Two paleontologists, Dr. Tyson and Dr. Rees, disagree over the interpretation of certain footprints that were left among other footprints in hardened volcanic ash at site G. Dr. Tyson claims they are clearly early hominid footprints since they show human characteristics: a squarish heel and a big toe immediately adjacent to the next toe. However, since the footprints indicate that if hominids made those prints they would have had to walk in an unexpected cross-stepping manner, by placing the left foot to the right of the right foot, Dr. Rees rejects Dr. Tyson' s conclusion. Which one of the following, if true, most seriously undermines Dr. Tyson's conclusion? ['The footprints showing human characteristics were clearly those of at least two distinct individuals.', "Footprints shaped like a human's that do not show a cross-stepping pattern exist at site M, which is a mile away from site G, and the two sets of footprints are contemporaneous.", 'When the moist volcanic ash became sealed under additional layers of ash before hardening, some details of some of the footprints were erased.', 'Certain species of bears had feet very like human feet, except that the outside toe on each foot was the biggest toe and the innermost toe was the smallest toe.']