Ancient Bones starneste dezbateri proaspete asupra primilor oameni din America

Cine au fost primii americani si cand si cum au ajuns aici? Timp de zeci de ani, arheologii au crezut ca stiu raspunsurile la aceste intrebari. Pe baza dovezilor disponibile, pareau vanatori mari de vanat din Asia, cunoscuti drept Clovis, au fost primii care au invartit acea pista, calcand pe masa de terenuri acum scufundate din Beringia pentru a intra in Lumea Noua in urma cu aproximativ 13.000 de ani.

Insa, incepand cu inceputul anilor 2000, semne ale unei prezente umane anterioare in America au inceput sa se dezvolte, erodand sprijinul pentru asa-numitul model Clovis. O noua intelegere a modului in care oamenii au cucerit in cele din urma Lumea Noua a inceput sa prinda contur: Homo sapiens a ajuns cu barca cu cel putin 15.000 de ani in urma, urmand coasta de vest a Americii.

Acum, oamenii de stiinta din spatele unei noi descoperiri cauta sa rescrie din nou povestea colonizarii umane din America – si intr-o maniera mult mai radicala. Intr-o lucrare publicata astazi in Nature, cercetatorii descriu oasele rupte ale unui mastodont (o ruda disparuta a elefantilor) si roci batute de pe un sit din sudul Californiei. Echipa sustine ca ramasitele demonstreaza oamenii in America in urma cu 130.000 de ani, in epoca Pleistocenului tarziu. Daca au dreptate, descoperirea ar putea pune sub semnul intrebarii presupunerea de multa vreme a lui H. sapiensa fost primul si singurul membru al familiei umane care a ajuns pe Lumea Noua, pentru ca are loc dintr-o perioada in care mai multe specii umane, inclusiv neandertalii, au cutreierat planeta. De asemenea, ar putea sugera ca arheologii au ratat o inregistrare de peste 100.000 de ani de oameni din aceasta parte a lumii. Dar anuntul s-a confruntat cu critici puternice din partea altor oameni de stiinta, care sustin diferit ca ramasitele nu reflecta neaparat activitatea umana si ca varsta lor este incerta.

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Paleontologii au excavat resturile la inceputul anilor ’90 de pe un sit din judetul San Diego, care a fost descoperit in cursul imbunatatirilor autostrazii la Calea de Stat 54. Cercetatorii au recuperat oasele unui numar de specii diferite de epoca de gheata de la niveluri stratigrafice diferite din sit. Pentru noul studiu, Steven Holen de la Muzeul de Istorie Naturala din San Diego si colegii sai s-au concentrat pe scheletul partial al unui mastodont masculin gasit in aceasta locatie, a numit site-ul Cerutti Mastodon pentru descoperitorul sau, co-autorul studiului Richard Cerutti, de asemenea la muzeu. Oasele membrelor mastodontului dovedesc rupturi distinctive numite fracturi in spirala care se invart in jurul axei lungi a osului. Astfel de fracturi apar de obicei atunci cand se aplica forta pe osul proaspat. Capetele unora dintre oase au fost, de asemenea, rupte, si cateva mari, pietricele batute de piatra se aflau in apropiere. Cand echipa a rupt in mod experimental oasele din carcasele marilor mamifere din zilele noastre moderne folosind pietre de ciocan si nicovale, daunele rezultate s-au asemanat cu cele vazute pe oase si pe pietrisurile de pe site. Impreuna, modelul de deteriorare evident pe oase si pietre, precum si apropierea rocilor de oase sugereaza echipei umane zburau oasele cu rocile pentru a ajunge la maduva hranitoare din interior sau a face unelte osoase.

Nimic din toate acestea nu ar fi remarcabil in sine. Astfel de comportamente au fost bine documentate pe siturile arheologice din intreaga lume. Ceea ce face descoperirea o mare afacere este presupusa varsta a ramasitelor. Echipa a determinat varsta oaselor mastodontului prin aplicarea unei tehnici numite serie de uraniu, care foloseste descompunerea radioactiva a uraniului pentru a masura trecerea timpului. Rezultatele au indicat ca oasele sa aiba 130.000 de ani, sa dea sau sa dureze 9.

