( So ( ( when ( ( two ( host species ) ) ( have ( evolved ( from ( a ( common ancestor ) ) ) ) ) ) ) ( , ( ( ( the parasites ) ( that ( ( were ( originally present ) ) ( in ( that ( common ancestor ) ) ) ) ) ) ( ( evolve ( with ( the ( two ( new hosts ) ) ) ) ) . ) ) ) ) )
(ROOT (S (CC So) (SBAR (WHADVP (WRB when)) (S (NP (CD two) (NN host) (NNS species)) (VP (VBP have) (VP (VBN evolved) (PP (IN from) (NP (DT a) (JJ common) (NN ancestor))))))) (, ,) (NP (NP (DT the) (NNS parasites)) (SBAR (WHNP (WDT that)) (S (VP (VBD were) (ADJP (RB originally) (JJ present)) (PP (IN in) (NP (DT that) (JJ common) (NN ancestor))))))) (VP (VB evolve) (PP (IN with) (NP (DT the) (CD two) (JJ new) (NNS hosts)))) (. .)))
So when two host species have evolved from a common ancestor, the parasites that were originally present in that common ancestor evolve with the two new hosts.
(ROOT (S (NP (NP (DT The) (NN conclusion)) (SBAR (IN that) (S (NP (CD two) (NN animal) (NNS species)) (VP (VBD evolved) (PP (IN from) (NP (DT a) (JJ common) (NN ancestor))) (PP (IN in) (NP (JJ recent) (JJ geological) (NN history))))))) (VP (VBZ is) (ADVP (RBS best)) (VP (VBN supported) (PP (IN by) (NP (DT the) (NN observation))) (SBAR (IN that) (S (NP (DT the) (NNS species)) (VP (VBP have) (NP (JJ similar) (NN bone) (NN structure))))))) (. .)))
The conclusion that two animal species evolved from a common ancestor in recent geological history is best supported by the observation that the species have similar bone structure.