1. The oldest evidence for life on Earth

Simple life forms existed on the Earth 3,500 million years ago. Fossils thought to be evidence of once-living cells have been discovered in rocks of this age in Western Australia and South Africa. The fossils are of two types: finely layered structures called stromatolites; and microscopic spheroidal or thread-like structures. These fossils show that life originated within the first 1,000 million years of the Earth's existence.

Primitive cells, complex cells and multicellular organisms

Simple Cell. Artist: Kate Nolan. Source: Museum Victoria.

The very earliest organisms in the fossil record are cells of a very simple, primitive type where the DNA is free within the cell, and the cell is without internal structures. Cells of this type, called prokaryotes, are represented by bacteria and some other single-celled organisms living at the present day.

By 1,700 million years ago other cells had appeared, called eukaryotes. Most of the DNA of eukaryotes is contained in a nucleus,

Complex Cell. Artist: Kate Nolan. Source: Museum Victoria.

and the various functions of the cell are carried out by separate internal structures, some of which contain their own DNA. Eukaryotes compose over 99% of organisms living today, including all multicellular organisms. The appearance of eukaryotes was therefore one of the most significant evolutionary developments within the living world. Fossils of eukaryotes are known from Western Australia, in rocks dated at about 1,500 million years old, and from 800 million year old rocks from central Australia.

Multicellular Organism. Artist: Kate Nolan. Source: Museum Victoria.

Multicellular organisms evolved from single-celled eukaryotes. The first multi-cellular organisms were seaweeds, known from 1,300 million year old rocks from the United States. Multi-cellular animals, or metazoa, seem to have appeared somewhat later. The first evidence for them is found in rocks 900 million years old and consists of burrows made by worm-like animals, though the animals themselves are not preserved. The oldest known fossils of the bodies of multicellular animals are about 600 million years old.

Ediacaran fauna
In 1946 a South Australian geologist, Reg Sprigg, discovered fossil impressions of the oldest known multi-cellular animal, in the Ediacara Hills, north of Adelaide. Similar fossils have since been found at other localities in Australia, and in South Africa, Europe, Asia, North America and possibly South America. These fossils, which are known collectively as the Ediacaran fauna after the locality where they were first found, are all between 580 and 650 million years old. They include a variety of circular, frond-like or segmented organisms, all of them entirely soft-bodied. It is still uncertain what types of animals these fossils are represent. Some have been interpreted as jelly fish, worms, soft corals, echinoderms and arthropods, but some scientists now believe they may belong to types of organisms no longer surviving at the present day. It is clear, however, that the animals of the Ediacaran fauna are not the ancestors of the shelled animals that first appear in the fossil record in younger rocks of Cambrian age, around 545 million years ago.

Oxygen in the atmosphere
The Earth's original atmosphere was formed from gases released during cooling of the planet. It would have contained little or no oxygen, and so could not have supported animal life. Scientists agree that, as a result of photosynthesis, the level of oxygen in the atmosphere gradually increased during the Precambrian. Photosynthesis is a chemical process in which plants, and some bacteria, use sunlight to make chemicals (such as sugars) for energy, from carbon dioxide and water, at the same time releasing oxygen as a waste product. Present-day stromatolites are photosynthetic, and scientists think that stromatolites from 3,500 million years ago were too. Oxygen had apparently reached moderate levels in the Earth's atmosphere by 2,000 million years ago.

Stromatolites
Stromatolites are sheet-like, mound-like or columnar structures formed by primitive bacterial communities. These structures are made of a hard limestone-like substance, and range in size from a centimetre to several metres across. Inside, they are made of many fine concentric layers. Stromatolites are the most noticeable fossils known in Precambrian rocks, and were most common between 2500 million and 700 million years ago. They are rarer in younger rocks, probably due to the evolution of grazing invertebrates such as snails, which ate the bacteria depositing the stromatolites. Though rare at the present day, modern stromatolites can still be found in environments where grazing invertebrates are uncommon.

Banded iron formations
Banded iron formations are sedimentary rocks made up of alternating layers of iron-rich minerals and chert, a form of quartz. These formations are the source of most of the iron ore mined in the world, such as in the Hammersley Ranges in Western Australia. Banded iron formations are believed to have formed by iron minerals dissolved in sea water combining with oxygen, and falling out, or precipitating from the water. This is evidence that there was enough oxygen in the atmosphere at that time to cause iron minerals to precipitate. Banded iron formations reached their maximum development about 2,000 million years ago.