'Footprints' found in Mexico in 2003 that were first dated at 40,000 years old, have now been dated to 1.3 million years ago.
A second team of scientists have carried out different dating techniques.
Paul Renne, a geochronologist at the University of California, Berkeley, and his colleagues found the material to be 1.3million years old, much too old to be human footprints.
They used argon dating (a powerful technique that measures the decay rate of a form of argon in rocks) and palaeomagnetic analysis (measuring the magnetism of rocks).
Dr Silvia Gonzalez, from Liverpool John Moores University, and her team found the 'footprints' in an abandoned quarry near Puebla, 100 kilometres from Mexico City, and published their results in July 2005.
They used dating methods such as radiocarbon dating (measures the decay rate of the naturally occurring carbon 14) and an optically stimulated luminescence technique (measures the last time a material was exposed to sunlight or heat).
These techniques found the material to be about 40,000 years old. This would be the earliest evidence of humans in the Americas.
Renne's team argue that the techniques used in Dr Silvia Gonzalez's study may not be appropriate for the materials used and also say the 'footprints' are more likely to be digging marks from recent quarrying.
The debate is likely to continue for some time and more research will be needed to help clarify the different theories.