Group of British scientists claimed yesterday to have identified human footprints in central Mexico that are 40,000 years old, almost three times older than the most generally accepted evidence for human settlement in the Americas.
The team from universities in Liverpool, Bournemouth, and Oxford are convinced that the footprints are human and represent several adults and children who walked in freshly fallen volcanic ash in the Valsequillo Basin, about 80 miles south-east of Mexico City.
Working with international colleagues, they have applied dating techniques on the sediment itself and on finds including a land snail, a water snail and a mammoth tooth, all of which came back with an age of around 40,000 years.
The footprints had to be disentangled from animal tracks, more than 250 marks in all. Casts, which Matthew Bennett of Bournemouth University described as "unmistakably human" were produced from laser modelling at the site, employing a technique used to make industrial prototypes.
Although the findings were announced after two years of work at the British universities and elsewhere, they are controversial, defying the conviction of many scientists that humans arrived in the Americas not more than 15,000 years ago.
The tracks were made in gritty ash from the volcano, protected by later sediment layers, and underwater for long periods. The ash layer has since become as hard as concrete, and was locally quarried as a building material, which is how the footprints resurfaced.
Scientists partially excavated the quarry in the 1960s, and found ancient animal bones and hints of a very early habitation site, but with the technology of the day could not date them accurately.
The British team revisited the site two years ago. Silvia Gonzalez, from Liverpool John Moores University, first spotted the trail of marks, and described her instant conviction that they were human and very ancient as "like a thunderbolt in my mind". 

4万年前人类脚印将美洲人类史提前
　　英国科学家日前宣布，他们在墨西哥中部发现的人类脚印已有大约4万年的历史，从而推翻了此前有关美洲出现人类的历史不会超过1.5万年的结论。
　　据英国《卫报》7月5日报道，由英国利物浦大学、波恩茅斯大学和牛津大学的科学家组成的科研小组称，他们在墨西哥发现的250多个足迹中，有三分之二属于人类的脚印，其中包括成年人和小孩的脚印。这些脚印是当时人们在还未凝固的火山岩上行走时留下的。
　　科协家通过放射性碳鉴定技术和激光学鉴定技术断定，这些脚印属于人类，且约有4万年的历史。除此之外，科学家还发现了一只蜗牛、一只水蜗牛和一颗猛犸的牙齿，并断定它们也约有4万的历史。
　　2003年9月，在墨西哥中部一个湖边被废弃的采石场中的火山岩上，科学家发现了这些人类足迹，但当时许多科学家都坚信人类到达美洲的历史不会超过1.5万年。在经过近两年的争论后，这一发现在近日才正式被公开。其实早在上个世纪60年代，科学家就在此地发现了古代动物的遗骨和早期人类居住地的痕迹，但当时的鉴定技术无法测定出这些发现物的准确年代。
　　传统史学观点认为，大约1.2万年前的“冰河期”晚期，美洲人祖先从欧亚大陆经由“白令海峡”到达美洲。先前，也有科学家认为，人类来到美洲大陆的时间可能要比这一说法早上数千年。但是，上述脚印的发现证明，人类在4万年前曾居住在墨西哥中部。这推翻了史学家们长期以来深信不疑的美洲人历史不会超过1.5万年的结论。