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The writer of the passage thinks that unlike the Tasmanian devil, the human species is not likely to develop contagious cancer tumours, since ----.
A contagious tumour threatens to wipe out the famous
Tasmanian devil, a ferocious marsupial animal. Could
contagious cancers arise in humans, too? Given that
humans have great genetic diversity and can avoid
behaving in ways that would foster tumour transmission,
it might seem safe to assume that our species can
readily avoid the fate of the Tasmanian devil. Indeed, if
a person were bitten by an infected Tasmanian devil or
by a dog with the canine transmissible tumour, the
person’s genetic makeup, being so different from that of
the animals, would probably ensure a strong immune
response able to detect and kill the invading cells. Thus,
the bitten individual would not get sick or start
spreading the disease to others. There are grounds for
concern, though. Contagious cancers could, in theory,
arise in a group of great apes with low genetic diversity
because of population declines. If they were hunted by
human populations with many members having
impaired immunity, the close contact might enable
tumour cells to transfer to humans and then spread.
Such conditions exist where humans with a high HIV
prevalence hunt endangered apes. Although this
scenario is possible, we suspect that cross-species
transmission is not the most likely way that a
contagious cancer would arise in humans. We hold this
view in part because no known cases of cross-species
transmission of the dog cancer have occurred in nature,
although the disease has been experimentally
transferred to related canines in the laboratory.