Los ua pov thawj uas evolutionary hloov kuj tsis yog yuav mus ua lub noob, cia li qhib qhov ncauj ib tug ntxhw...
Ntev dhau los, kab tias txawm ua ntej cov pa cua celebrated sij hawm, yog ib tsev neeg zoo kawg thiab cov tsiaj nyob rau hauv teb chaws Africa. Zaj dabneeg no yog pib tej 10 lab lub xyoo dhau los ces tsev neeg hlob thiab pua. Ntawm peb lab (million) lub xyoos dhau los, ib ceg ntawm nws cov spilled rau cov teb chaws Europe thiab Asia. Raws li tej tsiaj tsiv mus territories tshiab, lawv txais tau mus climes dua qaum teb. Nws thiaj li, ib txhia hla tus choj Beringia, migrating ntawm cov teb chaws Asia qaum teb sab hnub tuaj mus rau hauv North America.
Nws suab ib zaj dabneeg paub. Muaj tseeb no yog txog peb yawg – lub keeb kwm neeg Asmeskas hauv lub Miocene, cov tseem ceeb hnyuj hnyo ntawm ancient sediments nyob Kenya fossils; qhov no tej pab pawg colonising teb chaws Europe thiab Asia; txoj kev mus kotaw mus rau hauv lub ntiaj teb tshiab. Tab sis, qhov no yuav tsis tau cov dab neeg uas hominins: ntawm australopithecines, paranthropines thiab Homo. Qhov no yog zaj dabneeg ntawm lub elephantines: ntawm mammoths, Loxodonta thiab Elephas.
Striking cov yam ntxwv nyob ntxhw – cov txwv thiab tusks – muaj nyob hauv cov pog koob yawg koob gomphothere los 20 lab lub xyoo dhau los. Rau ib tug tsiaj loj nrog tus caj dab luv luv, lub cev yog ib qho kev tsham, tas cov proboscideans tsuab nplooj thiab coj lawv mus rau lub qhov ncauj, yog li kev muab ib cov evolutionary uas kom zoo dua.
The development of a trunk and the transformation of incisors into tusks were accompanied by a change in the shape of the skull. Inside the mouth, the teeth were also changing. A short jaw left little room for a full set of molars, while the teeth needed to be able to sustain a long lifetime’s worth of heavy wear. Evolution provided a neat solution to both problems. Rather than having a whole set of premolars and molars crammed into the mouth at the same time – as in your mouth – there was just a single, large tooth occupying each side of the upper and lower jaw at any time. As this tooth wore down, another would be growing behind it, ready to slide into place when the worn-out tooth fell out, providing the animal with up to six sets of teeth in a lifetime.
The teeth of fossil gomphotheres and elephants preserve a signal of their diets. The ratio of different isotopes of carbon in the tooth enamel shows whether a particular individual was focusing more on browsing on leaves or eating grass. The grasslands of Africa first began to spread around 10 million years ago and isotope analysis reveals that late gomphotheres and early elephants switched to eating mainly grass around eight million years ago. In elephants, this switch is reflected in another change to their chewing teeth, which became three times as tall, with a proliferation of enamel ridges. But these adaptations to an abrasive diet appeared around five million years ago, three million years after that switch from soft leaves to tough grasses. With the degree of resolution we can achieve when looking far back into the past, it’s often difficult to know what came first – a change in behaviour or in anatomy. But in this case, it’s very clear: the changes to teeth lagged millions of year after the change in diet.
In our evolutionary narratives, the organism itself often seems to play a passive role: a powerless victim, almost, of changes to its environment or mutations in its genes. But the tale of the elephant’s tooth is somehow different, behaviour hloov kom meej meej precedes hloov hauv anatomy (and the underlying genetic instructions for tooth development). Perhaps we shouldn’t be surprised by this: developmental plasticity means that the final shape of an animal’s body is determined not only by DNA but also by external factors. And animals are more flexible in the way they interact with their environments than we sometimes assume. As elephants show, the source of novelty in evolution can come from behaviour rather than from genes.
It’s just possible that this type of change, originating with a change in behaviour, played an important role in human evolution. Around two million years ago, there was a large shift in body shape away from short legs, which first appears in Homo erectus. It’s likely that many of the new anatomical features, from longer legs to enlarged gluteal muscles and chunkier achilles tendons, are related to increased efficiency in running. If a group of humans began to run regularly, perhaps allowing them to hunt or scavenge more effectively, anatomical changes would follow, especially among the still-developing youngsters. Once running became an important part of behaviour, any mutations that enhanced it would be favoured. But the real source of novelty, saib tsam, was that change in behaviour and not a genetic mutation.
The great proboscideans that roamed the African landscapes where our own ancestors evolved remind us that evolutionary novelty doesn’t always originate in the genes.
Guardian.co.uk © saib xov xwm & Tawm txwv 2010