000 de ani – cu peste 100.000 de ani mai vechi decat cele mai vechi situri arheologice acceptate in mod obisnuit din America.

Astazi, situl Cerutti Mastodon se afla in mijlocul unui cadru urban. Dar cu 130.000 de ani in urma, in ultima perioada interglaciara, a fost un flux meandru intr-o campie inundata din apropierea coastei. Camile, lupii ingrozitori si capibara au cutreierat acolo. „A fost un loc foarte frumos pentru a trai”, a spus Holen la o teleconferinta de presa din 25 aprilie.

Daca Holen si colegii sai sunt corecti cu privire la varsta si natura descoperirilor, cercetatorii vor trebui sa-si regandeasca tot ceea ce au crezut ca stiau despre popularea Lumii Noi, inclusiv despre ce specie umana a fost prima care a colonizat-o. Majoritatea cercetatorilor sunt de acord ca oamenii au venit in America din nord-estul Asiei. In urma cu 130.000 de ani, autorii sustin ca H. sapiens, H. erectus, Neandertalii si Denisovanii (un grup cunoscut doar din ADN-ul antic recuperat din pestera Denisova din Siberia) ar fi putut fi prezenti in acea parte a lumii. Ar fi putut traversa Beringia pe jos inainte de 135.000 de ani in urma, cand nivelul marii era suficient de scazut. Altfel, ar fi putut calatori cu vaporul, urmand coastele Asiei, Beringiei si Americii de Nord pentru a ajunge la latitudinea sitului Cerutti Mastodon.

In timpul teleconferintei de presa, Holen a spus ca noua descoperire ar trebui sa incurajeze alti arheologi sa mearga sa caute mai multe site-uri din aceasta epoca – lucru pe care spune ca nu l-au facut anterior, pentru ca nimeni nu se astepta ca oamenii sa fie atat de devreme in America.

Expertii care nu sunt implicati in noul studiu au exprimat un scepticism profund in ceea ce priveste evaluarea echipei, in special afirmatia ca oasele rupte si pietrele batute reflecta activitatea umana. „Nu puteti impinge antichitatea umana in Lumea Noua in urma cu 100.000 de ani, bazata pe dovezi la fel de ambigue ca oasele rupte si pietrele nedescriptate – nu atunci cand provin dintr-o sapatura de salvare a autostrazii, facuta in urma cu 25 de ani, si nu aveti niciunul dintre cele detaliate dovezi tapononomice au cerut o cerere atat de grandioasa „, spune David Meltzer, de la Universitatea Metodistica de Sud, o autoritate cu privire la popasul din America.

That lack of taphonomic evidence—information about what happened to the remains between when they were deposited and when they were discovered—comes down to “the difference between paleontological and archaeological excavation,” says archaeologist Andy Hemmings of Florida Atlantic University, referring to the different approaches scientists use to unearth fossils as opposed to traces of material culture, which require more detailed provenience. “They didn’t map in every plottable object and pay attention to the relationships between items. Were pieces found 15 feet apart or 15 centimeters apart?” he says. Such information is important for reconstructing how the bones broke and what, if any, relationship existed between the bones and the rocks.

Although the researchers were able to experimentally reproduce the damage on the remains by processing fresh bone with stone tools, critics observe, the team did not rule out alternative causes. “It is one thing to show that broken bones and modified rocks could have been produced by people, which Holen and his colleagues have done. It is quite another to show that people, and people alone, could have produced those modifications. This, Holen [and his colleagues] have most certainly not done, making this a very easy claim to dismiss,” says archaeologist Donald Grayson of the University of Washington. Other commenters explained the team needs to look at many more fossil assemblages of large mammal bones, to see if natural causes could explain the breakage patterns evident in at the Cerutti Mastodon site.

