried out experiments which led to the synthesis of amino acids from a mixture of gases.
Following Miller's pioneer work large number of complex organic compounds have been synthesised, 'prebiotic synthesis', in the laboratory under allegedly 'primitive Earth' conditions.
Initial focus was on amino acid, protein and DNA synthesis by polymerisation but increasingly biologists believe that RNA-based organisms preceded DNA based life forms.
Aquatic environments - protection from UV; concentration of compounds?
Soil - protection from UV; moist; clay minerals as catalysts?
Hot springs/Submarine vents - energy source; modern sulphur-metabolising thermophilic bacteria are regarded by many as primitive/ancestral.
Extraterrestrial influences - some meteorites (carbonaceous chondrites) and interplanetary dust particles have been shown to contain complex organic molecules, even amino acids. This has led to suggestions that the Earth was 'seeded' from space.
A more rational explanation points to the ease of synthesis of these compounds of carbon and that there is a fundamental and universal chemical logic to life based on carbon.
What is the evidence for the earliest life on Earth?
There are 3 principal forms of evidence:
Microfossils - the remains of microbes preserved in sedimentary rocks
Stromatolites - organosedimentary structures that are formed by Cyanobacteria which are an important group of photosynthetic bacteria.
Organic and Isotopic Geochemistry - the chemistry of compounds and isotopes (C, O, and S) from ancient sedimentary rocks has provided some interesting evidence but it is very difficult to interpret.
First life - the balance of evidence
The presence of stromatolites in rocks 3.5 Ga old from southern Africa and Australia suggests that within 1Ga from its origin the Earth had become populated with organisms that:
were capable of motility
used chlorophyll-based photosynthesis and liberated oxygen to the atmosphere.
A fundamental evolutionary divide lies between the Prokaryotes, which are thought to be ancestral, to the more sophisticated Eukaryotes.
The main differences between these groups of organisms are in terms of the level of cellar organisation - which in evolutionary terms is very important.
Prokaryotes are essentially single-celled microrganisms with a simple cell structure that lacks nuclei and other specialised cell organelles.
All modern prokaryotes belong to the Bacteria which are highly diverse in terms of their metabolism and habitat.
Eukaryotes include single-celled and multicellular organisms with sophisticated nucleate cells.
Modern eukaryotes include plants, animals and all other non-bacterial organisms.
How did Eukaryotes evolve?
Symbiosis - intimate association between two dissimilar organisms eg:
Lichens - algal and fungal cells
Corals - contain algal cells
This theory maintains that the origin and evolution of eukaryotes was achieved by a series of symbioses between different types of prokaryote.
When did Eukaryotes evolve?
Fossil evidence is difficult to interpret. Most Precambrian microfossils are from silicified stromatolites.
Organic walled microfossils from marine shales exhibit a greater morphological diversity and include microfossils that are larger than the normal range of bacteria.
These
larger forms start to appear around 2.0 Ga microfossils so there is a sort of
consensus that eukaryotes had become established by at least 2.0
The
beginning of the demise of abundant stromatolites suggests that
early animal life had evolved by 1.0
From embryological studies biologist have believed that he earliest metazoa were probably segmented worm-like organisms.
The presence of simple tracks and trails preserved in sedimentary rocks by 600Ma is evidence for macroscopic animals - Metazoa.
By 570 Ma trace fossils were more elaborate and soft-bodied metazoa like those of the Ediacara fauna had evolved.
Animals with mineralised elements - 'skeletons' - had become established by 540 Ma and most of the major animal phyla had become established soon after this.
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