UNIT SIX 



How Did Life Begin? 



1 How did life begin? 



2 Did all kinds of living things begin at the same time? 



3 Is there life anywhere else in the universe besides on our earth? 



4 How con we tell about the kinds of life that there were in very early 



times? 



5 Has the earth always been populated with the same kinds of plants 



and animals? 



6 Can living things come into being today from non-living materials? 



7 Why are there not the same kinds of plants and animals in different 



parts of the world that have the same climate? 



8 What kinds of characteristics are inherited? What kinds are not? 



9 How can we tell that distinct kinds of plants or of animals are related? 

 10 How do we create new kinds of plants or animals? 



From what we know it is reasonable to believe that all things living today 

 are the direct descendants of similar plants and animals — that they came from 

 parents. And we may assume that these ancestors also came from parents, 

 and so on back for generations and for centuries. But this process of living and 

 reproducing similar offspring could not have been going on forever. For 

 there is good reason to believe that at some time in the past the conditions 

 on the surface of the earth were not suitable for any of the existing plants and 

 animals. There must, then, have been a time when there were no plants or 

 animals at all. What were the ancestors of present-day species? How did 

 life start in the first place? 



We expect different species of plants and animals to inhabit different 

 climates. But the tropical animals of Africa are different from the tropical 

 animals of America. And the inhabitants of the southern Temperate Zones 

 are different from those of the northern Temperate Zones. Did the ancestors 

 of these different groups come into existence separately? That may well have 

 been, for all we can tell. We are puzzled still further by another fact: al- 

 though these widely distributed species are different, they have in common 

 very much that is apparently unrelated to their conditions and modes of living; 

 and yet they seem to develop along the same basic pattern. 



We assume from our daily observations that every living thing reproduces 

 its own kind — that figs come from fig trees and kittens from cats. Human 

 children generally resemble their parents more than other members of the 

 species. Brothers and sisters resemble each other more than they do their 

 cousins. But then, even the offspring of the same parents are not exactly 



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