Neither is simple hammerstone/anvil technology alone what many experts expect to see at a 130,000-year-old site. James Adovasio of Florida Atlantic says butchery sites of comparable age from other parts of the world tend to contain incontrovertible stone tools. He notes that by this time period humans were master stone knappers, capable of creating a variety of sophisticated, sharp-edged tools for cutting and slicing. “The utter absence of these things here is, shall we say, perplexing,” he comments. Adovasio led the excavations at the controversial site of Meadowcroft in Pennsylvania that dates to perhaps 16,000 years ago.

The possibility archaic humans might have made it to the New World is another stumbling point for some critics. The Bering Strait was flooded 130,000 years ago, notes Jon Erlandson of the University of Oregon, a leading proponent of the coastal route model. “There’s some evidence that Homo erectus was able to cross a few small bodies of water, but no evidence that erectus, or Neandertals for that matter, could do long-range voyaging or that they had sophisticated boats like modern humans had when they colonized Australia.”

Species questions notwithstanding, if humans did enter the New World as early as Holen and his collaborators would have it, why is there such a yawning gap in the archaeological record between the Cerutti Mastodon remains and the next oldest sites in the Americas? “If there were people in San Diego 130,000 years ago, you have to explain why there weren’t any more of them there until 115,000 years after that,” Erlandson contends. He takes issue with the authors’ suggestion investigators simply have not been looking for remains that old, noting he and other archaeologists have been doing exactly that for quite some time, often through the same sort of construction-monitoring efforts that led to the discovery of the Cerutti Mastodon site. “I’ve done quite a bit of construction monitoring in the Santa Barbara area and we’ve carefully monitored excavations down to sediments of the same age. We were looking out for artifacts and didn’t find them,“ he says. “It boggles the mind that no one has found anything despite decades of geological monitoring.” Erlandson adds that there is a long history of people making claims for extraordinarily early sites in the Americas, including the site of Calico Hills in California, which the famed Kenyan paleoanthropologist Louis Leakey argued was perhaps 200,000 years old. But these claims have all been debunked.

Not only are there no other traces of humans in the Americas anywhere near 130,000 years old, there are also no any signs of human activity in the region from which humans are thought to have first entered the New World. “There is not a whisper of anything that age in northeast Asia,” observes archaeologist Robin Dennell of the University of Exeter in England, who studies the dispersal of human ancestors across Asia, Australia and the Americas. For his part, Dennell is not bothered by the team’s interpretation of the bones and stones as signs of human activity. But he is concerned about the dating. “The case for the site being 130,000 years old appears to rest on just three uranium-series dates,” he observes. “I’d want to see Cerutti Mastodon covered in more dates than a [date] palm tree before claiming it was in the last interglacial.”

Archaeological dating experts not involved in the research had mixed reactions to the study. “I think the dating is sound,” says geochronologist Rainer Grun of Griffith University in Australia. But geochemist Bonnie Blackwell of Williams College thinks the team could do more to bolster its case. Bone is spongy and uranium can be absorbed into it or leached out of it in ways that affect the accuracy of the dating results. She would like to see the mastodon teeth from the site dated using a technique called electron spin resonance (ESR), which looks at the electrons in the tooth enamel to estimate age. Blackwell has used a combination of uranium series and ESR to successfully date mastodon remains from the site of Hopwood Farm in Illinois.

“We need to leave our minds open. I admire these colleagues for sticking their necks out. They should be commended for doing that,” says archaeologist Tom Dillehay of Vanderbilt University, who fought for years to convince the archaeological community that remains from the controversial site of Monte Verde in Chile predate the Clovis culture. Today most scholars accept that Monte Verde dates back to around 15,000 years old, if not 18,000 to 20,000 years ago, as Dillehay would have it. “But more evidence is going to be needed” for something this early, he says of the claims for human activity at the Cerutti Mastodon site.

Hemmings agrees. “I’m all for hominins in the Americas by 130,000, but not on this evidence. There’s not enough to open the champagne.”