Footprints of 'first Americans'	科學家發現首批美洲人的足跡
People left traces of their presence in the sediments of a shoreline Human settlers made it to the Americas 30,000 years earlier than previously thought, according to new evidence. A team of scientists came to this controversial conclusion by dating human footprints preserved by volcanic ash in an abandoned quarry in Mexico. They say the first Americans may have arrived by sea, rather than by foot. The traditional view is that the continent's early settlers arrived around 11,000 years ago, by crossing a land bridge between Siberia and Alaska. Details of the latest findings were unveiled at the UK Royal Society's Summer Science Exhibition. Dr Silvia Gonzalez of Liverpool's John Moores University and her colleagues found the footprints in the quarry, some 130km (80 miles) south-east of Mexico City, in 2003. But they have only finished dating them this year. Dr Gonzalez was under no illusions that the finding would be controversial: "It's going to be an archaeological bomb and we're up for a fight," she said. The team used several methods to date a variety of material from the site near Puebla, Mexico, in order to be sure they were right about the age. The researchers used radiocarbon dating on shells and animal bones in the sequences and dated mammoth teeth by a technique called electron spin resonance. The sediments themselves were dated by optically stimulated luminescence. "Some lake sediments were incorporated into the ash and were baked. They look like small fragments of brick and these were the ones we dated in the footprint layer. They gave us a result of 38,000 years," Dr Gonzalez. (Agencies)	最新的證據顯示，人類移居美洲的歷史要比之前了解的提前3萬年。 對保存在火山灰中的人類足跡進行年代測定后，一個科學家小組得出了這一具有爭議性的結論。這些火山灰是在墨西哥一個廢棄的采石場被發現的。 他們表示，首批美洲人可能是經由海路抵達美洲的，而非徒步遷徙過去的。 傳統觀點認為，最初定居美洲的古人類大約在一萬一千年前跨越西伯利亞與阿拉斯加州之間的大陸橋抵達美洲。 英國皇家社會夏季科學展覽上公布了最新發現的詳細資料。 來自利物浦約翰·摩爾斯大學的西爾維亞·岡薩雷斯博士和她的同事們于2003年在采石場發現了這些足跡，但是他們今年才完成了足跡的年代測定。該采石場位于墨西哥城東南約130公里（80英里）的地方。 岡薩雷斯博士堅信這一發現將引起爭議，她說：”在考古學界看來，這無疑是一枚炸彈，我們將面臨一場戰爭。” 該小組運用多種方法對墨西哥普埃布拉附近遺址的各種物質進行了年代測定，以此確保他們得出的結果是正確的。 研究人員利用放射性碳測定法計算貝殼及動物骨骼的年代，用電子自旋共振法測量猛犸牙齒的年代，對沉積物本身則運用了光致發光技術進行檢測。 岡薩雷斯博士稱：“一些湖底沉積物被摻雜到火山灰中，之后被烘烤，這些看起來好象是磚塊的碎片具有三萬八千年的歷史，它們與足跡層屬于同一年代。” （中國日報網站譯）