Evolution does not affect our understanding of the need for atonement; that all humans are mortal, prone to sin, and in need of salvation. Brother Thomas and brother Roberts originally believed Adam and Eve were created mortal and prone to sin.      This did not affect their correct understanding of the atonement. They changed their views on this subject  without being re-baptized.
What about pre-adamites?
God showed His love and care to those humans who lived before Adam and Eve in the same way as today; He ‘causes the sun to rise on the evil and the good, and sends rain on the righteous and the unrighteous’ (Matthew 5:45), ‘giving you rain from heaven and fruitful seasons, satisfying you with food and your hearts with joy’ (Acts 14:17). However, since Adam was the first human to whom God revealed Himself, the pre-Adamites were ignorant and cannot be raised to judgment by a law they never knew; ‘all who have sinned apart from the law will also perish apart from the law’ (Romans 2:12), and ‘there is no accounting for sin when there is no law’ (Romans 5:13).
God did not fail to give them any opportunity to know Him. He revealed His existence to them, for ‘he did not leave himself without a witness by doing good’ (Acts 14:17), ‘because what can be known about God is plain to them, because God has made it plain’ (Romans 1:19), and that ‘since the creation of the world his invisible attributes – his eternal power and divine nature – have been clearly seen’ (Romans 1:20), so that ‘people are without excuse’ (Romans 1:20).
Paul states dogmatically that God made such arrangements ‘so that they would search for God and perhaps grope around for him and find him’ (Acts 17:27). It was possible for them to find God, and while they groped around He cared for them with the blessings of the natural world which the faithful also receive (Matthew 5:45).
Some object to the idea that God would have pre-Adamite humans to die in ignorance of His plan of salvation, on the basis that this contradicts God’s care and concern for His creation. Yet it is undeniable that most people since Adam have lived and died in complete ignorance of salvation. Scripture shows God works by personally revealing His covenant relationship to a small minority. Though some might come to know Him through preaching, God has left millions to die in ignorance of salvation.
Did Cain marry a sister?
Did one of Cain’s sisters willingly marry the man who had just murdered one of her brothers? If it is wrong to accept the existence of other people simply because the Bible does not mention them, then it is wrong to claim Cain married a sister the Bible never mentions. The idea that Adam and Eve already had more children by the time of Cain is contradicted by Eve’s statement that the birth of Seth comforted her after the death of Abel; ‘God has given me another child in place of Abel because Cain killed him’ (Genesis 4:25).
Paul made it clear that incest is so unnatural we do not need a commandment from God to know it is wrong; he said it is ‘not permitted even among the Gentiles’ (1 Corinthians 5:1-2). To Paul, even incest with a step-mother is so obviously unnatural and wrong that the unenlightened heathen realize it’s unacceptable, and followers of Christ should realize it’s wrong without needing God’s explicit revelation.
The Genesis flood
We are told the Nephilim ‘were on the earth in those days (and also after this)’ (Genesis 6:4), meaning they survived the flood; so flood was local, and did not destroy all people on the planet. Physical evidence confirms the flood was local, as brother Roberts acknowledged, and the Biblical evidence for the survival of other humans proves not all humans today are descended from Noah, and therefore not from Adam.
 This explicitly contradicts and excludes the false doctrine of Clean Flesh.
 ‘Adam’s nature was animal. Very good of its kind, as was the nature of all the other creatures. These did not sin, yet they returned to dust whence they came. So probably would Adam, if he had been left to the ordinary course of things as they were. But he would not have returned to dust if he had continued obedient. He would doubtless have been “changed in the twinkling of an eye” on eating of the Tree of Life. But, being disobedient, his sin determined his fate, and that of the creatures. It doomed them all to death according to law, and “nature” unchanged was permitted to take its course.’, Thomas, ‘The Bible Doctrine Concerning the Tempter Considered. No. II.’, The Herald of the Kingdom and Age to Come (2.8.181), 1852.
