Sterk­fontein is a cave sys­tem that has been exca­vat­ed by palaeon­tol­o­gists and archae­ol­o­gists since 1936, when the first hominid fos­sil was found here.

Archaeologists Skywalk At Sterk

Palaeon­tol­o­gists’ sky­walk at Ster­fontein Caves, allow­ing them to criss-cross the area effectively

Palaeoan­thro­pol­o­gy is the sci­en­tif­ic study of hominid fos­sils and their cul­tur­al mate­r­i­al and ori­gins. Sterk­fontein is home to one of the world’s longest run­ning palaeon­to­log­i­cal excavations.

Sterk­fontein has pro­duced some of the most famous hominid fos­sils in the world, togeth­er with a range of oth­er fos­sils of ani­mals and plants. The site has a very high con­cen­tra­tion of fos­sils – the high­est in the Cra­dle of Humankind – which, as a whole, has pro­duced more fos­sils of ear­ly hominids than any oth­er site on Earth.

Sterk­fontein has also yield­ed stone arte­facts that are up to almost 2-mil­lion years old, and are the old­est dat­ed stone tools in South­ern Africa.

Palaeon­tol­o­gist Dr Robert Broom began to recov­er fos­sils from lime min­ing activ­i­ties at Sterk­fontein in 1936. From 1945 onward, he and his col­league, John Robin­son, used con­trolled explo­sions to extract more fossils.

In 1957, Dr CK Bob” Brain of the Trans­vaal Muse­um found stone tools in a min­ers’ dump and Robin­son and Revil Mason used a grid sys­tem to exca­vate in the stone tool area. This grid sys­tem enabled them to record the hor­i­zon­tal and ver­ti­cal posi­tions of the exca­vat­ed fos­sils and artefacts.

Brain also dis­cov­ered stone arte­facts at Swartkrans that are between 1.7-million and 1.4-million years old, and are asso­ci­at­ed with remains of ear­ly Homo. In 1966, Ian Watt devel­oped a great­ly extend­ed grid sys­tem, before Pro­fes­sor Phillip Tobias and Alun Hugh­es began their excavations.

Remov­ing fos­sils from their rest­ing places

Fos­sils and arte­facts are extract­ed from the cave site by drilling and break­ing the brec­cia. The brec­cia is then chipped away from the fos­sils and stone tools with small chis­els and airscribes (a pow­er tool used to sep­a­rate fos­sils from rocks).

Areas of decal­ci­fied brec­cia (i.e. where the lime has been dis­solved away by ground­wa­ter and the brec­cia has bro­ken up) are exca­vat­ed with picks and shov­els and all the earth is sieved, so even small bones of rodents, insec­ti­vores, bats, lizards, frogs and birds can be recov­ered. Frag­ment­ed and crushed fos­sils can be recon­struct­ed after cleaning.

Since 1966, work at Sterk­fontein has been under­tak­en most­ly by sci­en­tists from the Uni­ver­si­ty of the Wit­wa­ter­srand, which owns the site. Exca­va­tions and dat­ing have been con­duct­ed by Pro­fes­sor Phillip Tobias, Alun Hugh­es, Pro­fes­sor Ron Clarke, Dr Kathy Kuman, Pro­fes­sor Tim Par­tridge and others.

These sci­en­tists have used sev­er­al meth­ods to exca­vate fos­sils from the Sterk­fontein caves and oth­er sites in the Cra­dle of Humankind World Her­itage Site.

How fos­sils are removed

Little Foot Small

Pro­fes­sor Ron Clarke work­ing on remov­ing the Lit­tle Foot” skele­ton from the rock it is embed­ded in

Fos­sils are care­ful­ly uncov­ered from the sur­round­ing brec­cia with del­i­cate airscribes. Lit­tle Foot” has been exca­vat­ed using airscribes, which oper­ate using com­pressed air.

To remove over­ly­ing lay­ers and expose cave brec­cia, jack­ham­mers and crow­bars can be used in sit­u­a­tions where sol­id brec­cia has first to be bro­ken open to expose fossils.

Decal­ci­fied sed­i­ments can be exca­vat­ed with trow­els. The sed­i­ments are sieved, and any bones and arte­facts recov­ered go to a lab­o­ra­to­ry for analysis.

Blocks of brec­cia can be removed for prepa­ra­tion in a lab­o­ra­to­ry. Pri­or to removal, the posi­tion of each block of brec­cia is record­ed in three dimen­sions, rel­a­tive to a grid. Fos­sils can be pre­pared in a lab­o­ra­to­ry using small, point­ed chis­els and light­weight ham­mers. Fine clean­ing of impor­tant fos­sils is done under a micro­scope with den­tal picks and airscribes.

