2:4 generations. “Generations” (Hebrew toledoth) is the word from which the book of Genesis gets its name. In the Septuagint it is rendered by the Greek genesis, which in Matthew 1:1 is translated “generation.” This is the first occurrence of the formula which marks the key subdivisions of the book: “These are the generations of...” The others are at Genesis 5:1; 6:9; 10:1; 11:10; 11:27; 25:19; 36:1,9; 37:2.
In all except this first one, the name of a specific patriarch is attached. Parallels with the terminology of the ancient Babylonian tablets indicate that these names are actually the signatures of the original writers of the particular tablets. That is, each of these primeval patriarchs kept the narrative records of his own generations, inscribing them on stone or clay tablets, then appending his name at the end, when he was ready to turn over the tablets and the task of writing the toledoth to the next in line. They eventually came down into Moses’ possession, who wrote the last section of Genesis (37:3ff), obtaining the information from “the sons of Jacob” (Exodus 1:1), as well as organizing and editing all the rest under divine inspiration, so that the entire collection finally became, in effect, the first of the five books of Moses. Since the first tablet (Genesis 1:1-2:4a) tells of events prior to the existence of any witness to record them, God Himself either wrote this section directly or specifically revealed it to Adam. It describes the generations of no person, therefore, but rather those of the cosmos itself.
2:4 in the day. As per the ancient Babylonian practice, the next tablet, beginning at 2:4b, keys in to the previous one by a phrase which both associates with the preceding histories and initiates the new narrative. The “day” of this verse does not necessarily refer to the entire creation week, as day-age theory advocates allege. It more likely refers to the first day of that week, when God created the earth and the heavens, as just stated in Genesis 2:4a, then proceeded also to “make” them through the rest of the six days.