God observes the heart (1Sa 16:7). He showed favour toward Abel because of his faith which was reflected in his offering the best to God. Christians must show his faith by his internal attitude (which only God can see) and his external actions (which both God and man can see).
Seth was chosen to replace the faithful Abel (Heb 11:4). Why was he chosen? Some believe that God chooses or elects purely on His grace. No one is worthy to be chosen. So God has absolute sovereignty to choose whomever He wants. This is of course true. However, we notice that the descendants of Seth (the chosen line) who had their actions recorded in the Bible were all faithful to God. Perhaps God chooses those whom He foreknew to be faithful. Only those who have faith will be chosen by God. We also notice that God showed particular favour to Enoch and Noah who had great faith and became examples for all Christians to follow (Heb 11:5-7).
From Adam to Noah, the average life span was 912 years (not counting the 365 years of Enoch who went to heaven without dying) (Gen 5:5-31; 9:28). From Shem to Abraham, the average was 317 years (Gen 11:10-31). Today, a 70-year old person is regarded as a very old person, usually with frail health. Moses in Ps 90:10 says, “The years of our life are seventy, or even by reason of strength eighty.” It is difficult to imagine how anyone can live as long as those patriarchs. Are there any good explanations to human longevity recorded in Genesis?
What are the normal explanations for the longevity of man in ancient times?
The longest living man was Methuselah who lived 969 years. This is about 8 times longer than the oldest human on record in the 20th century even though we now possess advanced technology to extend life. [Note that according the genealogies (presuming there are no gaps), Shem died at the age of 600, after the death of Abraham (at age 175), and could have witnessed the birth of Esau and Jacob.]
Legends: Some believe the record is fictional or legendary and does not reflect reality, just like legendary kings with reigns of thousands of years as recorded in ancient legends (such as the Babylonian writings).
Dynasties: Some believe that the duration after the birth of the sons (in Gen 5) indicates only the duration of his clan or family dynasties. For example, Gen 5:3-5: “When Adam had lived 130 years, he fathered a son in his own likeness, after his image, and named him Seth. The days of Adam after he fathered Seth were 800 years…Thus all the days that Adam lived were 930 years.” According to this explanation, Adam died some unknown time after his 130th year but his clan lived on for a total of 930 years. But this explanation would make no sense in the case of Enoch (Gen 5:23), as his grandson Lamech lived until at least 434 years (65+187+182) after the birth of Enoch but Enoch’s clan (according to this explanation) was described as living for only 365 years.
Shorter years: Some believe that the ancient “Hebrew” year must have been very much shorter than a year of today. However, there is not a trace of evidence to indicate that the ancient peoples counted their years significantly different from today (as the length of one year is normally based on the seasons). Further, Gen 5 records that both Enoch and Mahalalel became fathers in their 65th year. If 8 pre-Flood years roughly equalled say 1 year today, then both of them had their children at about age 8. In addition, the reference in Gen 6:3 that human life spans will be shortened to less than 120 years will then make no sense.
Different Earth: Some investigated the possibility that the Earth’s rotation period or the revolution period around the sun has slowed significantly but there is no such evidence. In addition, the survival of ancient people depended on their success in growing food and agricultural success depended on the length of seasons. There was never an agricultural reference to the different length of seasons in ancient times. All ancient records stated that a calendar year consisted roughly of 12 months of 30 days each.
Symbolic: Some believe the numbers were only a symbol of long life, perhaps to magnify the breadth of God’s blessing. But nowhere in Genesis was there any hint of the numbers being symbolic.
Genetics: Some believe that human beings were genetically more pure in this early time period, so there was less diseases to shorten their life spans. Another similar explanation is that sin had not yet achieved its full deleterious effects on the human society.
Populate the Earth: Some believe that God deliberately gave early people longer lives so that they had time to “fill the earth.”
The explanations are arranged in the order of the least likely (no.1) to the most likely (no.7).
Are there other indications that can support the existence of long life spans in ancient times?