 ‘Death and corruption, then, with reproduction, the characteristic of spring and summer, is the fundamental law of the physical system of the Six Days. Adam and Eve, and all the other animals born of the earth with themselves, would have died and gone to corruption, if there had been no transgression, provided that there had been no further interference with the physical system than Moses records in his history of the Six Day.’, Thomas, ‘Our Terrestrial System Before the Fall’, The Herald of the Kingdom and Age to Come (5.7.159), 1855.
 ‘Our friend imagines there was a change in the nature of Adam when he became disobedient. There is no evidence of this whatever, and the presumption and evidence are entirely the contrary way. There was a change in Adam’s relation to his maker, but not in the nature of his organization. What are the facts? He was formed from the dust a “living soul,” or natural body. His mental constitution gave him moral relation to God.’, Roberts, ‘The Relation of Jesus to the Law of Sin and Death’, The Christadelphian (6.578.85), 1869.
 ‘The phrase “sin in the flesh” is metonymical. It is not expressive of a literal element or principle pervading the physical organization. Literally, sin is disobedience, or the act of rebellion. The impulses that lead to this, reside in the flesh, and therefore come to be called by the name of the act to which they give birth. In determining first principles, we must be accurate in our conceptions. The impulses that lead to sin existed in Adam before disobedience, as much as they did afterwards; else disobedience would not have occurred. These impulses are in their own place legitimate enough. We can judge of this matter by experience, because the human nature under discussion is the human nature we have upon ourselves and see in operation around us. There is no such thing as essential evil or sin. Evil and sin are relative terms. There is no propensity but subserves a good purpose in its own place. Sin is forbidden use; evil, interference with desired conditions as a punishment of sin, sometimes flowing out of sin itself. The difficulty is to keep the impulses in the legitimate channel.’, ibid., p. 85.
 ‘Adam, before transgression, though a living soul (or natural body—1 Cor. 15:44–5), was not necessarily destined to die, as obedience would have ended in life immortal. After transgression, his relation to destiny was changed. Death (by sentence,) was constituted the inevitable upshot of his career. He was, therefore, in a new condition as regarded the future, though not in a new condition as regarded the actual state of his nature. In actual nature, he was a corruptible groundling before sentence, and a corruptible groundling after sentence; but there was this difference: before sentence, ultimate immortality was possible; after sentence, death was a certainty. This change in the destiny lying before him, was the result of sin.’, Roberts, ‘Apparent Contradictions Reconciled’, The Christadelphian (6.62.243), 1869.
 ‘That is, his disobedience evoked from God a decree of ultimate dissolution. This was the sentence of death, which, though effecting no change as regarded his constitution at the moment it was pronounced, determined a great physical fact concerning his future experience, viz., that immortality, by change to spirit nature, was impossible, and decay and decease inevitable. The sentence of death, therefore, appertained to his physical nature, and was necessarily transmitted in his blood, to every being resulting from the propagation of his own species.’ , ibid., p. 244.
 ‘Brother Roberts became much more conservative on this matter in after years, and so does everyone who, like him, has a great respect for the Word of God.’, Walker, ‘Was the Nature of Adam Changed After He Sinned in Eden?’, The Christadelphian (58.684.258), 1921.
 Speculation that Adam and Eve would surely have had more than three children by the time they had Seth when Adam was 105, is contra-indicated by the fact that Noah didn’t have children until he was 500 years old, and still had only three children by the time he was 600 years old (Genesis 5:32; 7:6). The fact is that when the Bible wants to refer to additional sons and daughters in the record of Adam, it does so explicitly; ‘during this time he had other sons and daughters’ (Genesis 5:4), ‘and he had other sons and daughters (Genesis 5:6, 10, 13, 16, 19, 22, 30).
 ‘it would seem reasonable to conclude from the principle already looked at – the divine sparingness of means – that the flood was co-extensive only with the Adamically-inhabited portion of the globe.’, Roberts, ‘The Visible Hand of God’, The Christadelphian (18.205.308), 1881; however brother Roberts still believed all humans had perished in this local flood.