The Hebrew Phrase נִשְׁמַת חַיִּים
In Gen. 2:7, it is written,
And Yahveh God formed the man from the dust of the earth, and He blew into his nostrils the (נִשְׁמַת חַיִּים), and the man became a living soul.
וַיִּיצֶר יַהְוֶה אֱלֹהִים אֶת הָאָדָם עָפָר מִן הָאֲדָמָה וַיִּפַּח בְּאַפָּיו נִשְׁמַת חַיִּים וַיְהִי הָאָדָם לְנֶפֶשׁ חַיָּה
Whatever נִשְׁמַת חַיִּים (nishmat chayyim) is, it is written that Yahveh "blew"1 it into the man's nostrils, at which point "the man became a living soul" (וַיְהִי הָאָדָם לְנֶפֶשׁ חַיָּה), or a living creature. Likewise, it is written that man's (נְשָׁמָה) is in his nostrils.2 Furthermore, when the Israelites conquered the cities that Yahveh gave them, they were commanded to "not let any (נְשָׁמָה) live."3 Here, נְשָׁמָה is used as a synecdoche for a person, since the נְשָׁמָה is the vital constituent.
Aubrey Rodway Johnson wrote,4
Hence, when a young boy fell sick, he died when "there was no (נְשָׁמָה) left in him."5 The contexts in which נְשָׁמָה occurs suggests that it is simply the breath breathed in and out through the nostrils (and mouth), and it is called the נִשְׁמַת חַיִּים or "breath of life" in Gen. 2:7 because only living creatures possess "breath" or נְשָׁמָה. Hence, the presence of the נְשָׁמָה, "breath," is indicative of life.
The Hebrew Phrase רוּחַ חַיִּים
In Gen. 6:17 and also 7:15, a different phrase is used to describe that which was possessed by "all flesh under heaven" that was destroyed by the flood: רוּחַ חַיִּים (ruach chayyim). Being different words seems to suggest that נִשְׁמַת חַיִּים and רוּחַ חַיִּים are different things. After all, why use different words if they are the same thing? On the contrary, different words are often used in the Hebrew scriptures to describe the same concept when the different words are synonyms.
Aubrey Rodway Johnson wrote,6
Regarding נְשָׁמָה and רוּחַ, the first proof that they are synonyms is derived from their usage in Hebrew parallelisms. In his commentary on Gen. 2:7, Carl Friedrich Keil wrote,7
It is true, נְשָׁמָה generally signifies the human soul, but in chap. vii. 22 נִשְׁמַת־רוּחַ חַיִּים is used of men and animals both; and should any one explain this, on the ground that the allusion is chiefly to men, and the animals are connected per zeugma, or should he press the ruach attached, and deduce from this the use of neshamah in relation to men and animals, there are several passages in which neshamah is synonymous with ruach (e.g. Isa. xlii. 5; Job xxxii. 8, xxxiii. 4)... (bold-faced emphasis mine)
By the (נְשָׁמָה) of God they perish, and by the (רוּחַ) of His nostrils they are consumed. Job 4:9
All the while my (נְשָׁמָה) is in me, and the (רוּחַ) of God is in my nostrils. Job 27:3
The (רוּחַ) of God made me, and the (נְשָׁמָה) of the Almighty gave me life. Job 33:4
In addition, Isa. 42:5, regarding which Aubrey Rodway Johnson wrote,8
Like the נְשָׁמָה, the רוּחַ is also blown into someone, causing them to live.9 Also like the נְשָׁמָה, the רוּחַ is present in the nostrils.10 It, like נְשָׁמָה, is followed by חַיִּים ("life") because it is a vital constituent in living creatures.
The Hebrew Phrase נִשְׁמַת רוּחַ חַיִּים
While Gen. 7:22 is the only verse that contains the phrase נִשְׁמַת רוּחַ חַיִּים, the phrase נִשְׁמַת רוּחַ without חַיִּים occurs in 2 Sam. 22:16 and Psa. 18:15. As was previously demonstrated, נְשָׁמָה and רוּחַ are synonymous. Thus, the double genitive construction נִשְׁמַת רוּחַ חַיִּים could be understood as, "the (נְשָׁמָה), that is to say, the (רוּחַ חַיִּים)," where נְשָׁמָה stands in apposition to רוּחַ חַיִּים, and thus, רוּחַ חַיִּים (ruach chayyim) = נְשָׁמָה (neshama), just as נִשְׁמַת חַיִּים (nishmat chayyim) (used elsewhere in the Tanakh; cp. Gen. 2:7) could theoretically stand in apposition to רוּחַ (ruach).
Oldest Extant Manuscript
As far as the oldest manuscript is concerned, it would probably be the Samaritan Pentateuch as the earliest extant Masoretic texts (those with diacritics) are dated around the 9th-10th century A.D.
According to August Freiherr von Gall in Der Hebräische Pentateuch der Samaritaner, there is no difference between the Samaritan Pentateuch and the Masoretic text as far as the letters themselves are concerned (Samaritan Pentateuch lacks diacritics).
Gen. 2:7 contains the phrase נשמת חיים (nishmat chayyim):
Gen. 6:17 contains the phrase רוח חיים (ruach chayyim):
Gen. 7:15 contains the phrase רוח חיים (ruach chayyim):
Gen. 7:22 contains the phrase נשמת רוח חיים (nishmat ruach chayyim):
1 cp. Isa. 54:16
2 Isa. 2:22
3 Deu. 20:16 cp. Jos. 10:40, 11:11, 11:14; 1 Kings 15:29; Psa. 150:6
4 p. 27, footnote 6
5 1 Kings 17:17
6 p. 27
7 p. 78
8 p. 29
9 Eze. 37:9
10 Job 27:3; Lam. 4:20
Johnson, Aubrey Rodway. The Vitality of the Individual in the Thought of Ancient Israel. 1964. Reprint. 2nd ed. Eugene: Wipf and Stock, 2006.
Keil, Carl Friedrich. Commentary on the Old Testament. Vol 1. 1900. Reprint. Trans. Martin, James. Grand Rapids: Eerdmans, 1986.
Der Hebräische Pentateuch der Samaritaner. Ed. von Gall, August Freiherr. 1918. Reprint. Berlin: De Gruyter, 2010.