The problem is actually more serious yet. According to Genesis 4, Cain not only found a wife, but by the time of the birth of his son Enoch, he found enough people to help him build and populate a city. Furthermore, Adam and Eve did not give birth to Seth until after Cain murdered Abel and was banished to Nod.
Resolving the Population Problem
The first step in the solution of this problem is to recognize that Adam and Eve had many more children than Cain, Abel, and Seth. Genesis 5:4 says that "after Seth was born, Adam lived 800 years," and that "he had other sons and daughters." In fact, the genealogy of Genesis 5 records that every descendant of Adam down to Lamech had "other sons and daughters." These other sons and daughters were born to men even older than 187 years. Considering the long life spans recorded in Genesis 5 (which I take seriously based on evidences described in The Genesis Question, pp. 117-122) and assuming that couples remained reproductive for about half their lifetime, the possibility existed for a veritable population explosion. In fact, the world's population could have approached a few billion by the time of Adam's death at the age of 930 (see table).
|Expected Population Growth in Adam's Lifetime|
|According to Genesis 5, life spans from Adam to Noah averaged 912 years. Each of the patriarchs mentioned had "other sons and daughters" in addition to the sons recorded by name. The table calculations are based on:|
|year||reproducing couples||children born||total population|
According to the simple mathematics, if Cain waited to marry until he was about 200 years old, he probably had several women to choose from, providing some migrated eastward to Nod with other family members. If he waited another 200 years to build a city, he could have had at least a few thousand people to help him, again assuming some migration occurred. It is not entirely impossible that Cain had sisters from whom to take a wife even before his banishment and the birth of Seth. The text does not tell us.
Archeological evidence indicates that population growth did not take off explosively in the pre-Flood era. What happened? My view is that the sin of Cain eventually grew out of control. Murder may well have become the number one cause of death. This scenario would explain the very strong language God used with Noah in Genesis 9:6, commanding Noah's descendants to exercise whatever means necessary up to the death penalty to restrain the sin of murder.
The latter part of the fourth chapter of Genesis suggests, however, that the sin of murder took several generations to reach catastrophic proportions. Thus, the numbers suggested above for Cain's marriage candidates and fellow citizens are not likely to have been significantly affected.
Resolving the "Incest" Problem
Given that we are all descended from Adam and Eve, either Cain or one of his brothers must have married a sister. This would seem to violate the commands recorded in the book of Leviticus forbidding marriage between brothers and sisters. The Levitical laws, however, must be considered in their proper historical context.
Though the book of Genesis condemns sexual relations between children and their parents, it nowhere prohibits a man from marrying his sister or niece. Abraham, for example, married his half-sister without compunction. Not until the time of Moses were laws established forbidding a man from marrying a sister or niece. The timing of this command makes perfect sense biologically, for genetic defects as a result of intra-family marriage would not begin to crop up until after the first few dozen generations.
The Mystery of Cain's Mark
Genesis 4 tells us that God found it necessary to put a mark on Cain as a warning to other humans not to take vengeance upon him. The necessity of this mark indicates that the population of the human race had reached (or would reach) such a level that mistaking Cain for someone else would be a problem.
The text says nothing about what kind of mark Cain received or about its being passed on to his progeny. I see no basis for believing that any one of the races of man carries the mark of Cain.
A "Last Days" Warning
In many places Scripture speaks of horrible wickedness again overtaking humanity in the "last days." One indicator that these last days may be approaching is the fact that for the first time since the Flood, murder has become the prime cause of death for the human race. In case you wonder about my statistic, it includes the murder of living humans before their birth. According to the widely published data on abortion, one individual in three experiences death by murder in the United States, exceeding the toll taken by both heart disease and cancer. The figures for the rest of the world's nations are typically worse. In the case of Eastern Europe, Russia, and the People's Republic of China, the figures are much worse.
Amazingly, God holds back His judgment upon such rampant evil and widespread lack of repentance, even among believers. He patiently waits for those who will yet throw themselves upon His mercy and receive His grace. May we who are His show the way until He comes.
Other resources that may be of interest:
The Genesis Question book by Hugh Ross
Are There Gaps in the Biblical Genealogies? article by William Henry Green
Who Was Adam? book by Fazale Rana