And our kinship lines tie in a lot more recently than Noah! This fact is the history of our families. When historical demographers (people who study populations of the past) talk about drastic losses of population during the Black Plague, our ancestors were the survivors. That suffering was their suffering, and the rebuilding was their work. If we have all gone different ways along in the great movements in history, it is because our ancestors did what they did that the great movements happened at all.

Thus the history of populations is the history of families and their individual members. But since we don't have all their stories, we have to imagine them from the broad patterns - the statistics that human beings leave behind.

How many people have lived on the earth?

This question is very hard for the historical demographer to answer, since regular census taking started only a few centuries ago. Some very early censuses, such as when Moses counted the men of Israel who were "able to go forth to war" (Num 1:3), were sporadic and covered only a small part of the world's population.

But by making some assumptions about general trends in population, demographers have made some rough estimates of the number of people who have lived on the earth. One approach to this problem yields an estimate of about 70 billion people since the beginning of history.

However, this estimate and other guesses by historical demographers assume that man has lived on the earth for a least a million years. These demographers also do not take into account the great flood, which left only Noah and his family to repopulate the earth. If we assume that Adam and Eve began their mortal existence about 4000 BC and the great flood occurred about 2400 BC, we get a picture of population growth quite different from that of most demographers. As the population growth shows, world population probably grew in two expanding cones rather than one - one from Adam and Eve and one from Noah following the flood. According to this pattern, a rough estimate of the total number of people ever on the earth is around 51 billion people. This means that 8 percent of the people who have ever lived on the earth are currently living (just over 4 billion) - a sizable proportion, considering the time that has passed since man was introduced to the earth.

Since populations are made of people, all people are part of some kinship line, the patterns of change in populations of villages, nations, continents, or the entire world are simply the growth or interruption of various kinship lines over the years. The cone of descendants diagram shows that the descendants of a particular couple are a miniature population, growing like the expanding cone pattern of general population growth.

But there is one very important difference between the history of a kinship line and the history of a whole population: while plagues, wars, and famines make only slight dents in worldwide population growth, they have tremendous impact on individual kinship lines.

For example, the Black Plague that struck Europe in 1720 and 1721 probably killed over 20 million people - completely wiping out thousands of families.

This huge loss of life seems even greater when we think of how many descendants those 20 million people would have had if the plague had not occurred. An earlier epidemic in England cut the population of England by 40 percent - almost half - between 1348 and 1377.

Some families were hit hard by the plague, while others came through with few losses. In one English village nearly half the deaths in the 1546-17 Black Plague epidemic occurred in only 8 percent of the families. Many families were probably left without any children to carry on the family line. Wars, famines, and other disasters have had the same effect many times in history.

Kinship lines can also die out quietly through childlessness of married couples, deaths of children before they are able to marry, and adults who never marry. Suspecting that few of the great men of the past had living descendants, Francis Galton, a nineteenth century mathematician, made a study and figured out how likely it is for a kinship line to die out in a certain number of generations. The results are quite surprising.

For example, in the United States in 1960 there was about an 80 percent chance that a man would have no descendants with his last name thirteen generations later.

The chance of a kinship line dying out depends on the death rate of the society and the number of children in each family. Even in societies where couples have many children there is a 20 to 30 percent chance that a family line will die out after ten generations or so. What does this mean in the search for our ancestors? Many of the family lines existing, for example, in the year 1700 have no male descendants bearing the family name living today. This means that most of the family lines today are descended from fewer than 60 percent of the family lines that existed two centuries ago.

As one demographer said, "Either one's line is extinguished or one has hundreds of thousands of descendants." This principle points out how important the Lord's promise was when he said that Abraham's seed would be multiplied "as the stars in the heaven, and as the sand which is upon the sea shore" (Gen 22:17) and that his seed would continue until the end of the earth (Gen 22:18).