What is the Cause of Mortality?

According to clean flesh mortality and sin can be separated.

"Who said that mortality can only be affirmed of sin? That is totally wrong. Mortality exists where sin has not entered. Does a lobster die because of sins? Do fish die because their flesh is defiled by sin?" (Clean Flesh teacher quoted in Logos, 1971, p. 207; Reprinted in the Atonement book published by Graeham Mansfield)

What have Christadelphians historically taught?

"Mortality means sin: sin in the flesh." (AJ, Logos, 1971, p. 209)

"In our own day, as recent painful experience has made us aware, a class of believers are treading the same dangerous ground, in teaching that the flesh of Jesus was destitute of that which, in the flesh of his brethren, constitutes the cause or source of mortality." (Robert Roberts, The Christadelphian, 1873, p. 551)

"14. —Peter says 'he bore our sins in his own body on the tree.'—(1 Peter 2:24; Is. 53:6.) Does this mean the very acts of disobedience themselves or their effects? As the former is inadmissible, it must be the latter. If he bore their effects in his body, was not his body mortal, which is the effect of sin?" (Robert Roberts, The Christadelphian, 1873, p. 462)

"So here was a change in physical constitution consequent upon sin, which at last resulted in death. No man understands the balance of life even in mortality. We cannot hope, therefore, this side the Kingdom of God to understand the precise interference with the balance of life that was Adam’s before sin entered the world. All we know (because we are told it by God through an apostle) is that death in relation to man is 'by sin.'" (The Christadelphian, 1907, p. 456-457).

"For those, however, who are prepared to go a little more deeply into the subject, we commend the words of our correspondent. We are dealing with Divine ideas, and the Bible is the text-book which must define our terms. There, the word 'mortal' is only used in relation to the sin-state. It appears many times, but is mainly translated 'man'. The sense in which the word is Scripturally used is revealed in such places as Rom. 6: 12 or 2 Cor. 4: 11" (H. P. Mansfield, Logos, 1951, p. 57)

"His character was spotless; but as being the seed of the woman, of whom no clean flesh can be born (Job 25:4), and seed of Abraham, which is not immaculate, be it virgin or Nazarite,his nature was flesh and blood (Heb. 2:14), which Paul styles 'sinful flesh,' or flesh full of sin, a physical quality or principle which makes the flesh mortal; and called 'sin'because this property of flesh became its law, as the consequence of transgression. 'God made Jesus sin for us, who knew no sin; that we might be made the righteousness of God in him.'—(2 Cor. 5:21.)" (Herald of the Kingdom and Age to come, 1855, p. 51; Reprinted in The Christadelphian, 1873, 1917)
"Who shall deliver me from the body of this death'—-from this law of sin and death to which my body is subject? There is but one that can deliver, even Jesus Christ the Lord, who partook of flesh and blood that through death he might destroy this law of sin and death from the body, that is, diabolos." (John Thomas, Herald of the Kingdom and Age to Come, 1852, p. 182; Reprinted in The Christadelphian 1873, p. 484)
"To say that Adam was mortal before he sinned is really to contradict this, and to affirm that death did not come 'by sin,' but only accelerated death; that the man would have died in any case, even apart from sin, being 'mortal,' but that sin caused him to die quicker. But nevertheless it is objected that sin did not physically change his flesh—as though disease and death did not involve physical change! We have shown (Christadelphian, January, p. 24) the Bible usage of the term 'mortal.' No brother has any authority to invent a new meaning under profession of superior enlightenment (Shield, February, p. 27). In the mouths of inspired men 'mortal' means 'obnoxious to death,' and they teach that man is 'obnoxious to death' by sin." (The Christadelphian, 1905)
Then, again, the man is described as being very good (Gen. 1:31); and, as Adam had not formed a character at that time, it could only be in relation to his physical make-up; but, after disobeying, he is described as knowing good and evil; therefore, the law of mortality became a fixed law of his members, flesh made mortal because of sin, and, therefore, called sin in the flesh, or sin’s flesh, the result of which is exhibited in Mark 7:21–22: Jas. 1:14: Rom. 7:18." (C.C. Walker, The Christadephian, 1914)