The top 50 centimeters of soil on Tiliga Island contain bones from the native birds eaten by the islanders since the first human immigration to the island 3, 000 years ago. A comparison of this top layer with the underlying 150 centimeters of soil -- accumulated over 80, 000 years -- reveals that before humans arrived on Tiliga, a much larger and more diverse population of birds lived there. Thus, the arrival of humans dramatically decreased the population and diversity of birds on Tiliga. Which one of the following statements, if true, most seriously weakens the argument? ['Many of the bird species that disappeared from Tiliga did not disappear from other, similar, uninhabited islands until much later.', 'Bones from bird species known to have been eaten by the islanders were found in the underlying 150 centimeters of soil.', 'The bird species known to have been eaten by the islanders had few natural predators on Tiliga.', 'The arrival of a species of microbe, carried by some birds but deadly to many others, immediately preceded the first human immigration to Tiliga.']