Plant Science Portal 
Beginnings of Life
 Plant Evolution Tour: Part I of XV
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Scientists believe that the world's continental crust started to develop about 4200 Ma (4.2 billion years ago) and by 1900Ma, a large landmass called Rodinia had formed on the earth.

At about 1000 Ma, Rodinia split up to form three smaller landmasses, West Gondwana (modern-day South America and West Africa), East Gondwana (most of Australia, eastern Antarctica, India and Southern Africa) and Laurasia (Greenland, North America, Baltica and Siberia).

The Earth's surface would initially have been very hot owing to the thin crust which would have poorly insulated against the warmth generated from the earth's centre. The crust is thought to have developed at around 3000Ma, and was composed of igneous rock (from volcanos), weathered sediments and remnants of the millions of meteorites which were bombarding the earth at the time. Because of the regular bouts of volcanic activity, the atmosphere contained very high concentrations of carbon dioxide (CO2) and methane (CH4). This would have created a significant "glasshouse effect", and in combination with the thermal heat generated from the Earth's core meant that the world was a lot warmer than in current times with global temperatures averaging between 30°C and 50°C. In addition, the concentration of oxygen (O2) is thought to have been <1% of the earth's atmosphere. Other gases in the atmosphere present in low concentrations at the time included hydrogen, hydrogen cyanide, hydrogen sulphide and formaldehyde.

It was in this harsh environment that it is thought the first cells evolved. For this to occur, organic matter would have had to form first and there are two theories as to how this happened.

The first was proposed by Miller (1953) who found under laboratory conditions that hydrogen, carbon dioxide, ammonia and methane gases, when heated with water and charged with electricity for 24 hours converted much of the carbon into other organic compounds such as sugars, amino acids, purines, and pyrimidines - compounds which are all essential building blocks of life. It was hypothesised that these conditions replicated the earth's environment at the time, and if organic molecules could be formed in 24 hours, then it was probable that over a much longer period the first cells developed in similar conditions.

The second theory states that the first organic material was imported with meteorites onto the planet from space, however this seems less probable.

For cells to evolve, membranes would have had to be created and it is thought that tidal pools (the "primordial sea") are the most likely location for this. The waters would have contained amphiphilic organic compounds. Amphiphilic compounds contain a hydrophobic (water-repelling) and hydrophilic (water-soluble) ends which would have formed spherical clumps (with the hydrophobic ends pointing to the centre of the sphere and binding to each other). Any liquid on the inside of the spherical clump would have been isolated from the surrounding solution, and it is considered this could have been the beginnings of a 'protocell'.

Prokaryotes: The first living organisms

The first organisms were likely to be prokaryotes which still exist today (see picture). These are the simplest life forms usually ranging in size between 1-10 μm in length and are single-celled. They reproduce by expanding then dividing into two. Stromatolites dating back to 3500 Ma provide evidence for this. Stromatolites are composed of layers of sediment in light and dark shades which are thought to be the fossilised remains of primitive bacterial and/or algal mat communities. Stromatolites are still being produced today in the intertidal zone at Shark Bay in Western Australia, and are composed of alternating layers of calcium carbonate (CaCO3) and microorganisms (algae).
Stromatolites growing in Shark Bay, WA, Australia.
Stromatolites growing in Shark Bay, WA, Australia. Photo by Robert Paul  Young, licenced via Creative Commons.
Cherts and shales have also provided evidence for the world's first life forms. Some of the most significant cherts are also located in Western Australia; the Warrwoona Group. Carbonaceous cherts there date back to 3500-3300 Ma, and contain filamentous and conical structures which are likely to be early life forms, probably photoautotrophic cyanobacteria. This is very significant for a study of plant evolution because plants derive their energy from photosynthesis; a process where light is converted into chemical energy and carbon dioxide into oxygen. The presence of cyanobacteria dating back to 3500 Ma indicates the age of photosynthesis.

There are much younger deposits throughout the world dating back to the Proterozoic (2500-540 Ma) which show an increasing degree of diversification (creation of new species) and biological sophistication over time.

Eukaryotes

The majority of modern life forms are eukaryotes. These differ from prokaryotes because they have a membrane-bound nucleus in their cells which contains their DNA (genetic information). Eukaryotes also have membrane-bound organelles (special parts of a cell with specific biological function), can be multicellular, and often reproduce sexually rather than through simple mitotic division like prokaryotes.

It is suggested that the eukaryotes didn't necessary 'evolve' all of their organelles, but that some organelles may have started out as separate prokaryotic organisms which were 'engulfed' in an "endosymbiotic" process. There is evidence for this in modern day plant cells, as mitochondria and chloroplasts each have their own DNA which is different from the nuclear DNA of the organism they belong to. And this DNA has similarity with the DNA of other primitive prokaryotic organisms.

Plants, animals, fungi and Protista (algaes) are all eukaryotes. The earliest fossil evidence for eukaryotes dates back to 2700 Ma. Looking at the sheer number of eukaryotic organisms which have evolved since, this is a very significant event! The oldest fossil which is significant in a discussion of plant evolution is an alga called Grypania which is 2100 Ma. Algae were the precursors to plants.

The number of life forms altered considerably between 1000-540 Ma when several glacial periods (ice ages) occurred and led to significant numbers of extinctions. Nevertheless, there was always a relatively quick renewal in the number of species. Because algae are photosynthetic organisms, and their numbers increased over time, oxygen was produced and this significantly increased the atmospheric concentration of O2 in the Earth's atmosphere. This had a major impact on the life that the Earth could support.
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