34 



modern methods for "sequencing" or determining the detailed 

 arrangement, one after the other, of nucleotides in nucleic acids and 

 the amino acids in proteins. Since the information for the sequence 

 of amino acids in proteins is carried in the nucleic acid hereditary 

 material, the sequences of both these polymer classes reflect evolu- 

 tionary changes handed down from organism to organism. Sequenc- 

 ing the proteins or the nucleic acids allows the estimation of genea- 

 logical relationships among organisms presently alive. We can trace 

 relationships among a group of organisms back to a common ances- 

 tor of that group that lived hundreds of millions or even billions of 

 years ago. 



The way this is done is to compare the sequences of related 

 proteins or nucleic acids from a number of organisms. The degree of 

 difference or diversity between these sequences is determined. The 

 information concerning the degree of diversity among sequences can 

 be used to estimate the order and perhaps the relative times of 

 divergence of species from their ancestral relatives (fig. III-4). We can 

 estimate how old a particular family of organisms is by determining 

 how diverse the sequences within chosen molecules are among its 

 member species. If a family exhibits a relatively high level of 

 sequence diversity, it is held to have existed as a group for a rela- 

 tively long time. 



THE KINGDOMS OF LIFE 



Genealogical relationships traditionally have been defined by 

 such characteristics of organisms as shape, photosynthetic ability, 

 and mode of cell reproduction. Now we use the biochemical record 

 to define groups in terms of shared genetic information. For many 

 years, all life was held to be divided into only two major kingdoms: 

 the plant kingdom, studied by botanists, and the animal kingdom, 

 studied by zoologists. More recently we recognized that all plant and 

 animal cells exhibit fundamental properties not shared by bacterial 

 cells. The present view groups all cellular life into two major divi- 

 sions: the prokaryotes and the eukaryotes. Prokaryotic cells, all bac- 

 terial, are generally small, simple, relatively undifferentiated cells. 

 Eukaryotic cells, which make up all the plants and animals, fungi, 



