It was Adam, as he is the one who first received the commandment from God.
Romans 5:14 Nevertheless, death reigned from the time of Adam to the time of Moses, even over those who did not sin by breaking a command, as did Adam, who was a pattern of the one to come.
That consequences are:
- Sin entered the world
- Adam ushered the sin, sorrow and all the trouble we have right now.
Romans 5:12 Therefore, just as sin entered the world through one man, and death through sin, and in this way death came to all men, because all sinned--
"Death" here is not only physical death, but also spiritual death. It's surprising that even if you don't teach a child to sin, she/he commits a sin.
The reason we are dying because Adam sinned against God. But, Jesus came so we will have life.
Jesus is the way the truth and the life.
James 1:15 Then, after desire has conceived, it gives birth to sin; and sin, when it is full-grown, gives birth to death.
Romans is the gospel of grace. Salvation is free but not cheap.