Humans arrived in the Kaliko Islands about 7,000 years ago, and within 3,000 years most of the large mammal species that had lived in the forests of the Kaliko Islands had become extinct. Yet humans cannot have been a factor in the species' extinctions, b

In this argument the speaker concludes that humans could not have been a factor in the extinction of large mammal species in the Kaliko islands 3,000 years ago. To justify this conclusion, the speaker points out that no evidence exists that humans hunted or had other significant contact with these mammals. The speaker also points out that while archeologists have found bones of discarded fish in the islands, they have not found any discarded mammal bones there. For three reasons, this evidence lends little credibility to the speaker's argument.

First, the argument concludes too hastily that humans could not have had any significant contact with these mammals. In relying on the lack of physical evidence such as bones, the speaker overlooks the possibility that humans exported mammals--particularly their bones--during this time period. Without ruling out this alternative explanation for the disappearance of these species from the islands, the speaker cannot justify the conclusion that humans were not a factor in their extinction from the islands.

Secondly, the argument relies on the assumption that without significant contact with these other species humans could not have been a factor in their extinction. But the speaker provides no evidence that this is the case. Moreover, perhaps humans drove these other species away from their natural habitat not by significant contact but merely by intruding on their territory. Or perhaps humans consumed the plants and animals on which these species relied for their subsistence. Either scenario would explain how humans could have been a factor in the extinction of these species despite a lack of significant contact.

Thirdly, the speaker assumes that the bones of fish that archeologists have found discarded on the island were discarded by humans, and not by some other large mammal. However, the speaker provides no evidence to substantiate this assumption. Given other possible explanations for these discarded fish bones, this evidence in itself lends little credible support to the speaker's theory about the extinction of large species of mammals.

In conclusion, the argument is unconvincing as it stands. To strengthen it, the speaker must rule out the possibility that humans exported the bones of these other species. To better evaluate the argument, we would need more information about the diet of humans and of the now-extinct mammals during that time period; particularly, we would need to know whether those other mammals also fed on the fish whose discarded bones have been found on the islands.