Piec­ing to­gether the bushy branches of our fam­ily tree

A tiny bone from Lit­tle Foot’s skele­ton adds fresh in­sights into what our an­ces­tors could do

Pretoria News - - FRONT PAGE - AMÉLIE BEAUDET Beaudet is a post-doc­toral fel­low at Wits Univer­sity.

IN HIS book, Won­der­ful Life, Pro­fes­sor Stephen Jay Gould, an evo­lu­tion­ary bi­ol­o­gist, palaeon­tol­o­gist and widely-read pop­u­lar sci­ence au­thor, de­scribed the evo­lu­tion of life in the fol­low­ing way: life is a co­pi­ously branch­ing bush, con­tin­u­ally pruned by the grim reaper of ex­tinc­tion, not a lad­der of pre­dictable progress.

Study­ing Aus­tralo­p­ithe­cus, an ex­tinct ho­minin genus that rep­re­sents a branch of our fam­ily tree, is an ex­cel­lent way to make more sense of our bushy fam­ily tree, and un­der­stand how species emerge, evolve and dis­ap­pear.

We don’t know the iden­tity of Homo’s di­rect an­ces­tor, but the most likely can­di­date is one of the Aus­tralo­p­ithe­cus species that lived in Africa be­tween 2 and 4 mil­lion years ago.

But it’s dif­fi­cult to study the bi­ol­ogy and his­tory of Aus­tralo­p­ithe­cus, the fos­sil record for the genus is too frag­men­tary. There have been some ma­jor and ex­cit­ing finds along the way. For ex­am­ple, the dis­cov­ery of a par­tial Aus­tralo­p­ithe­cus – later nick­named Lucy – in Ethiopia in 1974 pro­vided valu­able in­for­ma­tion. But Lucy’s skele­ton is only 40% com­plete and lacks im­por­tant el­e­ments – like a com­plete skull.

A more com­plete skele­ton, named Lit­tle Foot by re­searchers, of­fers sci­en­tists a chance to fill in the gaps.

A num­ber of stud­ies have been done on the skele­ton over the past few years. My col­leagues and I have added to this body of knowl­edge in a pa­per that ex­plores Lit­tle Foot’s first cer­vi­cal ver­te­bra, also called the at­las.

Our pa­per sheds light on an im­por­tant part of Aus­tralo­p­ithe­cus’s anatomy. It helps us un­der­stand bet­ter how the an­cient ho­minins lived. The find­ings sug­gest this spec­i­men could climb and move in trees. But it might also have been able to walk on the ground. That echoes the re­sults of a pre­vi­ous study we con­ducted, which fo­cused on Lit­tle Foot’s in­ner ear. The same study also sup­ports the hy­poth­e­sis of a late emer­gence of hu­man brain metabolism.

This sort of re­search brings us closer to our ori­gins and con­trib­utes to a thor­ough por­trait of the main char­ac­ters in hu­man evo­lu­tion­ary his­tory. It also il­lus­trates Gould’s de­scrip­tion of our evo­lu­tion as a “co­pi­ously branch­ing bush”.

The first skele­tal el­e­ments of Lit­tle Foot were un­earthed from the Sterk­fontein Caves near Jo­han­nes­burg in 1994 and 1997. The caves are among the rich­est sites of fos­sil re­mains in the world, and form part of what’s known as the Cra­dle of Hu­mankind.

After 20 years of metic­u­lous ex­ca­va­tions by Ron Clarke and his team, Lit­tle Foot turned out to be the most com­plete Aus­tralo­p­ithe­cus skele­ton dis­cov­ered: it is more than 90% in­tact. The spec­i­men has been dated as 3.67 mil­lion years old.

Var­i­ous anatom­i­cal stud­ies have been re­cently con­ducted on Lit­tle Foot. For in­stance, we’ve vir­tu­ally repli­cated the in­ner sur­face of the brain­case to de­liver in­for­ma­tion about brain size, shape and or­gan­i­sa­tion. We’ve also stud­ied the shape of the in­ner ear, which is part of the bal­ance sys­tem. The find­ings told us more on Lit­tle Foot’s brain and be­hav­iour.