The rapid rise of ancient civilizations in the Fertile Crescent of Mesopotamia was the implicit evidence of human longevity. Long life spans would promote a high rate of advance in a most efficient way. A long life would give an individual opportunity to accumulate knowledge and experience, reducing the necessity of starting from scratch. In contrast, post-Flood peoples show evidences of starts and stops and restarts in their technological and cultural progress.
The change in dietary law after the Flood correlates with a change in life expectancy. In Gen 9:2-3, God told Noah after the Flood that the people need no longer to restrict their diet to green plants. They could now eat animals. A diet that includes meat adds higher concentrations of heavy elements to the body (anywhere from 10 to 10,000 times more than a vegetarian diet). While these heavy elements may help the body, they are also detrimental to health, even life-threatening, if they accumulate in the body for a few hundred years. But the health risk becomes negligible for people living only 120 years or less.
What about vegetarians of today? If diet is a main factor for longevity, then why would today’s vegetarians not live longer than non-vegetarians? The reason is because they already have heavy elements in their bodies at birth. In fact, the complete abstention from meat may create health risks because our bodies do need the supply of heavy elements. Some vegetarians have been known to die of mulnutrition.
What are the possible reasons to explain the shorter life span after the Flood?
The more popular theory explaining the long life spans before the Flood is the canopy theory. It presumes that Gen 2:6 describes “streams” or “mist” coming out of the ground watering the Garden of Eden. This canopy of mist (water vapour) would have shielded man from various forms of life-shortening radiation so that longevity was possible. With the coming of the Flood, the climate of the Earth underwent a tremendous change so that this canopy disappeared after the Flood.
However, there are 4 problems: [a] There is no historical or scientific evidence for the existence of such a canopy; the theory is speculative at best. [b] Any canopy substantial enough to protect man from radiation would either collapse or dissipate into outer space. [c]In theory, such covering would set up a powerful greenhouse effect that no ice or liquid water would remain on Earth to sustain life. [d] Although the vapour canopy would provide some protection against ultraviolet radiation, it would not impede the hard cosmic rays.
Of course, any theoretical objections can be countered by the argument of God’s providence.
More recent efforts to find an explanation start with the identification of possible factors that limit human life span. 13 factors were identified:  war and murder,  accidents,  disease,  famine or inadequate nutrition,  metabolic rate,  inadequate exercise,  stress,  chemical carcinogens,  ultraviolet radiation,  solar X-ray radiation,  radioisotope decay radiation,  cosmic radiation,  apoptosis.
Based on historical observations, the last 3 are the more probable factors: [a] radiation from radioisotopes (e.g. uranium, radium, thorium) in igneous rocks, [b] cosmic radiation (cosmic rays from outer space), and [c] apoptosis (biochemically “programmed” cell death).
How do the 3 probable factors affect life expectancy?
Radiation from radioisotopes:
It is possible that pre-Flood people lived in geographic locations well isolated from igneous rocks and were therefore exposed to minimal radiation from radioisotopes. However, those people who grow up and live all their lives in these areas (that is, far from igneous rocks) should have at least significantly longer life expectancy. This has not happened.
The effect that large amount of radiation significantly shortens human life spans has been clearly demonstrated by the effects of nuclear bombs in Hiroshima and Nagasaki in 1945, and the explosion of the nuclear reactor in Chernobyl, Ukraine in 1986.
The increase in the amount of cosmic radiation, if there is any, would explain why all man in the world will be affected. Astronomers discovered that harmful cosmic rays can come from quasars, black holes, neutron stars, novae (star explosions), supernovae (giant star explosions) and their remnants.
[a] Quasars are so extremely distant that their cosmic ray flux is small.
[b] The black holes and neutron stars in our galaxy actually result from supernovae.
[c] Novae occur hundreds of times more frequently than supernovae, but novae are over 10,000 times less powerful. Consequently, the radiation from novae is much less damaging to man.
[d] Supernovae are by far the prime contributor to cosmic rays incident on Earth.
In 1996, two astronomers determined from empirical evidence that most of the cosmic rays striking Earth come from a recent, nearby supernova.
The only supernova eruption that could possibly be implicated in generating the cosmic rays is the Vela supernova, about 1300 light-years away. [It emits 30 times more cosmic radiation than the 2 supernovae that happened in AD1016 and in AD1054.] Its damage to the ozone layer would have increased ultraviolet radiation by 2 to 10 times. Various estimates on the timing of the supernova range from 9000 BC to 35,000 BC.