Fos­silised remains, such as micro­fau­na (the bones of small mam­mals and bats), can also be released from cal­ci­fied brec­cia by care­ful­ly soak­ing chunks of the rock in weak acetic acid. This dis­solves the cal­ci­um car­bon­ate matrix hold­ing the del­i­cate fossils.

How do palaeoan­thro­pol­o­gists know how old their dis­cov­er­ies are?

Researchers use a num­ber of tech­niques to date the cave deposits.

Palaeomagnetism

Palaeo­mag­net­ism is a method used to date deposits at Sterk­fontein and oth­er Cra­dle of Humankind sites. It is based on the fact that the Earth’s mag­net­ic field has reversed at var­i­ous, pre­cise­ly known moments in the Earth’s his­to­ry. Since elec­tri­cal­ly charged par­ti­cles align them­selves to the mag­net­ic pole, sci­en­tists can deter­mine age esti­mates of fos­sils by analysing in which direc­tion par­ti­cles at var­i­ous points in the sed­i­ments are aligned. Recent work on sed­i­ments in Sterk­fontein Mem­ber 4, includ­ing sed­i­ments asso­ci­at­ed direct­ly with Mrs Ples”, sug­gest that this fos­sil is about 2.1-million years old.

Ura­ni­um series dating

Ura­ni­um series dat­ing relies on the analy­sis of the chang­ing ratios of ura­ni­um and lead in the cave deposits. This dat­ing method has been used at Sterk­fontein to date infills and stalagmites.

Cos­mo­genic iso­tope dating

Cos­mo­genic iso­tope dat­ing involves analy­sis of iso­topes of ele­ments such as beryl­li­um and alu­mini­um. This method has been used to obtain a date of about 4.1-million years for the Lit­tle Foot” skele­ton, although palaeo­mag­net­ism dat­ing places it slight­ly younger (about 3.3-million years ago).

Comparison

Esti­mates of the age of hominids from the Cra­dle of Humankind can be obtained by com­par­ing them and asso­ci­at­ed ani­mal remains with sim­i­lar fos­sils from East Africa that were found in asso­ci­a­tion with vol­canic ash deposits. The East African sed­i­ments can be dat­ed accu­rate­ly using tech­niques such as car­bon dat­ing, which entails the analy­sis of iso­topes of radioac­tive potas­si­um (K) and sta­ble argon (Ar). The nat­ur­al decay rate of the potas­si­um iso­topes rel­a­tive to argon can be used as a mea­sure of elapsed time. Unfor­tu­nate­ly, this dat­ing tech­nique can­not be applied to South African fos­sils as they are not asso­ci­at­ed with vol­canic mate­ri­als. But by draw­ing com­par­isons, sci­en­tists believe that Mem­ber 5 from Sterk­fontein, in which ear­ly Homo remains have been found, is between 1.5-million and 1.9-million years old, while Mem­ber 4 is slight­ly older.

Cut­ting edge of science

Palaeontologist

A palaeon­tol­o­gist at work in the laboratory

Spe­cif­ic ages of the dif­fer­ent mem­bers of Sterk­fontein are dif­fi­cult to deter­mine, as meth­ods of dat­ing and knowl­edge of con­tex­tu­al evi­dence are con­stant­ly being refined and updat­ed. The Uni­ver­si­ty of the Witwatersrand’s Pro­fes­sor Tim Par­tridge, a world expert on the issue, explains: A date obtained one month may be com­plete­ly dif­fer­ent to that cal­cu­lat­ed next month. We’re on the cut­ting edge of sci­ence, so things are bound to change all the time.”

Age of Sterk­fontein deposits

At Sterk­fontein, there are six Mem­bers (sed­i­men­ta­ry units or lay­ers) of rock, con­tain­ing fos­sils dat­ing between 4-mil­lion and 1.5-million years old, giv­ing us an extra­or­di­nary time­line of fau­na and flo­ra devel­op­ment dur­ing the time of our hominid ances­tors. The six mem­bers have been clas­si­fied by the Uni­ver­si­ty of the Witwatersrand’s Pro­fes­sor Tim Par­tridge, a geol­o­gist and dat­ing expert, from old­est to youngest. 

Mem­ber 1 – old­est mem­ber, more than 4-mil­lion years old

Mem­ber 2 – about 4.2-million to 3.3-million years old

Mem­ber 3 – about 3.3-million to 2.5-million years old

Mem­ber 4 – about 2.5-million to 2.1-million years old

Mem­ber 5 – about 1.9-million to 1.5-million years old

Mem­ber 6 – less than 200,000 years old

Mem­bers 1, 2 and 3 are under­ground, in the Sil­ber­berg Grot­to of Sterkfontein.

Mem­bers 4, 5 and 6 are vis­i­ble from the surface.