Utterly fascinating. But the last paragraph in the article puts the lie to the title -- pigs and horses don't have this feature. Last I checked, both mammals.
There is no indefinite plural article in English. The title is perfectly consistent with the content of the article, if potentially a bit misleading.
"If not for a virus, none of us would ever be born."
if horses and pigs are a-ok without it, then the assertion that in its absence we would have no chance of existence seems a bit odd