That is not entirely true, for linguists know too much about language to suggest that there ever was, literally, a single language from which all Indo-European languages descended.
( That ( ( ( ( ( is not ) ( entirely true ) ) , ) ( for ( linguists ( ( know ( ( too much ) ( about language ) ) ) ( to ( suggest ( that ( there ( ever ( ( ( ( was , ) literally ) , ) ( ( a ( single language ) ) ( ( from which ) ( ( all ( Indo-European languages ) ) descended ) ) ) ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT That)) (VP (VBZ is) (RB not) (ADJP (RB entirely) (JJ true)) (, ,) (SBAR (IN for) (S (NP (NNS linguists)) (VP (VBP know) (ADVP (ADVP (RB too) (RB much)) (PP (IN about) (NP (NN language)))) (S (VP (TO to) (VP (VB suggest) (SBAR (IN that) (S (NP (EX there)) (ADVP (RB ever)) (VP (VBD was) (, ,) (ADVP (RB literally)) (, ,) (NP (NP (DT a) (JJ single) (NN language)) (SBAR (WHPP (IN from) (WHNP (WDT which))) (S (NP (DT all) (JJ Indo-European) (NNS languages)) (VP (VBD descended))))))))))))))) (. .)))
It's unlikely there is only one parent language to all Indo-European languages.
( It ( ( 's ( unlikely ( there ( ( is only ) ( ( one ( parent language ) ) ( to ( all ( Indo-European languages ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (PRP It)) (VP (VBZ 's) (ADJP (JJ unlikely) (SBAR (S (NP (EX there)) (VP (VBZ is) (ADVP (RB only)) (NP (NP (CD one) (NN parent) (NN language)) (PP (TO to) (NP (DT all) (JJ Indo-European) (NNS languages))))))))) (. .)))