The Earth was born in a ball of fire in the bright, hot, ear­ly solar sys­tem, about 4.6-billion years ago.

As the Earth cooled, a pri­ma­ry broth” formed over the thin crust to cre­ate the first oceans, and gas­es exhaled from with­in – main­ly water vapour and car­bon diox­ide – formed the first prim­i­tive atmos­phere. The Earth in those far-off times was devoid of life.

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This is an artist’s con­cept of the young Earth being bom­bard­ed by aster­oids. (Source: NASA Flickr)

The Earth’s crust is made up of a mosa­ic of rigid plates” which float on a man­tle of hot, plas­tic rock. Since the Earth formed, these plates have been mov­ing rel­a­tive to one anoth­er in a process known as tec­ton­ic shift. Around 250-mil­lion years ago, plate move­ments had giv­en rise to a super-con­ti­nent named Pan­gaea, which was sur­round­ed by a vast ocean.

This super-con­ti­nent broke up about 200-mil­lion years ago to form a giant south­ern con­ti­nent called Gond­wana and a giant north­ern con­ti­nent called Laurasia.

Gond­wana and Laura­sia start­ed break­ing up around 135-mil­lion years ago. By about 65-mil­lion years ago, at the time the last dinosaurs died out, our world start­ed to look like some­thing we would recog­nise, though North Amer­i­ca and Europe were still joined in the north, and Aus­tralia and Antarc­ti­ca were still joined in the south.

This ongo­ing process of tec­ton­ic shift has meant not only that the con­ti­nents con­tin­ue to move – on aver­age a few cen­time­tres a year or at rough­ly the rate fin­ger­nails grow – but also that dif­fer­ent parts of the Earth’s sur­face have been cov­ered by water at var­i­ous times, and that some of the area now under the sea was once dry land.

Some of the ear­li­est forms of life evolved in the sea about 3.8-billion years ago. These were sin­gle-celled bac­te­ria and cyanobac­te­ria, also known as blue-green algae, and were sim­i­lar to the black algae that some­times grow in swim­ming pools.

Gondwana

Gond­wana (Source: Wikicommons)

Over time, these bac­te­ria trapped fine sed­i­ments and formed lay­ers that built up on top of each oth­er to form mound­ed and lam­i­nat­ed liv­ing struc­tures called stro­ma­to­lites. Stro­ma­to­lites are still form­ing in some places, such as in the shal­low waters of Shark Bay off the west­ern coast of Aus­tralia. The stro­ma­to­lite fos­sil record is the only evi­dence we have of life on Earth for the first sev­en-eighths of the planet’s existence.

Sci­en­tists have found fos­sil traces of stro­ma­to­lites which are about 3.5-billion years old – some of the world’s old­est – near Bar­ber­ton in South Africa’s Mpumalan­ga Province. They were pro­duced by life forms sim­i­lar to mod­ern-day algae, and were formed in ancient, shal­low, saline waters, along what was once a rocky coast­line. Stro­ma­to­lite fos­sils of a sim­i­lar age have also been found in north-west­ern Aus­tralia and Green­land. Fos­sils of stro­ma­to­lites have also been dis­cov­ered at Sterkfontein.

By about 2-bil­lion years ago, these organ­isms, which through pho­to­syn­the­sis and res­pi­ra­tion absorb car­bon diox­ide and release oxy­gen, had begun to oxy­genate the Earth’s atmosphere.

As liv­ing organ­isms evolved they became increas­ing­ly com­plex and diverse. From those first sim­ple, sin­gle-celled bac­te­ria, the vari­ety of life explod­ed. Today, there are about 1.5-million doc­u­ment­ed species of liv­ing things, out of an unknown total of per­haps some­thing between 30-mil­lion and 100-mil­lion species.

Dolomite pre­serves fos­sil stro­ma­to­lites rep­re­sent­ing some of the ear­li­est known forms of life on Earth, which emerged in the oceans.

Jlee7795

Inside the Sterk­fontein Caves

With­in the dolomite are lay­ers of chert, a hard, grey, sed­i­men­ta­ry rock. The chert at Sterk­fontein con­tains rip­ple marks which, along with the stro­ma­to­lites, bear tes­ti­mo­ny to the ancient inland sea that once cov­ered the area.

The dolomitic lime­stone con­tains cal­ci­um car­bon­ate, mag­ne­sium car­bon­ate and small amounts of man­ganese, iron and sil­i­ca. Caves were to form by a process of solu­tion in these rocks much lat­er (prob­a­bly from around 20-mil­lion years ago, dur­ing the Miocene peri­od). The sil­i­ca formed the hard bands of chert with­in the dolomite.

The dis­solved cal­ci­um car­bon­ate was lat­er rede­posit­ed as sta­lac­tites and sta­lag­mites; this was what even­tu­al­ly drew lime min­ers to the caves in 1896.

About 2.6-billion years ago, in what is known as the Neoar­chaen era, the area around the Cra­dle of Humankind lay under an inland sea. Dolomitic lime­stone formed as sed­i­ment in this shal­low sea, and is the pre­dom­i­nant rock in the hun­dreds of caves in the area today.