Gen 11 indicates that the change in life span did not happen instantaneously. Life spans dropped from Noah (950 years) gradually down to Nahor (148 years). This may indicate the shortening of human life span as a result of increasing effects of cosmic radiation from Vela.
Recent research in human cells discovered a cell phenomenon called “apoptosis” or “programmed cell death” (PCD). Apparently, our cells are designed to shut down after a certain number of cell regenerations. Because of this, no matter how healthy and safe a lifestyle a person leads, he or she will not live beyond about 120 years.
Apoptosis is scientifically described as the disintegration of cells into membrane-bound particles that are then eliminated by phagocytosis (engulfing of cells by bacteria) or by shedding. It is also called predestined cell suicide which happens when cells are no longer useful. The significance of PCD has been recognized since 1972.
In the case of cells that do not regenerate such as brain cells, their apopotosis causes loss of body functions leading to gradual degeneration of the organism and eventual death.
This apoptosis may seem a curse but it is also a blessing in disguise because it is found to be a powerful force in limiting the development and spread of cancers and tumours.
As we now live in a environment that induces cancer (from radiation, chemicals, etc.), we need apoptosis. Without it, we would be much more susceptible to cancer and life span will probably even be shorter.
Gen 6 states that God acted purposefully to shorten human life spans. One beneficial effect of this is the limitation of the spread of human wickedness. One can only imagine how much evil a wicked person can do with a life span of 900 years. It is possible that God chose something like the Vela supernova to reduce human life span and at the same time designed apoptosis to mediate its effect in order to achieve His plan.
Who were the “sons of God” and “daughters of man” in Gen 6:2?
There are 4 possibilities:
 Just another name for ordinary men and women:
Man was created with the breath of God and males could be called the sons of God.
Woman was created from Adam’s rib so females could be called the daughters of man.
Therefore, this verse simply describes normal human marriages.
The event apparently resulted in God’s decision (punishment) in Gen 6:3. Ordinary marriages should not cause such a reaction from God.
How would such marriages give birth to “mighty men of old” in Gen 6:4?
 Reference to the male descendants of Seth marrying the female descendants of Cain:
Descendants of Seth revered God and were therefore called the “sons of God”. (Note that Gen 4:25—5:32 says nothing bad about the line of Seth.) Descendants of Cain did not revere God and were therefore called the daughters of man. (Gen 4:1-24 says nothing good about the line of Cain, except their cultural achievements.) Since the mothers normally exert greater influence over the children when they are young. These families (with mothers from Cain’s line) were consequently corrupted. Moral decline was the result of the Sethite family marrying outside its godly heritage.
In OT, sons may refer to members of a group; sons of God refer to those belonging to God. Hebrew elohim can be translated as a genitive of quality, meaning “godly sons”.
OT calls man “children of God” (Ex 4:22-23; Dt 14:1; 32:5); the term is more appropriate for Seth’s descendants.
Genesis illustrates how religious intermarriages (spouses from different religions) resulted in calamity for the righteous (Gen 28:1; 34:1ff; 38:1ff).
OT reference of “sons of God” is restricted to the Israelites, never to the descendants of Seth.
If Gen 6:1 uses “man” for the whole mankind, why would Gen 6:2 uses the same word for Cain’s descendants?
Would the marriages between believers and non-believers result in God’s wrath?
Were there absolutely no intermarriages between the sons of Seth and the daughters of Cain before ch.6?
 Reference to human leaders marrying ordinary women:
The verse refers to 3 possible sins: [a] The leaders saw pretty daughters of common people and married them by force or coercion; they were involved in polygamy and were arrogant like Lamech in Gen 4:19. [b] The leaders participated in worshipping false gods and had sexual relations with prostitutes. [c] The leaders took the virginity of pretty women previously engaged to other men.
