MERGING OF ANCESTRAL LINES 485 



blood. But through the five hundred thousand or million years of human 

 existence one can imagine cycles in which mergings alternate with spreads; 

 as populations have "increased" and "diminished," when viewed in descend- 

 ing direction. 



In order that the number of ancestors of a person, or a group, may actu- 

 ally decrease from generation to generation, so many sibs must appear in 

 some one generation that the next preceding generation will have a loss of 

 over half the number it would have had without merging, and this must hap- 

 pen not only in one generation, 'but be a frequent or continuous process. 

 The condition under which this must usually have occurred is when a small 

 group separated from its kin, went as pioneers to a completely isolated and 

 favorable area, and there increased so rapidly that couples often had several 

 fertile children. The later generations, however numerous they may be, 

 will all be descended from the few pioneers but from none of the contem- 

 poraries of the latter. In the earlier hundreds of thousands of years of 

 human existence there must have been many times when small groups mi- 

 grated to another habitat in which they throve, thus giving to the pioneers 

 a high value as multiple ancestors. 



An increasing population means larger families, and that means (when 

 looking backward) the merging of ancestral lines. This is an essential part 

 of the evolution of life on the earth, but the question to us is; did the de- 

 scendants of a particular pioneer group survive? If not, then the pioneers 

 were not permanent ancestors and cease to interest us. 



Was there ever a time when the ancestors of all mankind were reduced to 

 a single couple? If so, these two alone must have been the fertile pioneers 

 of a surviving group while all their kin, and all other contemporary human 

 groups — if such existed — died out. All our mixed lines of ancestry must 

 have merged into the two unbreakable lines of all male and all female, each 

 running back to this single pair. But even if this has happened, with the 

 four parents of this couple the process of ancestral spread appears again: 

 one cycle has ended and a new one has started. 