Lit­tle Foot’s first cer­vi­cal ver­te­bra, or at­las, is nearly in­tact and rep­re­sents a key com­po­nent of Aus­tralo­p­ithe­cus’s bi­ol­ogy be­cause it con­nects the skull with the rest of the skele­ton. It also plays a role in how blood is supplied to the brain via the ver­te­bral ar­ter­ies.

By study­ing it we’ve been able to un­der­stand more about how Aus­tralo­p­ithe­cus moved, specif­i­cally their heads and necks, and the blood flow that ir­ri­gated their brains. We turned our at­ten­tion to it in a bid to con­firm or con­tra­dict pre­vi­ous find­ings and to find out more about Aus­tralo­p­ithe­cus.

The cra­nial base was filled with sed­i­ments. These were phys­i­cally re­moved and the skull was scanned us­ing a tech­nique called mi­cro­to­mog­ra­phy at Wits Univer­sity.

This imag­ing tech­nique is far more ac­cu­rate than the clas­si­cal med­i­cal imag­ing tools and pro­vided us with high-res­o­lu­tion images of the ver­te­bra that could be vir­tu­ally ex­tracted from the sed­i­ments.

Our main find­ings cen­tred on Lit­tle Foot’s lo­co­mo­tion – the way it moved; the way this evolved over time; and, its brain metabolism.

First, what we dis­cov­ered about Lit­tle Foot’s head and neck move­ments in­di­cates that this spec­i­men could climb and move in trees, but this does not ex­clude the pos­si­bil­ity that it might also have walked on the ground. This find­ing is in ac­cor­dance with re­sults from our pre­vi­ous study about Lit­tle Foot’s in­ner ear.

Sec­ond, we com­pared the anatomy of Lit­tle Foot’s ver­te­bra with two other Aus­tralo­p­ithe­cus spec­i­mens. One came from the same site as Lit­tle Foot, but from a ge­o­log­i­cal unit that is younger. The sec­ond spec­i­men was found in the 1970s in Hadar, Ethiopia – the same site where Lucy was dis­cov­ered.

The at­las of Lit­tle Foot is sim­i­lar to the one of the Aus­tralo­p­ithe­cus spec­i­men from Ethiopia. The ad­di­tional spec­i­men from South Africa, which comes from the ge­o­log­i­cally younger de­posits of Sterk­fontein, is more hu­man-like. The ob­ser­va­tions could in­di­cate that some ear­lier species of Aus­tralo­p­ithe­cus might have spent much more time in trees than the later rep­re­sen­ta­tives of the genus.

Fi­nally, our es­ti­ma­tion of blood flow sup­ply­ing Lit­tle Foot’s brain shows that the en­er­getic costs of Aus­tralo­p­ithe­cus’s brain were lower than those es­ti­mated in mod­ern hu­mans. This could be due to Aus­tralo­p­ithe­cus’ rel­a­tively small brain, a diet that in­cor­po­rated less meat (and so pro­vided less en­ergy), or be­cause other or­gans re­quired more en­ergy.

This con­firms the late emer­gence of the hu­man-like brain metabolism that pre­vi­ous stud­ies sug­gested.

There is more work to be done on Lit­tle Foot’s skele­ton – we are plan­ning more stud­ies us­ing the var­i­ous tools of­fered by “vir­tual pa­le­oan­thro­pol­ogy”. These stud­ies and oth­ers will help us to shed more light on a cru­cial part of hu­man an­ces­tory’s fam­ily tree. | The Con­ver­sa­tion


LIT­TLE Foot’s skull, with the ar­row on the right-hand im­age in­di­cat­ing the spec­i­men’s at­las. The Con­ver­sa­tion

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