Try predicate calculus (first order logic).
"All humans have a common ancestor" can be translated to:
Ex.Ay human(y) & ancestor(x,y)
which means there exists an x such that for all humans,
x is an ancestor (appears in y's genetic lineage by definition).
x is the "common ancestor"
Actually, now that I look at it, x is our last (latest) common ancestor,
and all of her ancestors. But the statement, as written, says there is at
least one.
Not much can be validly inferred from this statement alone. Just paraphrases
with negations, disjunctions and implications thrown in such as:
Ex.~Ey human(y) -> ~ancestor(x,y)
translation: there is an x such that there isn't a human who doesn't have
x as an ancestor.
-- David McFadzean david@lucifer.com Memetic Engineer http://www.lucifer.com/~david/ Church of Virus http://www.lucifer.com/virus/