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A map detail­ing fos­sil sites in the Cra­dle of Humankind

The search for fos­sils begins with geo­log­i­cal sur­veys. Some areas are more like­ly to yield fos­sils than oth­ers, and researchers nor­mal­ly con­cen­trate their efforts on regions that have good, fos­sil-bear­ing rock such as the dolomitic lime­stone of the Cra­dle of Humankind and the ancient lake beds of East Africa.

But a good mea­sure of luck is also need­ed some­times, as was the case in many of the Cra­dle of Humankind sites, which were first explored by miners.

Hun­dreds of palaeon­to­log­i­cal sites in South Africa have been exposed by min­ers. But sad­ly, many have been destroyed in the min­ing process.

Site prepa­ra­tion

When a poten­tial site is iden­ti­fied, it is sur­veyed using tech­nol­o­gy that pro­duces three-dimen­sion­al maps and plans of poten­tial fos­sil-bear­ing areas. Finds are plot­ted on vir­tu­al maps using a dig­i­tal Geo­graph­i­cal Infor­ma­tion Sys­tem (GIS).

Exca­va­tion

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The Almost Human” exhi­bi­tion takes you through the exca­va­tion for Homo nale­di

The type of exca­va­tion method used depends on the type of sed­i­ment, or matrix, hold­ing the fossils.

At Sterk­fontein, many of the fos­sils are encased in hard brec­cia, a matrix con­sist­ing of mud or sand and stone frag­ments cement­ed togeth­er by cal­ci­um car­bon­ate. This brec­cia is exca­vat­ed using jack­ham­mer drills dri­ven by com­pressed air. Blocks are then bro­ken away by dri­ving wedges into the holes.

Tech­ni­cians care­ful­ly uncov­er fos­sils from brec­cia blocks by remov­ing the sur­round­ing rock using del­i­cate drills, includ­ing pow­er tools called airscribes, which use com­pressed air. Lit­tle Foot is being exca­vat­ed in this way.

Areas of decal­ci­fied brec­cia deposits (i.e. where the lime has been dis­solved by ground water) are exca­vat­ed with picks and shov­els and care­ful­ly sieved so that even small bones of rodents, insec­ti­vores, bats, lizards, frogs and birds can be recov­ered. Frag­ment­ed and crushed fos­sils can be recon­struct­ed after cleaning.

Jack­ham­mers, crow­bars and sledge­ham­mers are used to remove dolomite and lay­ers of cal­ci­um car­bon­ate to expose cave brec­cia when there is no chance of dam­ag­ing fossils.

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 or dig­i­tal 3D site plan.

Tech­ni­cians care­ful­ly remove fos­sils from the brec­cia. 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, includ­ing micro­fau­na (the bones of small mam­mals and bats), can be released from hard, cal­ci­fied brec­cia by care­ful­ly soak­ing chunks in baths of weak acetic acid. This dis­solves the cal­ci­um car­bon­ate matrix hold­ing the del­i­cate fossils.

Spec­i­mens are num­bered and cat­a­logued with ref­er­ence to their posi­tion ver­ti­cal­ly and hor­i­zon­tal­ly in the site. Pho­tog­ra­phy and mak­ing draw­ings of strati­graph­ic sec­tions are impor­tant activ­i­ties dur­ing the excavation.

In the laboratory

After a fos­sil has been pre­pared and cleaned, it is stud­ied in the laboratory.

There are many ways of exam­in­ing fos­sils in detail. Sig­nif­i­cant fos­sils can be mea­sured using callipers.

Binoc­u­lar light micro­scopes are used to look at sur­face fea­tures of bones.

These can show cut marks or car­ni­vore dam­age. A scan­ning elec­tron micro­scope (SEM) is used for detailed, high mag­ni­fi­ca­tion analy­sis, for exam­ple, to study microwear on teeth, yield­ing data on diet.

Trace ele­ment and iso­tope analy­sis of soils and fos­sils pro­vides infor­ma­tion about the environment.

Iso­tope analy­sis of tooth enam­el can also indi­cate the diets of ani­mals that lived mil­lions of years ago.

DNA analy­sis has the poten­tial to add infor­ma­tion on the nature of rela­tion­ships between animals.

Sci­en­tif­ic tech­niques: At the cut­ting edge

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Under­stand the sci­ence behind fos­sil dat­ing at Maropeng’s Almost Human” exhibition

Sci­en­tists date fos­sils using var­i­ous techniques.

Researchers are able to get a rough time-frame for fos­sils by relat­ing them to the rock lay­ers (strati­graph­ic sequences) in which they are found.