“Sons of God” can refer to 2 kinds of leaders: [a] The word “God” (Heb. elohim) can mean a ruler (Ps 82:6-7; ESV, NIV: “gods”). The term is used to magnify the Davidic kings who were also described as “sons of God” (2Sa 7:14). [b] The word “God” can also mean a judge (Ex 21:6; 22:8) because they dispensed the justice of God.
The existence and development of hierarchy and leaders could be a result of increasing population in Gen 6:1.
“Daughters of man” could refer to the temple prostitutes (see Hos 4:10-14). This passage was written to warn Israelites against prostitution in worshipping false gods.
“Violence” in Gen 6:11 may be referring to the use of violence by leaders in seizing women.
The event here may be a warning to events during the Israeli exodus. The Israelis were involved in sexual immorality with Moabite women (Ex 25:1-2) and were punished.
Why would the marriages between leaders and common citizens or the temple prostitutes give birth to mighty men of old in Gen 6:4? Counter explanations for this difficulty: [a] The people in Gen 6:4 were not the children from the marriages in Gen 6:2. These people simply existed in that time period. [b] “Mighty men of old” and “men of renown” did not refer to giants but only to princes and warriors, just like Gen 10:8 describes Nimrod as “a mighty man” and “a mighty hunter”.
The sin involved in such marriages should not be greater than those in ch.3 and ch.11. Why would it lead to God’s destruction of mankind by the Flood? Counter explanation for this difficulty: The sin is arrogance and polygamy (note the plural for daughters), corresponding to Lamech’s arrogance and polygamy.
 Reference to fallen angels marrying human females:
This is probably the oldest opinion known, advocated among Jews (such as Josephus) and in OT Pseudipigrapha 1 Enoch 6—11. It was also supported by early Christian writers.
Some take a compromise view that “sons of God” were human leaders possessed by fallen angels (combining explanations 3 and 4).
OT refers angels as “sons of God” (Job 1:6; 2:1; 38:7; Ps 29:1; 89:7). The Alexandrian Septuagint also translated this verse as “the angels of God”. There are also indirect references linking fallen angels to the Flood (1Pe 3:19-20; 2Pe 2:4-5). Jude also mentioned about fallen angels in situations similar to this chapter (Jude 1:6-7). Early Church Fathers also explained the term as fallen angels.
2Pe 2:4-14 appears to describe the pre-Flood events linking corruption of angels (v.4) and sexual corruption (v.7,14). Objection to this explanation: only v.5 describes pre-Flood events.
If fallen angels married human women intending to spread immortality to the Earth, then it would explain why God said in Gen 6:3 that He will limit man’s life span.
There was a legend describing how mighty men were children of gods and human women (also in the Greek mythology).
If that was the sin of fallen angels, why did God destroy human beings? Counter explanations for this difficulty: [a] The women might be willing participants. [b] This is only one example of the numerous sins on Earth. [c] The fallen angels might have been punished too.
Angels are not corporeal beings and are probably non-sexual (neither male or female). Jesus said in Mt 22:29-30 and Mk 12:25 that angels do not marry. Heb 1:7,14 also says that angels are spirits. They do not possess any DNA for reproduction. Counter explanations for this difficulty: [a] Jesus was referred to loyal angels and were about end of the world. [b] Fallen angels could possess human beings who could then marry. (Mt 8:16,28; 12:43-45; Mk 1:23,32) This, however, has a difficulty: would the DNA be changed because of demonic possession?
There is not one case of a demon impregnating a woman being documented in history. Also, if that happened before the Flood, what can stop them from not repeating it now? Counter explanations for this difficulty: [a] When Jesus encountered evil spirits, they seemed to be afraid of being sent to the place of darkness and chains which Jude described as “the Abyss” (Gr. tartarus). Their inclination to cause a woman to bear their offsprings might be restrained by the threat of the terrible penalty of being confined to the Abyss. [b] After the Flood, “giants” were mentioned in other OT books, the last incidence being in 1Ch 20:8. Yet since the time of David, we see no evidence of suggestion of their return. One possible explanation is that the threat of consignment to the Abyss for angels who cross a sexual boundary was instituted at or after David’s time. However, it should be noted that this is pure speculation.
Conclusion: The largest number of commentators support the 2nd explanation although the 3rd explanation seems the best.