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))))))))))))))) (. .)))
There is active research in the linguistics community to determine the origins of Indo-European languages.
( There ( ( is ( ( active research ) ( in ( the ( linguistics ( community ( to ( determine ( ( the origins ) ( of ( Indo-European languages ) ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (EX There)) (VP (VBZ is) (NP (NP (JJ active) (NN research)) (PP (IN in) (NP (DT the) (JJ linguistics) (NN community) (S (VP (TO to) (VP (VB determine) (NP (NP (DT the) (NNS origins)) (PP (IN of) (NP (JJ Indo-European) (NNS languages))))))))))) (. .)))