Where fos­sils are found in asso­ci­a­tion with vol­canic ash deposits, as they are at East African sites, their age can be deter­mined using potas­si­um-argon dat­ing. This method is based on the fact that after vol­canic rock cools some of the radioac­tive iso­tope potas­si­um-40 (K-40) decays to the gas argon-40 (Ar-40), which is then trapped with­in the rock. By mea­sur­ing the pro­por­tion of K-40 rel­a­tive to trapped Ar-40 in a sam­ple, the date when the rock cooled can be established.

Ura­ni­um Series Dat­ing is also based on the decay of radioac­tive iso­topes, in this case by mea­sur­ing the pro­por­tions of ura­ni­um to lead or ura­ni­um to heli­um in an ancient sample.

Elec­tron Spin Res­o­nance (ESR) dating

Elec­tron Spin Res­o­nance (ESR) dat­ing is based on nat­ur­al radioac­tiv­i­ty. By mea­sur­ing the build-up of elec­trons trapped inside crys­tals after they were formed, frag­ments of tooth enam­el can be dated.

Cos­mo­genic dating

Cos­mo­genic dat­ing is based on the find­ing that when cos­mic rays from out­er space reach the Earth they cre­ate cos­mo­genic iso­topes. In quartz they pro­duce mea­sur­able amounts of the iso­topes beryl­li­um-10 and alu­mini­um-26. The rel­a­tive pro­por­tions of these iso­topes with­in buried rocks can be used to estab­lish when they were last exposed to the atmosphere.

Palaeo­mag­net­ism

Palaeo­mag­net­ism is based on the fact that the Earth’s mag­net­ic field wan­ders” and has reversed at known peri­ods in the dis­tant past. The align­ment of fine iron par­ti­cles with­in sed­i­ments, rel­a­tive to the mag­net­ic field pre­vail­ing when the sed­i­ments were laid down, can be deter­mined and used as a mea­sure of age rel­a­tive to known peri­ods of reversed polarity.

Radio­car­bon dating

Radio­car­bon dat­ing is of lim­it­ed val­ue at the Cra­dle of Humankind since it can­not be applied reli­ably to sam­ples that are old­er than about 50,000 years. This tech­nique is based on the fact that all liv­ing organ­isms have a mix­ture of sta­ble 12C and radioac­tive 14C iso­topes (absorbed from the atmos­phere). After death, organ­isms do not absorb 14C any longer, and the remain­ing radioac­tive iso­topes decay at a known con­stant rate. By mea­sur­ing the pro­por­tion of 14C rel­a­tive to 12C in an ancient organ­ic sam­ple, it is pos­si­ble to cal­cu­late its age.

X-rays and CT scans

Researchers can study the inter­nal anato­my of fos­sils with­out dam­ag­ing the mate­r­i­al by using X-ray analy­sis (radi­og­ra­phy).

CT scans (using com­put­ed tomog­ra­phy) pro­vide detailed inter­nal images of fos­sils. For exam­ple, CT images revealed that the roots of the wis­dom teeth of Mrs Ples” were still open at the time of death, indi­cat­ing that this indi­vid­ual was prob­a­bly an adolescent.

Cave taphon­o­my

Taphon­o­my is the study of process­es which relate to the death of organ­isms, their bur­ial and decay. It is impor­tant to under­stand these to inter­pret fos­sil sites correctly.

Bones were once parts of liv­ing ani­mals. After an ani­mal dies, it nat­u­ral­ly decays where it is buried. Taphon­o­my includes the inves­ti­ga­tion of things like the age of the indi­vid­u­als which the bones once belonged to, and how weath­er­ing has affect­ed them.

Dr Bob Brain, a zool­o­gist at the Trans­vaal Muse­um, under­took pio­neer­ing tapho­nom­ic work on fos­sils at Swartkrans in the Cra­dle of Humankind. At issue was an idea devel­oped by Pro­fes­sor Ray­mond Dart of the Uni­ver­si­ty of the Wit­wa­ter­srand, who iden­ti­fied the Taung Skull, that the ear­ly hominids were ruth­less hunters. In the course of exam­in­ing near­ly 240,000 fos­sils over 25 years, Brain hypoth­e­sised that some of the hominid and ani­mal bones were food remains left by large cats. The ear­ly hominids were not the hunters, but the hunted!

Palaeoe­col­o­gy

Palaeoe­col­o­gy is the study of how organ­isms – includ­ing hominids – relat­ed with their envi­ron­ment in the past.

The pro­por­tions of species rep­re­sent­ed in a fos­sil assem­blage may sug­gest much about the nature of past environments.

Pri­mate studies

Of all the liv­ing pri­mates, humans are said to be most sim­i­lar to chim­panzees, in terms of DNA.

There are a num­ber of ways in which mod­ern pri­mates can be stud­ied to mod­el the lives of extinct hominids. Researchers can study liv­ing apes and try to find behav­iour that is also found in humans which might have been present in a com­mon ances­tor and/​or ear­ly hominids.

Return to the Exhi­bi­tion Guide.